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The effects of skiing velocity on mechanical aspects of diagonal crosscountry skiing a
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d
Erik Andersson , Barbara Pellegrini , Øyvind Sandbakk , Thomas Stüggl
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& Hans-Christer Holmberg
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Swedish Winter Sports Research Centre, Mid Sweden University, Östersund, Sweden b
CeRiSM, Research Center for Sport, Mountain and Health, Rovereto, Italy c
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Faculty of Exercise and Sport Science, University of Verona, Verona, Italy d
Centre for Elite Sports Research, Norwegian University of Science and Technology, Trondheim, Norway e
Department of Sport Science and Kinesiology, University of Salzburg, Salzburg, Austria f
Swedish Olympic Committee, Stockholm, Sweden Published online: 23 Jun 2014.
To cite this article: Erik Andersson, Barbara Pellegrini, Øyvind Sandbakk, Thomas Stüggl & HansChrister Holmberg (2014) The effects of skiing velocity on mechanical aspects of diagonal crosscountry skiing, Sports Biomechanics, 13:3, 267-284, DOI: 10.1080/14763141.2014.921236 To link to this article: http://dx.doi.org/10.1080/14763141.2014.921236
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Sports Biomechanics, 2014 Vol. 13, No. 3, 267–284, http://dx.doi.org/10.1080/14763141.2014.921236
The effects of skiing velocity on mechanical aspects of diagonal cross-country skiing ERIK ANDERSSON1, BARBARA PELLEGRINI2,3, ØYVIND SANDBAKK4, ¨ GGL1,5, & HANS-CHRISTER HOLMBERG1,6 THOMAS STO ¨ stersund, Sweden, 2CeRiSM, Swedish Winter Sports Research Centre, Mid Sweden University, O 3 Research Center for Sport, Mountain and Health, Rovereto, Italy, Faculty of Exercise and Sport Science, University of Verona, Verona, Italy, 4Centre for Elite Sports Research, Norwegian University of Science and Technology, Trondheim, Norway, 5Department of Sport Science and Kinesiology, University of Salzburg, Salzburg, Austria, and 6Swedish Olympic Committee, Stockholm, Sweden
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(Received 3 September 2013; accepted 10 April 2014)
Abstract Cycle and force characteristics were examined in 11 elite male cross-country skiers using the diagonal stride technique while skiing uphill (7.58) on snow at moderate (3.5 ^ 0.3 m/s), high (4.5 ^ 0.4 m/s), and maximal (5.6 ^ 0.6 m/s) velocities. Video analysis (50 Hz) was combined with plantar (leg) force (100 Hz), pole force (1,500 Hz), and photocell measurements. Both cycle rate and cycle length increased from moderate to high velocity, while cycle rate increased and cycle length decreased at maximal compared to high velocity. The kick time decreased 26% from moderate to maximal velocity, reaching 0.14 s at maximal. The relative kick and gliding times were only altered at maximal velocity, where these were longer and shorter, respectively. The rate of force development increased with higher velocity. At maximal velocity, sprint-specialists were 14% faster than distance-specialists due to greater cycle rate, peak leg force, and rate of leg force development. In conclusion, large peak leg forces were applied rapidly across all velocities and the shorter relative gliding and longer relative kick phases at maximal velocity allow maintenance of kick duration for force generation. These results emphasise the importance of rapid leg force generation in diagonal skiing.
Keywords: Cycle characteristics, kinetics, Nordic skiing
Introduction The diagonal stride is the most common uphill technique used in classical cross-country skiing. The technique shows similarities to running with its diagonal arm and leg motion. In contrast to running, propulsive forces in diagonal skiing are generated by both the arms and legs and the leg thrust is followed by a gliding phase on the contralateral leg (Nilsson, Tveit, & Eikrehagen, 2004). Velocity of skiing can be elevated by increasing the product of cycle rate and cycle length. The velocity of the diagonal stride on snow, both on flat and slightly uphill (2.58) terrain, is adapted primarily by altering the cycle rate (Nilsson Correspondence: Erik Andersson, Swedish Winter Sports Research Centre, Department of Health Sciences, Mid Sweden ¨ stersund, Sweden, E-mail: [email protected] University, 83125 O q 2014 Taylor & Francis
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et al., 2004; Va¨ha¨so¨yrinki et al., 2008). In sprint skiing, faster skiers utilise higher cycle rates, than slower skiers, when using the diagonal stride technique (Zory, Barberis, & Rouard, 2005). Moreover, in order to maintain leg thrust duration and momentum while increasing diagonal roller-skiing velocity from high to maximal, on a steeper incline (78), skiers adopt a higher frequency running diagonal stride technique without gliding, i.e. overcompensate by a substantially higher cycle rate at a shorter cycle length (Sto¨ggl, Mu¨ller, Ainegren, & Holmberg, 2011). This adaptation is considered as a strategy to maintain enough kick time for force generation at maximal velocity. The role of cycle rate, cycle length, and time for force generation at different velocities has not been investigated at a typical incline (7.58) for diagonal skiing on snow. Based on previous studies from roller-skiing (Sto¨ggl et al., 2011) and on-snow skiing (Nilsson et al., 2004; Va¨ha¨so¨yrinki et al., 2008), it may be speculated that an increased cycle rate at a long cycle length is the main factor for increasing a high sub-maximal velocity up to a maximal velocity. During diagonal skiing on a slight uphill (2.58), Va¨ha¨so¨yrinki et al. (2008) demonstrated that the relative propulsion provided by the legs (in relation to the arms) increased at higher velocities. In addition, a higher performance level in diagonal roller-skiing at a 98 incline is related positively to the skier’s ability to generate large leg forces over short durations (Lindinger, Go¨pfert, Sto¨ggl, Mu¨ller, & Holmberg, 2009). As diagonal skiing velocity increases, the time during which force can be applied through poling and/or leg thrust is reduced. A 29% decrease in kick time has been reported from slow to maximal velocities, reaching only 0.12 s at maximal velocity. The shorter kick times at higher velocities were also associated with larger average kick forces (Va¨ha¨so¨yrinki et al., 2008). These observations clearly emphasise that the importance of a rapid leg force generation increases at higher diagonal skiing velocities. To avoid complications due to changing weather conditions, many recent investigations on the diagonal stride have been performed indoors on a treadmill using roller-skis (Ainegren, Carlsson, Laaksonen, & Tinnsten, 2012; Lindinger et al., 2009; Sto¨ggl & Mu¨ller, 2009; Sto¨ggl et al., 2011). Roller-skiing with ratcheted wheels enables static friction that is several times greater than skiing on snow, which allow skiers to exert high propulsive forces without ski-slipping, and is therefore less dependent on a skier’s technical skills (Ainegren et al., 2012). Thus, the ideal technique for diagonal roller-skiing may differ from the ideal technique for skiing on snow, and generalising results from the former to the latter may be questionable. Therefore, it may be important to compare the results of the studies conducted on snow to those that used roller-skiing in the laboratory. To date, only one study has examined the effects of different velocities on kinematics/ kinetics of diagonal skiing on snow (Va¨ha¨so¨yrinki et al., 2008). This study, however, was performed on an incline of 2.58 where elite skiers would normally use the kick double poling technique (Go¨pfert, Holmberg, Sto¨ggl, Mu¨ller, & Lindinger, 2013). Therefore, the purpose of this study was to analyse pole and leg cycle characteristics and kinetics of elite crosscountry skiers using diagonal stride technique at different velocities on a 7.58 incline on snow. The aims were to assess the mechanics of velocity adaptations and compare the diagonal stride technique of cross-country skiers who specialise in sprint versus distance races. It was hypothesised that: (1) elite skiers would adapt to higher skiing velocities by increasing both cycle rate and cycle length up to a high velocity, thereafter increasing cycle rate even further and slightly reducing cycle length up to maximal velocity, (2) the rate at which pole and leg forces are applied would increase with skiing velocity, and (3) sprintspecialised skiers would reach the highest maximal velocities by utilising a substantially higher cycle rate than distance-specialised skiers.
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Methods Participants
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Eleven elite male Norwegian cross-country skiers, including one Olympic and World Champion and two members of the Norwegian National Team (mean ^ SD: age 23.5 ^ 4.8 years, body height 1.81 ^ 0.06 m, body mass 77.4 ^ 6.9 kg, VO2max 72.1 ^ 4.6 ml/kg/min), volunteered to participate in the study. The participants consisted of four sprint-specialists, four distance-specialists, and three all-round skiers who performed equally well in sprint and distance races. This study was approved by the Regional Ethical Review Board of Umea˚ University and the participants were fully informed of all procedures prior to providing their written consent. Study design All skiers were tested on a 50 m uphill slope (7.58) on snow using the diagonal stride technique with the data from the final 20 m section used for analyses. Skiers were instructed to employ a gliding phase at all velocities (i.e., they were not allowed to switch to a runningtype diagonal skiing). Each skier performed the test at maximal velocity first (100%), followed by high velocity (80% of maximal), and finally moderate velocity (65% of maximal) at last. Trials in which the skier deviated from the predetermined velocity by more than 10% were repeated. All participants were familiar with the selected skiing velocities as they were regularly used in training sessions. The moderate velocity corresponded to an intensity that is normal for 10 –30 km competitions, whereas the high and maximal velocities corresponded to intensities that are appropriate for sprint competitions. Prior to testing, the skier performed a 15 min warm-up at 60 – 75% of maximal heart rate (measured by Polar S610, Polar Electro Oy, Kempele, Finland) and each trial was separated by 6 min of recovery with light activity (approximately 50% of maximal heart rate). Skiers utilised their own racing skis and all skis were prepared by the same professional ski technician using wax appropriate for the environmental conditions, in order to standardise grip and gliding properties. All trials were performed on the same day under stable weather conditions (light wind, an air temperature of 1 –28C, a snow temperature of 08C, and a relative humidity of 92 – 96%). The track was machine groomed on the evening prior to testing. Kinematics All trials were recorded using one panning camera (Panasonic NV-GS 280, Panasonic, Osaka, Japan). The shutter of the camera was set at 1/500 s to ensure high clarity and the frame rate of the video was 50 Hz. The video analysis was carried out using Dartfish Pro Suite 4.5 (Dartfish Ltd, Fribourg, Switzerland). For synchronisation of pole forces, leg forces, and video data, a simultaneous lift and push down of the right ski and pole was performed before and after each trial, with the skier standing still. The skiers started from standing at the base of the 50 m slope and accelerated to the required velocity. One pair of photocells (IVAR, LL Sport, Mora, Sweden) were placed over the final 20 m to obtain skiing times and velocities. The pole and leg movements were divided into gliding, thrust, and swing phases as illustrated in Figure 1. Cycle time was defined as the period from the start of one right pole plant to the subsequent right pole plant. The poling cycle was divided into a poling phase (ground contact) and a swing phase (tip of the pole in the air). The gliding and leg swing
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Figure 1. Sequence of actions (1–7) and time course of cycle and leg force characteristics associated with the diagonal stride skiing technique. a ¼ unloading phase (leg force minima); b ¼ peak pole force; c ¼ peak leg force; PLP, pre-loading phase; KP, kick phase. Data are presented for one subject skiing at high velocity and are mean values of five successive time normalised cycles.
times were defined according to the force data. The kick time was defined as the period starting when the ski was stationary on the ground, obtained from the video recording, and ending when the plantar force signal dropped to zero. The pre-loading time was calculated as the ski-loading time (based on the force data) minus the kick time. The gliding, pre-loading, kick, and leg swing phases are illustrated in Figure 1. In each trial, kinematic parameters were averaged over five consecutive movement cycles. Kinetics The poles used for force measurements were adjustable in length, and each athlete used their preferred pole length. The ground reaction forces directed along the pole were measured at a rate of 1500 Hz by a 15 g strain gauge load cell (Hottinger Baldwin Messtechnik GmbH, Darmstadt, Germany) fitted into a short (80 mm), lightweight (65 g) aluminium body mounted directly below the pole grip. Pole force data were amplified by a telemetric recording system (TeleMyo 2400T G2, Noraxon, Scottsdale, AZ, USA) and transferred to a laptop computer via an A/D converter card. The vertical plantar pressure for each leg was recorded at 100 Hz using a Pedar Mobile System (Novel GmbH, Munich, Germany). The total plantar foot area was divided into forefoot and rear-foot at 50% of foot length, and inside-foot and outside-foot at 50% of foot width. The relative force impulses of the forefoot and inside-foot were calculated by dividing
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the impulses of the forefoot and inside-foot by the ski-loading force impulse, respectively. The pole and plantar system was validated as described previously by Holmberg, Lindinger, Sto¨ggl, Eitzlmair, and Mu¨ller (2005) and the insole was calibrated with a Pedar calibration device. All kinetic parameters were analysed over the same five movement cycles as those used for kinematic analyses.
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Data analysis International Ski Federation (FIS) sprint and distance point rankings were used to classify each skier into sprint-specialised or distance-specialised groups. FIS points indicate a skier’s rank relative to the top-ranked skier in the world, who has 0 point, such that lower points indicate a better ranking. A skier was defined as a sprint or distance specialist if the ratio of the lowest to the highest FIS points was less than 55% and as an all-round skier if the ratio was 65% or higher. Accordingly, the points in sprint and distance races were, respectively, 38 ^ 21 and 110 ^ 14 for the sprint-specialised skiers ( p , 0.05), 97 ^ 22 and 47 ^ 18 for the distance-specialised skiers ( p , 0.05), and 85 ^ 34 and 77 ^ 27 for the all-round skiers. The cycle time, cycle rate (the reciprocal of cycle time), cycle length (the product of cycle time and skiing velocity), poling time, arm swing time, gliding time, pre-loading time, kick time, and leg swing time were determined. Relative time phases (% of cycle time) for poling, arm swing, gliding, pre-loading, kick, and leg swing were calculated by dividing the durations for the separate phases by the cycle time. The average right and left pole and leg forces during the five skiing cycles were calculated for each participant. The pole and leg force impulses over one movement cycle were determined as the average value of the right and left poling and ski-loading phases, respectively. The absolute peak force, leg force minima, and time to peak force were determined from the pole and leg kinetics (Figure 1). The leg force parameters were calculated over the entire phase of ski-loading (i.e., the pre-loading and kick phases together). The relative peak force was calculated by dividing the absolute force by the body weight (BW) and expressed as a percentage (%BW). The relative times to peak pole and leg force were calculated by dividing the times required to attain the maximal value by the total poling and leg thrust time, respectively. The rate of force development was calculated by dividing the peak force by the time to peak force during the ski-loading phase. All data were processed using the IKE-Master Software (IKE-Software Solutions, Salzburg, Austria) and MS Office Excel 2007 (Microsoft Corporation, Redmond, WA, USA). Statistical analyses Data were checked for normality using the Kolmogorov–Smirnov analysis and are presented as mean ^ standard deviation (SD). One-way repeated measures ANOVA tests with Bonferroni a correction were used for analysing differences in the variables measured at the three skiing velocities. A Mann –Whitney U test was carried out to identify differences in cycle and force characteristics at the different velocities between sprint-specialised and distance-specialised skiers. The level of statistical significance was set at a ¼ 0.05. All analyses were performed with SPSS 17.0 (SPSS Inc., Chicago, IL, USA). Results The cycle characteristics at the three different velocities of skiing are presented in Table I. The skiers reached a maximal velocity of 5.6 ^ 0.6 m/s and the moderate and high velocities were 62 ^ 4% and 80 ^ 4% of the maximal velocity, respectively. The cycle rate increased continuously as the velocity increased ( p , 0.05 for all pair-wise comparisons), while cycle
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Velocity
Velocity (m/s) Cycle time (s) Poling time (s) Arm swing time (s) Gliding time (s) Pre-loading time (s) Kick time (s) Leg swing time (s) Relative poling time (%) Relative arm swing time (%) Relative glide time (%) Relative pre-loading time (%) Relative kick time (%) Relative leg swing time (%)
Moderate (3.5 m/s)
High (4.5 m/s)
Maximal (5.6 m/s)
3.5 ^ 0.3bc 1.15 ^ 0.07bc 0.45 ^ 0.07bc 0.70 ^ 0.04bc 0.42 ^ 0.05bc 0.07 ^ 0.01 0.19 ^ 0.03bc 0.47 ^ 0.06bc 39 ^ 4 61 ^ 4 36 ^ 3c 6^1 17 ^ 2c 41 ^ 5c
4.5 ^ 0.4ac 1.01 ^ 0.10ac 0.38 ^ 0.04ac 0.63 ^ 0.09ac 0.36 ^ 0.08ac 0.06 ^ 0.01 0.16 ^ 0.02ac 0.43 ^ 0.06ac 38 ^ 4 62 ^ 4 36 ^ 5c 6^1 16 ^ 2c 43 ^ 5c
5.6 ^ 0.6ab 0.69 ^ 0.09ab 0.27 ^ 0.03ab 0.41 ^ 0.07ab 0.19 ^ 0.07ab 0.05 ^ 0.01 0.14 ^ 0.02ab 0.32 ^ 0.05ab 40 ^ 3 60 ^ 3 27 ^ 8ab 7^1 20 ^ 2ab 46 ^ 7ab
Notes: The values presented are M ^ SD (n ¼ 11). The letters indicate statistically significant differences from moderate (a), high (b), or maximal (c) velocity ( p , 0.05).
length increased from moderate to high velocity with a decrease from high to maximal velocity (both p , 0.05) (Figure 2). The absolute poling, arm swing, gliding, kick, and leg swing times decreased as velocity increased ( p , 0.05 for all pair-wise comparisons), while the time for pre-loading did not show any significant differences across the different velocities. The relative poling and arm swing times remained unchanged across all velocities (approximately 39% of cycle time for poling and 61% for arm swing) and the relative gliding,
Figure 2. Cycle rate and cycle length at three different diagonal skiing velocities. Data expressed as M ^ SD. The letters indicate statistically significant differences from moderate (a), high (b), or maximal (c) velocity ( p , 0.05).
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kick, and leg swing times remained unchanged from moderate to high velocity (approximately 36%, 22%, and 42% of cycle time, respectively). However, at maximal velocity the relative gliding time was 9% shorter and the relative kick and swing times 4% and 3% longer than at high velocity ( p , 0.05 for all pair-wise comparisons). The relative time for pre-loading was not significantly different between the three velocities. Force characteristics at the three different skiing velocities are presented in Table II. The relative and absolute peak pole forces increased 98% from moderate to maximal velocity (13% vs 26% of BW [100 vs 197 N]), while the time to peak pole force decreased by 79%, down to 42 ms ( p , 0.05 for all pair-wise comparisons) (Figure 3A). The relative and absolute peak leg force was unchanged across velocities (approximately 190% of BW [1500 N]), while time to peak leg force decreased by 26% from moderate to maximal velocity, from 148 to 109 ms (Figure 3B). The minimum leg force before the phase of ski-loading did not change significantly across velocities (approximately 26% of BW [200 N]). The relative time to peak pole force decreased from 51% to 11% of poling time ( p , 0.05), while the relative time to peak leg force remained unchanged (approximately 59%) from moderate to maximal velocity. The rate of force development during the poling phase was five- to six-fold higher at maximal than moderate velocity and the development of leg force during the ski-loading phase was 39% faster ( p , 0.05 for all pair-wise comparisons). The pole and leg force impulses remained unchanged from moderate to high velocity, but decreased from high to maximal velocity ( p , 0.05). The pole to leg force impulse ratio was not significantly different between the three velocities. During gliding, most of the force was distributed on the inside rear-foot, with low loading on the forefoot and a distinct shift of loading to the inside forefoot during the ski-loading phase (Figure 4A). During the gliding phase, 75% and 25% of the force impulse were on the rear-foot and forefoot, respectively (Figure 4B), and during the ski-loading phase the corresponding distributions were 10% and 90% (Figure 4C) The distributions were not significantly different between the three velocities. Cycle characteristics for the sprint-specialised and distance-specialised skiers are presented in Table III. On average, the sprint-specialised skiers were 10% and 14% faster at high and maximal velocities, respectively, than the distance-specialised skiers (both p , 0.05), with no differences between groups at moderate velocity. The sprint-specialised Table II. Force characteristics of diagonal skiing. Velocity
Peak pole force (N) Minimum leg force prior SL (N) Peak leg force (N) Relative time to peak pole force (%) Relative time to peak leg force (%) RFD for the poling phase (kN/s) RFD for the leg during the SL phase (kN/s) Pole force impulse (Ns) Leg force impulse (Ns) Pole to leg force impulse ratio (%)
Moderate (3.5 m/s)
High (4.5 m/s)
Maximal (5.6 m/s)
100 ^ 24 221 ^ 93 1531 ^ 217 51 ^ 23c 57 ^ 7 1.0 ^ 0.9bc 10.1 ^ 2.4bc 22 ^ 4c 235 ^ 39c 10 ^ 3
118 ^ 36 181 ^ 69 1538 ^ 184 34 ^ 19c 59 ^ 6 1.6 ^ 1.5ac 12.6 ^ 2.3ac 22 ^ 5c 215 ^ 27c 10 ^ 2
197 ^ 44ab 205 ^ 68 1448 ^ 187 11 ^ 2ab 61 ^ 7 5.8 ^ 1.5ab 13.9 ^ 2.4ab 18 ^ 2ab 174 ^ 21ab 10 ^ 2
bc
ac
Notes: The values presented are M ^ SD (n ¼ 11). Abbreviations: SL ¼ ski-loading; RFD ¼ rate of force development. The letters indicate statistically significant differences from moderate (a), high (b), or maximal (c) velocity ( p , 0.05).
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Figure 3. (A) Relative peak pole force, time to peak pole force, (B) relative peak leg force and time to peak leg force, at three different diagonal skiing velocities. Data expressed as M ^ SD. The letters indicate statistically significant differences from moderate (a), high (b), or maximal (c) velocity ( p , 0.05).
skiers utilised a 5% longer cycle length at high velocity, and a 22% higher cycle rate at maximal velocity, compared with distance-specialised skiers (both p , 0.05) (Figure 5). For both groups, kick time decreased similarly with increasing velocity and the poling time decreased similarly from moderate to high velocity, but was 13% shorter for the sprintspecialised skiers at their higher maximal velocity ( p , 0.05). Both groups reduced their relative gliding time and demonstrated longer relative kick and leg swing times as the velocity increased from high to maximal. However, at maximal velocity, the sprint-specialised
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group exhibited a shorter relative gliding phase and a longer relative pre-loading phase than the distance-specialised group (both p , 0.05). Force characteristics for the sprint-specialised and distance-specialised skiers are presented in Table IV. The relative peak pole force was similar for the two groups. The minimum leg force during the unloading phase was approximately 40% lower at both high and maximal velocity for the sprint-specialised group compared to the distance-specialised group (both p , 0.05) and the relative peak leg force was 11% higher for the sprint-
Figure 4. (A) Force distribution on inside rear-foot, outside rear-foot, inside forefoot, and outside forefoot; data are presented for one subject using diagonal skiing at high velocity and are averaged over five successive cycles; PLP, preloading phase; KP, kick phase; (B) relative force impulse distribution on inside rear-foot, outside rear-foot, inside forefoot, and outside forefoot during the gliding phase; and (C) the ski-loading phase. Data expressed as M ^ SD (B–C).
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Figure 4. (continued)
Figure 5. Cycle rate and cycle length adaptations from moderate to maximal diagonal skiing velocities for sprintspecialised and distance-specialised cross-country skiers. Data expressed as M ^ SD. § and * indicate a statistically significant difference between the groups at high and maximal velocity, respectively ( p , 0.05).
specialised group compared to the distance-specialised group ( p , 0.05). The times to peak pole and peak leg force did not differ between the groups, which was also the case for the rate of force development during the poling phase. The rate of leg force development during skiloading was similar between the groups at moderate and high velocity, but was 18% higher for the sprint-specialised group at maximal velocity ( p , 0.05). No difference in any force impulse variables was observed between the groups.
3.5 ^ 0.1 0.43 ^ 0.05 0.74 ^ 0.05 0.44 ^ 0.04 0.07 ^ 0.01 0.20 ^ 0.03 0.46 ^ 0.07 37 ^ 1 63 ^ 1 38 ^ 4 6^1 17 ^ 3 39 ^ 3
3.5 ^ 0.3 0.46 ^ 0.07 0.68 ^ 0.03 0.42 ^ 0.05 0.07 ^ 0.01 0.20 ^ 0.02 0.45 ^ 0.06 40 ^ 4 59 ^ 5 37 ^ 3 6^1 18 ^ 2 40 ^ 5
4.8 ^ 0.1* 0.37 ^ 0.03 0.65 ^ 0.03 0.40 ^ 0.04 0.06 ^ 0.01 0.15 ^ 0.02 0.41 ^ 0.03 36 ^ 3 64 ^ 3 39 ^ 3 6^1 15 ^ 2 40 ^ 4
SS (4.8 m/s)
High
4.4 ^ 0.2 0.39 ^ 0.05 0.68 ^ 0.07 0.41 ^ 0.03 0.06 ^ 0.01 0.17 ^ 0.03 0.43 ^ 0.06 37 ^ 3 63 ^ 3 38 ^ 2 5^1 16 ^ 2 40 ^ 6
DS (4.4 m/s)
Notes: The values presented are M ^ SD (SS, n ¼ 4; DS, n ¼ 4). * p , 0.05 in comparison to the DS skiers.
Velocity (m/s) Poling time (s) Arm swing time (s) Glide time (s) Pre-loading time (s) Kick time (s) Leg swing time (s) Relative poling time (%) Relative arm swing time (%) Relative glide time (%) Relative pre-loading time (%) Relative kick time (%) Relative leg swing time (%)
DS (3.5 m/s)
SS (3.5 m/s)
Moderate
Velocity
6.2 ^ 0.1* 0.26 ^ 0.02* 0.37 ^ 0.03 0.17 ^ 0.03* 0.05 ^ 0.01 0.13 ^ 0.01* 0.28 ^ 0.03* 41 ^ 2 59 ^ 1 26 ^ 4* 8 ^ 1* 21 ^ 2 45 ^ 10
SS (6.2 m/s)
Maximal
Table III. Cycle characteristics for sprint-specialised (SS) and distance-specialised (DS) skiers using the diagonal stride skiing technique.
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5.5 ^ 0.1 0.30 ^ 0.02 0.46 ^ 0.09 0.24 ^ 0.02 0.04 ^ 0.01 0.15 ^ 0.01 0.33 ^ 0.02 40 ^ 6 60 ^ 6 31 ^ 2 5^1 20 ^ 3 44 ^ 5
DS (5.5 m/s)
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12 ^ 2 33 ^ 4 192 ^ 21 142 ^ 81 144 ^ 25 1.0 ^ 0.5 9.6 ^ 3.0
12 ^ 3 23 ^ 3 212 ^ 4 272 ^ 75 162 ^ 43 0.5 ^ 0.2 10.0 ^ 1.8
16 ^ 2 19 ^ 3* 215 ^ 16 118 ^ 36 127 ^ 25 1.8 ^ 0.7 13.5 ^ 2.4
SS (4.8 m/s)
High
12 ^ 2 33 ^ 3 186 ^ 28 143 ^ 62 143 ^ 45 1.1 ^ 0.6 10.6 ^ 3.0
DS (4.4 m/s)
25 ^ 4 20 ^ 4* 195 ^ 10* 45 ^ 9 112 ^ 21 6.1 ^ 1.6 14.1 ^ 1.4*
SS (6.2 m/s)
21 ^ 2 32 ^ 4 176 ^ 9 51 ^ 10 117 ^ 31 4.4 ^ 1.7 11.9 ^ 1.3
DS (5.5 m/s)
Maximal
Notes: The values presented are M ^ SD (SS, n ¼ 4; DS, n ¼ 4). Abbreviations: SL ¼ ski-loading; RFD ¼ rate of force development. * p , 0.05 in comparison to the DS skiers.
Peak pole force (% BW) Minimum leg force prior SL (N) Peak leg force (% BW) Time to peak pole force (ms) Time to peak leg force (ms) RFD for the poling phase (kN/s) RFD for the leg during the SL phase (kN/s)
DS (3.5 m/s)
SS (3.5 m/s)
Moderate
Velocity
Table IV. Force characteristics for sprint-specialised (SS) and distance-specialised (DS) skiers using the diagonal stride skiing technique.
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Discussion and implications The main findings of the current study were the following: (1) elite cross-country skiers using diagonal stride increase their velocity from moderate to high with both a higher cycle rate and a longer cycle length, and at maximal velocity, the cycle rate is substantially higher at a shortened cycle length; (2) the relative poling and arm swing times were unchanged across all the three velocities, while the relative gliding, kick, and leg swing times were unchanged from moderate to high velocity, with decreased relative gliding, increased relative kick, and leg swing times at maximal velocity; (3) the peak pole force almost doubled from moderate to maximal velocity (100 to 197 N) and was applied considerably earlier at maximal velocity, while the peak leg force remained unchanged across velocities; (4) the rates of force development during the poling and the ski-loading phases were 6.0- and 1.4-fold higher, respectively, at maximal than at moderate velocity; (5) skiers loaded primarily their rear-foot during the gliding phase with a distinct shift to the forefoot during the ski-loading phase; (7) in comparison to distance-specialised skiers, sprint-specialised skiers reached higher maximal velocities at higher cycle rates, which was associated with shorter relative phases of gliding and higher rate of leg force development.
Cycle characteristics Employing an incline similar to that used in the present study, Sto¨ggl et al. (2011) reported that in diagonal roller-skiing, increases from high to maximal velocity involved a substantial increase in cycle rate at a shortened cycle length, which is in agreement with the present data and our hypothesis. Previous studies examining velocity alterations during diagonal skiing on flat and slight uphill (2.58) terrain on snow have shown that increases in velocity were mainly associated with increases in cycle rate, with only slightly increased or maintained cycle length up to maximal velocity (Nilsson et al., 2004; Va¨ha¨so¨yrinki et al., 2008). Both the current study and that of Sto¨ggl et al. (2011) were performed on steeper inclines (78) and the overcompensation in cycle rate with a decreased cycle length when increasing from high to maximal velocity was probably related to the additional work against gravity. The advantage of a substantially higher cycle rate at a shortened cycle length at maximal velocity is that it allows velocity to be maintained throughout the movement cycle (Hoffman, Clifford, & Bender, 1995). However, the higher cycle rate is likely to increase the metabolic cost, due to the higher internal work (Frederick, 1992). Finally, the current study confirms the importance of high cycle rates for achieving high maximal skiing velocities, which has also been verified for race performance in classical sprint cross-country skiing (Zory et al., 2005). Absolute durations of the poling and the kick phases decreased as velocity increased. From moderate to high velocity, poling and kick times both decreased by 16% to 0.38 s and 0.16 s, respectively. However, from high to maximal velocity, the poling time decreased substantially more than the kick time (29% vs 13%). Thus, the time sufficient for leg force application was reached at the high velocity, as indicated by both the smaller decrease of the leg force application time and the reduced leg force impulse at the maximal velocity. The kick times observed across the different velocities are in agreement with earlier findings reported for slight uphill diagonal skiing (Va¨ha¨so¨yrinki et al., 2008), and the kick time at maximal velocity is comparable to the contact times reported for sprint running uphill (98) (Weyand, Sternlight, Bellizzi, & Wright, 2000). Thus, our findings emphasise the importance of fast and substantial leg force generation due to the limited time available for force application. The relative durations of poling and arm swing were unchanged between all velocities and similar to the corresponding times for diagonal skiing on flat terrain (Nilsson et al., 2004).
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However, the relative durations of gliding, kick, and leg swing changed from high to maximal velocity (9% shorter gliding, 4% longer kick, and 3% longer leg swing), which differs from earlier observations by Nilsson et al. (2004), who reported unchanged relative phases up to maximal velocity. These differences at maximal velocity might be related, at least in part, to the uphill skiing in the present study, which required the skiers to overcome a greater component of gravitational force along the slope gradient, which likely influenced the skiing technique. The prolonged relative kick phase at a decreased relative gliding time may be related to a temporal constraint, i.e., the minimal time period required for leg force generation when increasing velocity from high to maximal. To obtain an even higher cycle rate while maintaining this critical kick time, the skiers had to shorten their absolute and relative phases of gliding, thereby adapting their technique towards a running style of diagonal skiing (albeit without completely abandoning the gliding phase, which they were instructed to maintain). Furthermore, Sto¨ggl et al. (2011) proposed that extensive vertical movement of the centre of mass may be a prerequisite for generating sufficient leg force over a limited time during fast diagonal skiing. Finally, the shorter time available for force generation at higher skiing velocities accentuates the importance of explosive strength and sprint qualities, enabling maximal rates of force generation. This would allow a higher skiing velocity, where the skier starts to shorten cycle length and overcompensate with a substantially higher cycle rate. In addition, a skiing technique with well-timed vertical oscillations might be beneficial for force generation at high skiing velocities. Force characteristics At moderate velocity, the peak pole force was in accordance with Bjo¨rklund, Sto¨ggl, and Holmberg (2010) and Lindinger et al. (2009), who reported peak pole forces of approximately 15% of BW (110 N) for diagonal roller-skiing at velocities of 3.1 m/s (6.58 and 98 incline, respectively). However, the peak pole force was two-fold higher at maximal than at moderate velocity in the current study, reaching 25% of BW at maximal. The considerably higher peak pole forces at maximal velocity corroborate previous findings by Sto¨ggl et al. (2011) who reported forces of 29% of BW (215 N) for uphill (78) roller-skiing at maximal velocity. The peak pole forces during diagonal skiing at 3.1 m/s have been associated with high arm muscle activation (EMG), about 72% of maximal voluntary contraction (Bjo¨rklund et al., 2010). Therefore, it might be conjectured that the arm muscle activation in the current study was maximal or nearly maximal at the high and maximal skiing velocities due to the considerably higher (14 –86%BW) peak pole forces attained here. The timing of the pole force application changed across velocities, with shorter absolute and relative time to peak pole forces at the two higher velocities. This inverse pattern of a decreased poling time with an increased velocity has previously been observed by Sto¨ggl et al. (2011). For another skiing technique such as double poling, where the pole is planted at a smaller angle relative to vertical, the timing of the pole force application is likely more important than in diagonal skiing (Holmberg et al., 2005; Sto¨ggl & Holmberg, 2011). That is, the disadvantage of an early application of pole force is much greater for double poling compared to the diagonal technique, in which the pole is planted at a substantially smaller angle relative to the vertical. Furthermore, later application of pole force when using the diagonal technique probably enhances the effectiveness of the force by directing it more horizontally (Smith, 2002). However, a late application of pole force may result in higher unloading (‘body lift’) during the ski-loading phase and might thus reduce the friction (grip) and impair the generation of propulsive leg force.
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The peak leg forces and the timing of their application were similar for the three velocities, with peak forces of approximately two times of BW, which is in agreement with previous findings for diagonal roller-skiing (Lindinger et al., 2009; Sto¨ggl et al., 2011). Thus, only the rate of force development was altered at higher velocities, which is consistent with previous findings for running (Kram & Taylor, 1990) that faster velocities are related to the higher rates of force generation (Weyand et al., 2000). The substantial reduction in time to peak pole force in combination with the two-fold increase in peak pole force resulted in a six-fold faster rate of pole force development at maximal compared to moderate velocity. The faster leg force development during the skiloading phase at higher velocities was related primarily to a shorter time to peak force. This supports our second hypothesis that the rate of pole and leg force development would increase with higher skiing velocities. In comparison to Sto¨ggl et al. (2011), the present study demonstrated, at a similar maximal velocity, approximately 25% shorter time to peak pole and leg forces and higher rate of force development during poling (53%) and ski-loading (28%). It can be speculated that the importance of proper timing and the rate of vertical force generation might differ between roller-skiing and on-snow skiing. On one hand, the ratcheted wheel system on roller-skis generates static friction during the kick phase that is several times greater than that reported for skiing on snow (Ainegren et al., 2012; Komi, 1985; Komi & Norman, 1987; Va¨ha¨so¨yrinki et al., 2008). In addition, balancing on classic roller skis is more difficult than on snow as the skier needs to keep the ski aligned with the forward direction in the absence of a ski track. Furthermore, when skiing on snow, large vertical forces must be applied rapidly during pre-loading and at the beginning of the kick phase (Figure 1), in order to press down the grip-waxed ski camber to generate enough static friction for the propulsive kick phase (Komi, 1987; Komi & Norman, 1987). Effective kick phases on snow are likely to be much more dependent on good technique. Accordingly, skiers who train using diagonal roller skiing during the off-season may adopt techniques that are disadvantageous when skiing on snow. Distribution of leg force impulse During gliding, 76% and 24% of the force impulse were applied to the rear-foot and forefoot, respectively. This differs slightly from the corresponding values of 63% and 37% during diagonal stride roller-skiing reported by Lindinger et al. (2009). However, the latter study was conducted at a gradient of 98, in comparison to 7.58 in the present study, which may in part explain the slightly larger forefoot distribution compared to the present study. Additionally, differences between roller-skiing and on-snow skiing may be part of this discrepancy. A greater forefoot loading during roller-skiing might be due to two factors. First, forefoot loading of the roller-ski during the gliding phase may not influence the rolling friction to the same degree as it influences the ski-glide during gliding on snow, since less loading on the forefoot during gliding minimises the contact between grip-wax and snow, thereby reducing friction. Second, in comparison to rollerskiing, it might be more important for on-snow skiing to ‘push’ the ski forward during the pre-loading phase, thus placing the centre of mass over the base of support prior to the kick phase, in order to maximise the vertical force and avoid slipping. Together, these factors might result in a greater loading of the rear-foot, reflecting the greater need to push the ski forward before the kick phase. During leg thrust, 90% of the force was located on the forefoot, which was similar to previous findings by Lindinger et al. (2009).
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Comparison between sprint-specialised and distance-specialised skiers The sprint-specialised skiers were 10% faster than the distance-specialised skiers at the high velocity due to their ability to generate longer cycle lengths. The longer cycle lengths for the sprint-specialised skiers corroborate results from previous studies (Bilodeau, Rundell, Roy, & Boulay, 1996; Norman, 1989; Norman & Komi, 1987) that have reported high positive correlations between cycle length and skiing velocity in connection with distance events (10– 50 km). Furthermore, at maximal velocity the sprint-specialised skiers were 14% faster than the distance-specialised skiers, which were related to their ability to reach substantially higher cycle rates. This is in agreement to our hypothesis and in accordance with Zory et al. (2005) who demonstrated a positive correlation between cycle rate and diagonal sprint velocity. The present study demonstrates non-linear, concave upward and downward curves for cycle rate and cycle length, respectively, in agreement with the velocity adaptation in running (Hay, 2002). Moreover, Hay (2002) demonstrated that the maximal cycle length was reached at a velocity corresponding to approximately 85% of the maximal running velocity. The same relationship was observed here, whereby the fastest sprint-specialised skiers, when compared to distance-specialised skiers, generated longer cycle lengths at higher sub-maximal velocities before overcompensating by increasing cycle rate, and decreasing cycle length, up to maximal velocity (Figure 5). Regarding the comparison between sprint-specialised and distance-specialised skiers, there was no significant difference for peak pole forces, whereas the peak leg forces of the sprint-specialised skiers were 11% higher at the maximal velocity. Moreover, the sprintspecialised skiers demonstrated a 40% lower minimum force (i.e., greater unloading) preceding the leg thrust and an 18% higher rate of leg force development. The greater unloading in the sprint-specialised group reveals, indirectly, a higher position of the centre of mass at pole plant together with a rapid lowering of the centre of mass after the pole plant, which can be considered as a technical strategy to increase the effectiveness of the following kick phase (Norman, Caldwell, & Komi, 1985; Sto¨ggl et al., 2011). In conclusion, at maximal velocity, the sprint-specialised skiers exhibited greater and more rapid development of leg forces that were applied over shorter time periods. This might be related, at least in part, to larger vertical oscillations of the centre of mass, as indicated by the lower minimum force during the unloading phase, which previously has been verified as beneficial for leg force generation (Sto¨ggl et al., 2011).
Limitations In the present investigation, the leg forces analysed were the ground reaction forces directed perpendicular to the surface of the insole, while pole forces were measured along the pole. Therefore, it was not possible to extract vector components in vertical and forward directions nor possible to determine the relative contributions of poles and skis to propulsion at the different velocities. This is information that would have provided important additional insight. Another potential limitation is the insole bending during the kick phase, which could have altered the force measurements. However, since this bending of the ski boot occurred approximately halfway through the kick phase when no loading on the rear-foot was detected (Figure 4A), this effect is probably negligible. Moreover, since the pre-loading and kick phases were determined from video and force data collected at relatively low sampling frequencies (50 and 100 Hz, respectively), the values obtained are not exact. Although, the data obtained in the present study have shortcomings, they do provide a unique insight into the performance of diagonal skiing on snow by elite skiers.
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Cycle length and cycle rate increased from moderate to high velocity, and maximal velocity was attained by substantially elevating cycle rate while shortening cycle length. Kick times were considerably shorter than poling times at all velocities and the rate of force development increased with a higher velocity for both the poling and the ski-loading phases. These findings reveal that the development of leg force was considerably faster during diagonal skiing on snow than for roller-skiing. Therefore, when training on roller-skis athletes should focus on generating high vertical leg forces early during the kick in order to avoid a technique adaptation that might result in ‘slipping’ when skiing on snow. Altogether, these findings emphasise the importance of rapid leg force generation and highly coordinated motor skills for fast diagonal skiing.
Acknowledgements The authors would like to thank the participants and their coach (Per-Øyvind Torvik) for the cooperation and participation. The authors would also like to thank Mikael Sware´n, Jonas Danvind, and Espen Emanuelsen for assistance in collecting the data. Funding This study was supported financially by the Swedish National Centre for Research in Sports [grant number 2011:23] and the Swedish Olympic Committee [grant number 2011-111].
References Ainegren, M., Carlsson, P., Laaksonen, M. S., & Tinnsten, M. (2012). The influence of grip on oxygen consumption and leg forces when using classical style roller skis. Scandinavian Journal of Medicine & Science in Sports, 24, 301–310. doi:10.1111/sms.12006 Bilodeau, B., Rundell, K. W., Roy, B., & Boulay, M. R. (1996). Kinematics of cross-country ski racing. Medicine & Science in Sports & Exercise, 28, 128–138. Bjo¨rklund, G., Sto¨ggl, T., & Holmberg, H.-C. (2010). Biomechanically influenced differences in O2 extraction in diagonal skiing: Arm versus leg. Medicine & Science in Sports & Exercise, 42, 1899–1908. Frederick, E. C. (1992). Mechanical constraints on Nordic ski performance. Medicine & Science in Sports & Exercise, 24, 1010–1014. Go¨pfert, C., Holmberg, H.-C., Sto¨ggl, T., Mu¨ller, E., & Lindinger, S. J. (2013). Biomechanical characteristics and speed adaptation during kick double poling on roller skis in elite crosscountry skiers. Sports Biomechanics, 12, 154– 174. Hay, J. (2002). Cycle rate, length, and speed of progression in human locomotion. Journal of Applied Biomechanics, 18, 257– 270. Hoffman, M. D., Clifford, P. S., & Bender, F. (1995). Effect of velocity on cycle rate and length for three roller skiing techniques. Journal of Applied Biomechanics, 20, 465–479. Holmberg, H.-C., Lindinger, S., Sto¨ggl, T., Eitzlmair, E., & Mu¨ller, E. (2005). Biomechanical analysis of double poling in elite cross-country skiers. Medicine & Science in Sports & Exercise, 37, 807 –818. Komi, P. V. (1985). Ground reaction forces in cross-country skiing. In D. Winter, R. Norman, R. Wells, K. Hayes, & A. Patla (Eds.), Biomechanics IX-B (pp. 185–190). Champaign, IL: Human Kinetics. Komi, P. V. (1987). Force measurements during cross-country skiing. International Journal of Sport Biomechanics, 3, 370– 381. Komi, P. V., & Norman, R. (1987). Preloading of the thrust phase in cross-country skiing. International Journal of Sports Medicine, 8, 48–54. Kram, R., & Taylor, C. R. (1990). Energetics of running: A new perspective. Nature, 346, 265– 267. Lindinger, S. J., Go¨pfert, C., Sto¨ggl, T., Mu¨ller, E., & Holmberg, H.-C. (2009). Biomechanical pole and leg characteristics during uphill diagonal roller skiing. Sports Biomechanics, 8, 318 –333.
Downloaded by [Selcuk Universitesi] at 09:43 02 February 2015
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Nilsson, J., Tveit, P., & Eikrehagen, O. (2004). Effects of speed on temporal patterns in classical style and freestyle cross-country skiing. Sports Biomechanics, 3, 85– 107. Norman, R. W. (1989). Mechanical power output and estimated metabolic rates of Nordic skiers during Olympic competition. Journal of Applied Biomechanics, 5, 169–184. Norman, R. W., Caldwell, G., & Komi, P. V. (1985). Differences in body segment energy utilization between world class and recreational cross-country skiers. International Journal of Sport Biomechanics, 1, 253–262. Norman, R. W., & Komi, P. (1987). Mechanical energetics of world class cross-country skiing. Journal of Applied Biomechanics, 3, 353–369. Smith, G. A. (2002). Biomechanics of cross country skiing. In H. Rusko (Ed.), Cross country skiing: Olympic handbook of sports medicine and science (pp. 32–61). Malden, MA: Blackwell. Sto¨ggl, T., & Holmberg, H.-C. (2011). Force interaction and 3D pole movement in double poling. Scandinavian Journal of Medicine & Science in Sports, 21, e393–e404. Sto¨ggl, T., Mu¨ller, E., Ainegren, M., & Holmberg, H.-C. (2011). General strength and kinetics: Fundamental to sprinting faster in cross country skiing? Scandinavian Journal of Medicine & Science in Sports, 21, 791–803. Sto¨ggl, T. L., & Mu¨ller, E. (2009). Kinematic determinants and physiological response of cross-country skiing at maximal speed. Medicine & Science in Sports & Exercise, 41, 1476–1487. Va¨ha¨so¨yrinki, P., Komi, P. V., Seppala, S., Ishikawa, M., Kolehmainen, V., Salmi, J. A., & Linnamo, V. (2008). Effect of skiing speed on ski and pole forces in cross-country skiing. Medicine & Science in Sports & Exercise, 40, 1111–1116. Weyand, P. G., Sternlight, D. B., Bellizzi, M. J., & Wright, S. (2000). Faster top running speeds are achieved with greater ground forces not more rapid leg movements. Journal of Applied Physiology, 89, 1991–1999. Zory, R., Barberis, M., & Rouard, A. (2005). Kinematics of sprint cross-country skiing. Acta of Bioengineering and Biomechanics, 7, 87– 96.
Sports Biomechanics Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/rspb20
The effects of skiing velocity on mechanical aspects of diagonal crosscountry skiing a
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Erik Andersson , Barbara Pellegrini , Øyvind Sandbakk , Thomas Stüggl
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& Hans-Christer Holmberg
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Swedish Winter Sports Research Centre, Mid Sweden University, Östersund, Sweden b
CeRiSM, Research Center for Sport, Mountain and Health, Rovereto, Italy c
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Faculty of Exercise and Sport Science, University of Verona, Verona, Italy d
Centre for Elite Sports Research, Norwegian University of Science and Technology, Trondheim, Norway e
Department of Sport Science and Kinesiology, University of Salzburg, Salzburg, Austria f
Swedish Olympic Committee, Stockholm, Sweden Published online: 23 Jun 2014.
To cite this article: Erik Andersson, Barbara Pellegrini, Øyvind Sandbakk, Thomas Stüggl & HansChrister Holmberg (2014) The effects of skiing velocity on mechanical aspects of diagonal crosscountry skiing, Sports Biomechanics, 13:3, 267-284, DOI: 10.1080/14763141.2014.921236 To link to this article: http://dx.doi.org/10.1080/14763141.2014.921236
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Sports Biomechanics, 2014 Vol. 13, No. 3, 267–284, http://dx.doi.org/10.1080/14763141.2014.921236
The effects of skiing velocity on mechanical aspects of diagonal cross-country skiing ERIK ANDERSSON1, BARBARA PELLEGRINI2,3, ØYVIND SANDBAKK4, ¨ GGL1,5, & HANS-CHRISTER HOLMBERG1,6 THOMAS STO ¨ stersund, Sweden, 2CeRiSM, Swedish Winter Sports Research Centre, Mid Sweden University, O 3 Research Center for Sport, Mountain and Health, Rovereto, Italy, Faculty of Exercise and Sport Science, University of Verona, Verona, Italy, 4Centre for Elite Sports Research, Norwegian University of Science and Technology, Trondheim, Norway, 5Department of Sport Science and Kinesiology, University of Salzburg, Salzburg, Austria, and 6Swedish Olympic Committee, Stockholm, Sweden
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(Received 3 September 2013; accepted 10 April 2014)
Abstract Cycle and force characteristics were examined in 11 elite male cross-country skiers using the diagonal stride technique while skiing uphill (7.58) on snow at moderate (3.5 ^ 0.3 m/s), high (4.5 ^ 0.4 m/s), and maximal (5.6 ^ 0.6 m/s) velocities. Video analysis (50 Hz) was combined with plantar (leg) force (100 Hz), pole force (1,500 Hz), and photocell measurements. Both cycle rate and cycle length increased from moderate to high velocity, while cycle rate increased and cycle length decreased at maximal compared to high velocity. The kick time decreased 26% from moderate to maximal velocity, reaching 0.14 s at maximal. The relative kick and gliding times were only altered at maximal velocity, where these were longer and shorter, respectively. The rate of force development increased with higher velocity. At maximal velocity, sprint-specialists were 14% faster than distance-specialists due to greater cycle rate, peak leg force, and rate of leg force development. In conclusion, large peak leg forces were applied rapidly across all velocities and the shorter relative gliding and longer relative kick phases at maximal velocity allow maintenance of kick duration for force generation. These results emphasise the importance of rapid leg force generation in diagonal skiing.
Keywords: Cycle characteristics, kinetics, Nordic skiing
Introduction The diagonal stride is the most common uphill technique used in classical cross-country skiing. The technique shows similarities to running with its diagonal arm and leg motion. In contrast to running, propulsive forces in diagonal skiing are generated by both the arms and legs and the leg thrust is followed by a gliding phase on the contralateral leg (Nilsson, Tveit, & Eikrehagen, 2004). Velocity of skiing can be elevated by increasing the product of cycle rate and cycle length. The velocity of the diagonal stride on snow, both on flat and slightly uphill (2.58) terrain, is adapted primarily by altering the cycle rate (Nilsson Correspondence: Erik Andersson, Swedish Winter Sports Research Centre, Department of Health Sciences, Mid Sweden ¨ stersund, Sweden, E-mail: [email protected] University, 83125 O q 2014 Taylor & Francis
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et al., 2004; Va¨ha¨so¨yrinki et al., 2008). In sprint skiing, faster skiers utilise higher cycle rates, than slower skiers, when using the diagonal stride technique (Zory, Barberis, & Rouard, 2005). Moreover, in order to maintain leg thrust duration and momentum while increasing diagonal roller-skiing velocity from high to maximal, on a steeper incline (78), skiers adopt a higher frequency running diagonal stride technique without gliding, i.e. overcompensate by a substantially higher cycle rate at a shorter cycle length (Sto¨ggl, Mu¨ller, Ainegren, & Holmberg, 2011). This adaptation is considered as a strategy to maintain enough kick time for force generation at maximal velocity. The role of cycle rate, cycle length, and time for force generation at different velocities has not been investigated at a typical incline (7.58) for diagonal skiing on snow. Based on previous studies from roller-skiing (Sto¨ggl et al., 2011) and on-snow skiing (Nilsson et al., 2004; Va¨ha¨so¨yrinki et al., 2008), it may be speculated that an increased cycle rate at a long cycle length is the main factor for increasing a high sub-maximal velocity up to a maximal velocity. During diagonal skiing on a slight uphill (2.58), Va¨ha¨so¨yrinki et al. (2008) demonstrated that the relative propulsion provided by the legs (in relation to the arms) increased at higher velocities. In addition, a higher performance level in diagonal roller-skiing at a 98 incline is related positively to the skier’s ability to generate large leg forces over short durations (Lindinger, Go¨pfert, Sto¨ggl, Mu¨ller, & Holmberg, 2009). As diagonal skiing velocity increases, the time during which force can be applied through poling and/or leg thrust is reduced. A 29% decrease in kick time has been reported from slow to maximal velocities, reaching only 0.12 s at maximal velocity. The shorter kick times at higher velocities were also associated with larger average kick forces (Va¨ha¨so¨yrinki et al., 2008). These observations clearly emphasise that the importance of a rapid leg force generation increases at higher diagonal skiing velocities. To avoid complications due to changing weather conditions, many recent investigations on the diagonal stride have been performed indoors on a treadmill using roller-skis (Ainegren, Carlsson, Laaksonen, & Tinnsten, 2012; Lindinger et al., 2009; Sto¨ggl & Mu¨ller, 2009; Sto¨ggl et al., 2011). Roller-skiing with ratcheted wheels enables static friction that is several times greater than skiing on snow, which allow skiers to exert high propulsive forces without ski-slipping, and is therefore less dependent on a skier’s technical skills (Ainegren et al., 2012). Thus, the ideal technique for diagonal roller-skiing may differ from the ideal technique for skiing on snow, and generalising results from the former to the latter may be questionable. Therefore, it may be important to compare the results of the studies conducted on snow to those that used roller-skiing in the laboratory. To date, only one study has examined the effects of different velocities on kinematics/ kinetics of diagonal skiing on snow (Va¨ha¨so¨yrinki et al., 2008). This study, however, was performed on an incline of 2.58 where elite skiers would normally use the kick double poling technique (Go¨pfert, Holmberg, Sto¨ggl, Mu¨ller, & Lindinger, 2013). Therefore, the purpose of this study was to analyse pole and leg cycle characteristics and kinetics of elite crosscountry skiers using diagonal stride technique at different velocities on a 7.58 incline on snow. The aims were to assess the mechanics of velocity adaptations and compare the diagonal stride technique of cross-country skiers who specialise in sprint versus distance races. It was hypothesised that: (1) elite skiers would adapt to higher skiing velocities by increasing both cycle rate and cycle length up to a high velocity, thereafter increasing cycle rate even further and slightly reducing cycle length up to maximal velocity, (2) the rate at which pole and leg forces are applied would increase with skiing velocity, and (3) sprintspecialised skiers would reach the highest maximal velocities by utilising a substantially higher cycle rate than distance-specialised skiers.
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Methods Participants
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Eleven elite male Norwegian cross-country skiers, including one Olympic and World Champion and two members of the Norwegian National Team (mean ^ SD: age 23.5 ^ 4.8 years, body height 1.81 ^ 0.06 m, body mass 77.4 ^ 6.9 kg, VO2max 72.1 ^ 4.6 ml/kg/min), volunteered to participate in the study. The participants consisted of four sprint-specialists, four distance-specialists, and three all-round skiers who performed equally well in sprint and distance races. This study was approved by the Regional Ethical Review Board of Umea˚ University and the participants were fully informed of all procedures prior to providing their written consent. Study design All skiers were tested on a 50 m uphill slope (7.58) on snow using the diagonal stride technique with the data from the final 20 m section used for analyses. Skiers were instructed to employ a gliding phase at all velocities (i.e., they were not allowed to switch to a runningtype diagonal skiing). Each skier performed the test at maximal velocity first (100%), followed by high velocity (80% of maximal), and finally moderate velocity (65% of maximal) at last. Trials in which the skier deviated from the predetermined velocity by more than 10% were repeated. All participants were familiar with the selected skiing velocities as they were regularly used in training sessions. The moderate velocity corresponded to an intensity that is normal for 10 –30 km competitions, whereas the high and maximal velocities corresponded to intensities that are appropriate for sprint competitions. Prior to testing, the skier performed a 15 min warm-up at 60 – 75% of maximal heart rate (measured by Polar S610, Polar Electro Oy, Kempele, Finland) and each trial was separated by 6 min of recovery with light activity (approximately 50% of maximal heart rate). Skiers utilised their own racing skis and all skis were prepared by the same professional ski technician using wax appropriate for the environmental conditions, in order to standardise grip and gliding properties. All trials were performed on the same day under stable weather conditions (light wind, an air temperature of 1 –28C, a snow temperature of 08C, and a relative humidity of 92 – 96%). The track was machine groomed on the evening prior to testing. Kinematics All trials were recorded using one panning camera (Panasonic NV-GS 280, Panasonic, Osaka, Japan). The shutter of the camera was set at 1/500 s to ensure high clarity and the frame rate of the video was 50 Hz. The video analysis was carried out using Dartfish Pro Suite 4.5 (Dartfish Ltd, Fribourg, Switzerland). For synchronisation of pole forces, leg forces, and video data, a simultaneous lift and push down of the right ski and pole was performed before and after each trial, with the skier standing still. The skiers started from standing at the base of the 50 m slope and accelerated to the required velocity. One pair of photocells (IVAR, LL Sport, Mora, Sweden) were placed over the final 20 m to obtain skiing times and velocities. The pole and leg movements were divided into gliding, thrust, and swing phases as illustrated in Figure 1. Cycle time was defined as the period from the start of one right pole plant to the subsequent right pole plant. The poling cycle was divided into a poling phase (ground contact) and a swing phase (tip of the pole in the air). The gliding and leg swing
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Figure 1. Sequence of actions (1–7) and time course of cycle and leg force characteristics associated with the diagonal stride skiing technique. a ¼ unloading phase (leg force minima); b ¼ peak pole force; c ¼ peak leg force; PLP, pre-loading phase; KP, kick phase. Data are presented for one subject skiing at high velocity and are mean values of five successive time normalised cycles.
times were defined according to the force data. The kick time was defined as the period starting when the ski was stationary on the ground, obtained from the video recording, and ending when the plantar force signal dropped to zero. The pre-loading time was calculated as the ski-loading time (based on the force data) minus the kick time. The gliding, pre-loading, kick, and leg swing phases are illustrated in Figure 1. In each trial, kinematic parameters were averaged over five consecutive movement cycles. Kinetics The poles used for force measurements were adjustable in length, and each athlete used their preferred pole length. The ground reaction forces directed along the pole were measured at a rate of 1500 Hz by a 15 g strain gauge load cell (Hottinger Baldwin Messtechnik GmbH, Darmstadt, Germany) fitted into a short (80 mm), lightweight (65 g) aluminium body mounted directly below the pole grip. Pole force data were amplified by a telemetric recording system (TeleMyo 2400T G2, Noraxon, Scottsdale, AZ, USA) and transferred to a laptop computer via an A/D converter card. The vertical plantar pressure for each leg was recorded at 100 Hz using a Pedar Mobile System (Novel GmbH, Munich, Germany). The total plantar foot area was divided into forefoot and rear-foot at 50% of foot length, and inside-foot and outside-foot at 50% of foot width. The relative force impulses of the forefoot and inside-foot were calculated by dividing
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the impulses of the forefoot and inside-foot by the ski-loading force impulse, respectively. The pole and plantar system was validated as described previously by Holmberg, Lindinger, Sto¨ggl, Eitzlmair, and Mu¨ller (2005) and the insole was calibrated with a Pedar calibration device. All kinetic parameters were analysed over the same five movement cycles as those used for kinematic analyses.
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Data analysis International Ski Federation (FIS) sprint and distance point rankings were used to classify each skier into sprint-specialised or distance-specialised groups. FIS points indicate a skier’s rank relative to the top-ranked skier in the world, who has 0 point, such that lower points indicate a better ranking. A skier was defined as a sprint or distance specialist if the ratio of the lowest to the highest FIS points was less than 55% and as an all-round skier if the ratio was 65% or higher. Accordingly, the points in sprint and distance races were, respectively, 38 ^ 21 and 110 ^ 14 for the sprint-specialised skiers ( p , 0.05), 97 ^ 22 and 47 ^ 18 for the distance-specialised skiers ( p , 0.05), and 85 ^ 34 and 77 ^ 27 for the all-round skiers. The cycle time, cycle rate (the reciprocal of cycle time), cycle length (the product of cycle time and skiing velocity), poling time, arm swing time, gliding time, pre-loading time, kick time, and leg swing time were determined. Relative time phases (% of cycle time) for poling, arm swing, gliding, pre-loading, kick, and leg swing were calculated by dividing the durations for the separate phases by the cycle time. The average right and left pole and leg forces during the five skiing cycles were calculated for each participant. The pole and leg force impulses over one movement cycle were determined as the average value of the right and left poling and ski-loading phases, respectively. The absolute peak force, leg force minima, and time to peak force were determined from the pole and leg kinetics (Figure 1). The leg force parameters were calculated over the entire phase of ski-loading (i.e., the pre-loading and kick phases together). The relative peak force was calculated by dividing the absolute force by the body weight (BW) and expressed as a percentage (%BW). The relative times to peak pole and leg force were calculated by dividing the times required to attain the maximal value by the total poling and leg thrust time, respectively. The rate of force development was calculated by dividing the peak force by the time to peak force during the ski-loading phase. All data were processed using the IKE-Master Software (IKE-Software Solutions, Salzburg, Austria) and MS Office Excel 2007 (Microsoft Corporation, Redmond, WA, USA). Statistical analyses Data were checked for normality using the Kolmogorov–Smirnov analysis and are presented as mean ^ standard deviation (SD). One-way repeated measures ANOVA tests with Bonferroni a correction were used for analysing differences in the variables measured at the three skiing velocities. A Mann –Whitney U test was carried out to identify differences in cycle and force characteristics at the different velocities between sprint-specialised and distance-specialised skiers. The level of statistical significance was set at a ¼ 0.05. All analyses were performed with SPSS 17.0 (SPSS Inc., Chicago, IL, USA). Results The cycle characteristics at the three different velocities of skiing are presented in Table I. The skiers reached a maximal velocity of 5.6 ^ 0.6 m/s and the moderate and high velocities were 62 ^ 4% and 80 ^ 4% of the maximal velocity, respectively. The cycle rate increased continuously as the velocity increased ( p , 0.05 for all pair-wise comparisons), while cycle
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Velocity
Velocity (m/s) Cycle time (s) Poling time (s) Arm swing time (s) Gliding time (s) Pre-loading time (s) Kick time (s) Leg swing time (s) Relative poling time (%) Relative arm swing time (%) Relative glide time (%) Relative pre-loading time (%) Relative kick time (%) Relative leg swing time (%)
Moderate (3.5 m/s)
High (4.5 m/s)
Maximal (5.6 m/s)
3.5 ^ 0.3bc 1.15 ^ 0.07bc 0.45 ^ 0.07bc 0.70 ^ 0.04bc 0.42 ^ 0.05bc 0.07 ^ 0.01 0.19 ^ 0.03bc 0.47 ^ 0.06bc 39 ^ 4 61 ^ 4 36 ^ 3c 6^1 17 ^ 2c 41 ^ 5c
4.5 ^ 0.4ac 1.01 ^ 0.10ac 0.38 ^ 0.04ac 0.63 ^ 0.09ac 0.36 ^ 0.08ac 0.06 ^ 0.01 0.16 ^ 0.02ac 0.43 ^ 0.06ac 38 ^ 4 62 ^ 4 36 ^ 5c 6^1 16 ^ 2c 43 ^ 5c
5.6 ^ 0.6ab 0.69 ^ 0.09ab 0.27 ^ 0.03ab 0.41 ^ 0.07ab 0.19 ^ 0.07ab 0.05 ^ 0.01 0.14 ^ 0.02ab 0.32 ^ 0.05ab 40 ^ 3 60 ^ 3 27 ^ 8ab 7^1 20 ^ 2ab 46 ^ 7ab
Notes: The values presented are M ^ SD (n ¼ 11). The letters indicate statistically significant differences from moderate (a), high (b), or maximal (c) velocity ( p , 0.05).
length increased from moderate to high velocity with a decrease from high to maximal velocity (both p , 0.05) (Figure 2). The absolute poling, arm swing, gliding, kick, and leg swing times decreased as velocity increased ( p , 0.05 for all pair-wise comparisons), while the time for pre-loading did not show any significant differences across the different velocities. The relative poling and arm swing times remained unchanged across all velocities (approximately 39% of cycle time for poling and 61% for arm swing) and the relative gliding,
Figure 2. Cycle rate and cycle length at three different diagonal skiing velocities. Data expressed as M ^ SD. The letters indicate statistically significant differences from moderate (a), high (b), or maximal (c) velocity ( p , 0.05).
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kick, and leg swing times remained unchanged from moderate to high velocity (approximately 36%, 22%, and 42% of cycle time, respectively). However, at maximal velocity the relative gliding time was 9% shorter and the relative kick and swing times 4% and 3% longer than at high velocity ( p , 0.05 for all pair-wise comparisons). The relative time for pre-loading was not significantly different between the three velocities. Force characteristics at the three different skiing velocities are presented in Table II. The relative and absolute peak pole forces increased 98% from moderate to maximal velocity (13% vs 26% of BW [100 vs 197 N]), while the time to peak pole force decreased by 79%, down to 42 ms ( p , 0.05 for all pair-wise comparisons) (Figure 3A). The relative and absolute peak leg force was unchanged across velocities (approximately 190% of BW [1500 N]), while time to peak leg force decreased by 26% from moderate to maximal velocity, from 148 to 109 ms (Figure 3B). The minimum leg force before the phase of ski-loading did not change significantly across velocities (approximately 26% of BW [200 N]). The relative time to peak pole force decreased from 51% to 11% of poling time ( p , 0.05), while the relative time to peak leg force remained unchanged (approximately 59%) from moderate to maximal velocity. The rate of force development during the poling phase was five- to six-fold higher at maximal than moderate velocity and the development of leg force during the ski-loading phase was 39% faster ( p , 0.05 for all pair-wise comparisons). The pole and leg force impulses remained unchanged from moderate to high velocity, but decreased from high to maximal velocity ( p , 0.05). The pole to leg force impulse ratio was not significantly different between the three velocities. During gliding, most of the force was distributed on the inside rear-foot, with low loading on the forefoot and a distinct shift of loading to the inside forefoot during the ski-loading phase (Figure 4A). During the gliding phase, 75% and 25% of the force impulse were on the rear-foot and forefoot, respectively (Figure 4B), and during the ski-loading phase the corresponding distributions were 10% and 90% (Figure 4C) The distributions were not significantly different between the three velocities. Cycle characteristics for the sprint-specialised and distance-specialised skiers are presented in Table III. On average, the sprint-specialised skiers were 10% and 14% faster at high and maximal velocities, respectively, than the distance-specialised skiers (both p , 0.05), with no differences between groups at moderate velocity. The sprint-specialised Table II. Force characteristics of diagonal skiing. Velocity
Peak pole force (N) Minimum leg force prior SL (N) Peak leg force (N) Relative time to peak pole force (%) Relative time to peak leg force (%) RFD for the poling phase (kN/s) RFD for the leg during the SL phase (kN/s) Pole force impulse (Ns) Leg force impulse (Ns) Pole to leg force impulse ratio (%)
Moderate (3.5 m/s)
High (4.5 m/s)
Maximal (5.6 m/s)
100 ^ 24 221 ^ 93 1531 ^ 217 51 ^ 23c 57 ^ 7 1.0 ^ 0.9bc 10.1 ^ 2.4bc 22 ^ 4c 235 ^ 39c 10 ^ 3
118 ^ 36 181 ^ 69 1538 ^ 184 34 ^ 19c 59 ^ 6 1.6 ^ 1.5ac 12.6 ^ 2.3ac 22 ^ 5c 215 ^ 27c 10 ^ 2
197 ^ 44ab 205 ^ 68 1448 ^ 187 11 ^ 2ab 61 ^ 7 5.8 ^ 1.5ab 13.9 ^ 2.4ab 18 ^ 2ab 174 ^ 21ab 10 ^ 2
bc
ac
Notes: The values presented are M ^ SD (n ¼ 11). Abbreviations: SL ¼ ski-loading; RFD ¼ rate of force development. The letters indicate statistically significant differences from moderate (a), high (b), or maximal (c) velocity ( p , 0.05).
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Figure 3. (A) Relative peak pole force, time to peak pole force, (B) relative peak leg force and time to peak leg force, at three different diagonal skiing velocities. Data expressed as M ^ SD. The letters indicate statistically significant differences from moderate (a), high (b), or maximal (c) velocity ( p , 0.05).
skiers utilised a 5% longer cycle length at high velocity, and a 22% higher cycle rate at maximal velocity, compared with distance-specialised skiers (both p , 0.05) (Figure 5). For both groups, kick time decreased similarly with increasing velocity and the poling time decreased similarly from moderate to high velocity, but was 13% shorter for the sprintspecialised skiers at their higher maximal velocity ( p , 0.05). Both groups reduced their relative gliding time and demonstrated longer relative kick and leg swing times as the velocity increased from high to maximal. However, at maximal velocity, the sprint-specialised
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group exhibited a shorter relative gliding phase and a longer relative pre-loading phase than the distance-specialised group (both p , 0.05). Force characteristics for the sprint-specialised and distance-specialised skiers are presented in Table IV. The relative peak pole force was similar for the two groups. The minimum leg force during the unloading phase was approximately 40% lower at both high and maximal velocity for the sprint-specialised group compared to the distance-specialised group (both p , 0.05) and the relative peak leg force was 11% higher for the sprint-
Figure 4. (A) Force distribution on inside rear-foot, outside rear-foot, inside forefoot, and outside forefoot; data are presented for one subject using diagonal skiing at high velocity and are averaged over five successive cycles; PLP, preloading phase; KP, kick phase; (B) relative force impulse distribution on inside rear-foot, outside rear-foot, inside forefoot, and outside forefoot during the gliding phase; and (C) the ski-loading phase. Data expressed as M ^ SD (B–C).
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Figure 4. (continued)
Figure 5. Cycle rate and cycle length adaptations from moderate to maximal diagonal skiing velocities for sprintspecialised and distance-specialised cross-country skiers. Data expressed as M ^ SD. § and * indicate a statistically significant difference between the groups at high and maximal velocity, respectively ( p , 0.05).
specialised group compared to the distance-specialised group ( p , 0.05). The times to peak pole and peak leg force did not differ between the groups, which was also the case for the rate of force development during the poling phase. The rate of leg force development during skiloading was similar between the groups at moderate and high velocity, but was 18% higher for the sprint-specialised group at maximal velocity ( p , 0.05). No difference in any force impulse variables was observed between the groups.
3.5 ^ 0.1 0.43 ^ 0.05 0.74 ^ 0.05 0.44 ^ 0.04 0.07 ^ 0.01 0.20 ^ 0.03 0.46 ^ 0.07 37 ^ 1 63 ^ 1 38 ^ 4 6^1 17 ^ 3 39 ^ 3
3.5 ^ 0.3 0.46 ^ 0.07 0.68 ^ 0.03 0.42 ^ 0.05 0.07 ^ 0.01 0.20 ^ 0.02 0.45 ^ 0.06 40 ^ 4 59 ^ 5 37 ^ 3 6^1 18 ^ 2 40 ^ 5
4.8 ^ 0.1* 0.37 ^ 0.03 0.65 ^ 0.03 0.40 ^ 0.04 0.06 ^ 0.01 0.15 ^ 0.02 0.41 ^ 0.03 36 ^ 3 64 ^ 3 39 ^ 3 6^1 15 ^ 2 40 ^ 4
SS (4.8 m/s)
High
4.4 ^ 0.2 0.39 ^ 0.05 0.68 ^ 0.07 0.41 ^ 0.03 0.06 ^ 0.01 0.17 ^ 0.03 0.43 ^ 0.06 37 ^ 3 63 ^ 3 38 ^ 2 5^1 16 ^ 2 40 ^ 6
DS (4.4 m/s)
Notes: The values presented are M ^ SD (SS, n ¼ 4; DS, n ¼ 4). * p , 0.05 in comparison to the DS skiers.
Velocity (m/s) Poling time (s) Arm swing time (s) Glide time (s) Pre-loading time (s) Kick time (s) Leg swing time (s) Relative poling time (%) Relative arm swing time (%) Relative glide time (%) Relative pre-loading time (%) Relative kick time (%) Relative leg swing time (%)
DS (3.5 m/s)
SS (3.5 m/s)
Moderate
Velocity
6.2 ^ 0.1* 0.26 ^ 0.02* 0.37 ^ 0.03 0.17 ^ 0.03* 0.05 ^ 0.01 0.13 ^ 0.01* 0.28 ^ 0.03* 41 ^ 2 59 ^ 1 26 ^ 4* 8 ^ 1* 21 ^ 2 45 ^ 10
SS (6.2 m/s)
Maximal
Table III. Cycle characteristics for sprint-specialised (SS) and distance-specialised (DS) skiers using the diagonal stride skiing technique.
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5.5 ^ 0.1 0.30 ^ 0.02 0.46 ^ 0.09 0.24 ^ 0.02 0.04 ^ 0.01 0.15 ^ 0.01 0.33 ^ 0.02 40 ^ 6 60 ^ 6 31 ^ 2 5^1 20 ^ 3 44 ^ 5
DS (5.5 m/s)
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12 ^ 2 33 ^ 4 192 ^ 21 142 ^ 81 144 ^ 25 1.0 ^ 0.5 9.6 ^ 3.0
12 ^ 3 23 ^ 3 212 ^ 4 272 ^ 75 162 ^ 43 0.5 ^ 0.2 10.0 ^ 1.8
16 ^ 2 19 ^ 3* 215 ^ 16 118 ^ 36 127 ^ 25 1.8 ^ 0.7 13.5 ^ 2.4
SS (4.8 m/s)
High
12 ^ 2 33 ^ 3 186 ^ 28 143 ^ 62 143 ^ 45 1.1 ^ 0.6 10.6 ^ 3.0
DS (4.4 m/s)
25 ^ 4 20 ^ 4* 195 ^ 10* 45 ^ 9 112 ^ 21 6.1 ^ 1.6 14.1 ^ 1.4*
SS (6.2 m/s)
21 ^ 2 32 ^ 4 176 ^ 9 51 ^ 10 117 ^ 31 4.4 ^ 1.7 11.9 ^ 1.3
DS (5.5 m/s)
Maximal
Notes: The values presented are M ^ SD (SS, n ¼ 4; DS, n ¼ 4). Abbreviations: SL ¼ ski-loading; RFD ¼ rate of force development. * p , 0.05 in comparison to the DS skiers.
Peak pole force (% BW) Minimum leg force prior SL (N) Peak leg force (% BW) Time to peak pole force (ms) Time to peak leg force (ms) RFD for the poling phase (kN/s) RFD for the leg during the SL phase (kN/s)
DS (3.5 m/s)
SS (3.5 m/s)
Moderate
Velocity
Table IV. Force characteristics for sprint-specialised (SS) and distance-specialised (DS) skiers using the diagonal stride skiing technique.
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Discussion and implications The main findings of the current study were the following: (1) elite cross-country skiers using diagonal stride increase their velocity from moderate to high with both a higher cycle rate and a longer cycle length, and at maximal velocity, the cycle rate is substantially higher at a shortened cycle length; (2) the relative poling and arm swing times were unchanged across all the three velocities, while the relative gliding, kick, and leg swing times were unchanged from moderate to high velocity, with decreased relative gliding, increased relative kick, and leg swing times at maximal velocity; (3) the peak pole force almost doubled from moderate to maximal velocity (100 to 197 N) and was applied considerably earlier at maximal velocity, while the peak leg force remained unchanged across velocities; (4) the rates of force development during the poling and the ski-loading phases were 6.0- and 1.4-fold higher, respectively, at maximal than at moderate velocity; (5) skiers loaded primarily their rear-foot during the gliding phase with a distinct shift to the forefoot during the ski-loading phase; (7) in comparison to distance-specialised skiers, sprint-specialised skiers reached higher maximal velocities at higher cycle rates, which was associated with shorter relative phases of gliding and higher rate of leg force development.
Cycle characteristics Employing an incline similar to that used in the present study, Sto¨ggl et al. (2011) reported that in diagonal roller-skiing, increases from high to maximal velocity involved a substantial increase in cycle rate at a shortened cycle length, which is in agreement with the present data and our hypothesis. Previous studies examining velocity alterations during diagonal skiing on flat and slight uphill (2.58) terrain on snow have shown that increases in velocity were mainly associated with increases in cycle rate, with only slightly increased or maintained cycle length up to maximal velocity (Nilsson et al., 2004; Va¨ha¨so¨yrinki et al., 2008). Both the current study and that of Sto¨ggl et al. (2011) were performed on steeper inclines (78) and the overcompensation in cycle rate with a decreased cycle length when increasing from high to maximal velocity was probably related to the additional work against gravity. The advantage of a substantially higher cycle rate at a shortened cycle length at maximal velocity is that it allows velocity to be maintained throughout the movement cycle (Hoffman, Clifford, & Bender, 1995). However, the higher cycle rate is likely to increase the metabolic cost, due to the higher internal work (Frederick, 1992). Finally, the current study confirms the importance of high cycle rates for achieving high maximal skiing velocities, which has also been verified for race performance in classical sprint cross-country skiing (Zory et al., 2005). Absolute durations of the poling and the kick phases decreased as velocity increased. From moderate to high velocity, poling and kick times both decreased by 16% to 0.38 s and 0.16 s, respectively. However, from high to maximal velocity, the poling time decreased substantially more than the kick time (29% vs 13%). Thus, the time sufficient for leg force application was reached at the high velocity, as indicated by both the smaller decrease of the leg force application time and the reduced leg force impulse at the maximal velocity. The kick times observed across the different velocities are in agreement with earlier findings reported for slight uphill diagonal skiing (Va¨ha¨so¨yrinki et al., 2008), and the kick time at maximal velocity is comparable to the contact times reported for sprint running uphill (98) (Weyand, Sternlight, Bellizzi, & Wright, 2000). Thus, our findings emphasise the importance of fast and substantial leg force generation due to the limited time available for force application. The relative durations of poling and arm swing were unchanged between all velocities and similar to the corresponding times for diagonal skiing on flat terrain (Nilsson et al., 2004).
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However, the relative durations of gliding, kick, and leg swing changed from high to maximal velocity (9% shorter gliding, 4% longer kick, and 3% longer leg swing), which differs from earlier observations by Nilsson et al. (2004), who reported unchanged relative phases up to maximal velocity. These differences at maximal velocity might be related, at least in part, to the uphill skiing in the present study, which required the skiers to overcome a greater component of gravitational force along the slope gradient, which likely influenced the skiing technique. The prolonged relative kick phase at a decreased relative gliding time may be related to a temporal constraint, i.e., the minimal time period required for leg force generation when increasing velocity from high to maximal. To obtain an even higher cycle rate while maintaining this critical kick time, the skiers had to shorten their absolute and relative phases of gliding, thereby adapting their technique towards a running style of diagonal skiing (albeit without completely abandoning the gliding phase, which they were instructed to maintain). Furthermore, Sto¨ggl et al. (2011) proposed that extensive vertical movement of the centre of mass may be a prerequisite for generating sufficient leg force over a limited time during fast diagonal skiing. Finally, the shorter time available for force generation at higher skiing velocities accentuates the importance of explosive strength and sprint qualities, enabling maximal rates of force generation. This would allow a higher skiing velocity, where the skier starts to shorten cycle length and overcompensate with a substantially higher cycle rate. In addition, a skiing technique with well-timed vertical oscillations might be beneficial for force generation at high skiing velocities. Force characteristics At moderate velocity, the peak pole force was in accordance with Bjo¨rklund, Sto¨ggl, and Holmberg (2010) and Lindinger et al. (2009), who reported peak pole forces of approximately 15% of BW (110 N) for diagonal roller-skiing at velocities of 3.1 m/s (6.58 and 98 incline, respectively). However, the peak pole force was two-fold higher at maximal than at moderate velocity in the current study, reaching 25% of BW at maximal. The considerably higher peak pole forces at maximal velocity corroborate previous findings by Sto¨ggl et al. (2011) who reported forces of 29% of BW (215 N) for uphill (78) roller-skiing at maximal velocity. The peak pole forces during diagonal skiing at 3.1 m/s have been associated with high arm muscle activation (EMG), about 72% of maximal voluntary contraction (Bjo¨rklund et al., 2010). Therefore, it might be conjectured that the arm muscle activation in the current study was maximal or nearly maximal at the high and maximal skiing velocities due to the considerably higher (14 –86%BW) peak pole forces attained here. The timing of the pole force application changed across velocities, with shorter absolute and relative time to peak pole forces at the two higher velocities. This inverse pattern of a decreased poling time with an increased velocity has previously been observed by Sto¨ggl et al. (2011). For another skiing technique such as double poling, where the pole is planted at a smaller angle relative to vertical, the timing of the pole force application is likely more important than in diagonal skiing (Holmberg et al., 2005; Sto¨ggl & Holmberg, 2011). That is, the disadvantage of an early application of pole force is much greater for double poling compared to the diagonal technique, in which the pole is planted at a substantially smaller angle relative to the vertical. Furthermore, later application of pole force when using the diagonal technique probably enhances the effectiveness of the force by directing it more horizontally (Smith, 2002). However, a late application of pole force may result in higher unloading (‘body lift’) during the ski-loading phase and might thus reduce the friction (grip) and impair the generation of propulsive leg force.
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The peak leg forces and the timing of their application were similar for the three velocities, with peak forces of approximately two times of BW, which is in agreement with previous findings for diagonal roller-skiing (Lindinger et al., 2009; Sto¨ggl et al., 2011). Thus, only the rate of force development was altered at higher velocities, which is consistent with previous findings for running (Kram & Taylor, 1990) that faster velocities are related to the higher rates of force generation (Weyand et al., 2000). The substantial reduction in time to peak pole force in combination with the two-fold increase in peak pole force resulted in a six-fold faster rate of pole force development at maximal compared to moderate velocity. The faster leg force development during the skiloading phase at higher velocities was related primarily to a shorter time to peak force. This supports our second hypothesis that the rate of pole and leg force development would increase with higher skiing velocities. In comparison to Sto¨ggl et al. (2011), the present study demonstrated, at a similar maximal velocity, approximately 25% shorter time to peak pole and leg forces and higher rate of force development during poling (53%) and ski-loading (28%). It can be speculated that the importance of proper timing and the rate of vertical force generation might differ between roller-skiing and on-snow skiing. On one hand, the ratcheted wheel system on roller-skis generates static friction during the kick phase that is several times greater than that reported for skiing on snow (Ainegren et al., 2012; Komi, 1985; Komi & Norman, 1987; Va¨ha¨so¨yrinki et al., 2008). In addition, balancing on classic roller skis is more difficult than on snow as the skier needs to keep the ski aligned with the forward direction in the absence of a ski track. Furthermore, when skiing on snow, large vertical forces must be applied rapidly during pre-loading and at the beginning of the kick phase (Figure 1), in order to press down the grip-waxed ski camber to generate enough static friction for the propulsive kick phase (Komi, 1987; Komi & Norman, 1987). Effective kick phases on snow are likely to be much more dependent on good technique. Accordingly, skiers who train using diagonal roller skiing during the off-season may adopt techniques that are disadvantageous when skiing on snow. Distribution of leg force impulse During gliding, 76% and 24% of the force impulse were applied to the rear-foot and forefoot, respectively. This differs slightly from the corresponding values of 63% and 37% during diagonal stride roller-skiing reported by Lindinger et al. (2009). However, the latter study was conducted at a gradient of 98, in comparison to 7.58 in the present study, which may in part explain the slightly larger forefoot distribution compared to the present study. Additionally, differences between roller-skiing and on-snow skiing may be part of this discrepancy. A greater forefoot loading during roller-skiing might be due to two factors. First, forefoot loading of the roller-ski during the gliding phase may not influence the rolling friction to the same degree as it influences the ski-glide during gliding on snow, since less loading on the forefoot during gliding minimises the contact between grip-wax and snow, thereby reducing friction. Second, in comparison to rollerskiing, it might be more important for on-snow skiing to ‘push’ the ski forward during the pre-loading phase, thus placing the centre of mass over the base of support prior to the kick phase, in order to maximise the vertical force and avoid slipping. Together, these factors might result in a greater loading of the rear-foot, reflecting the greater need to push the ski forward before the kick phase. During leg thrust, 90% of the force was located on the forefoot, which was similar to previous findings by Lindinger et al. (2009).
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Comparison between sprint-specialised and distance-specialised skiers The sprint-specialised skiers were 10% faster than the distance-specialised skiers at the high velocity due to their ability to generate longer cycle lengths. The longer cycle lengths for the sprint-specialised skiers corroborate results from previous studies (Bilodeau, Rundell, Roy, & Boulay, 1996; Norman, 1989; Norman & Komi, 1987) that have reported high positive correlations between cycle length and skiing velocity in connection with distance events (10– 50 km). Furthermore, at maximal velocity the sprint-specialised skiers were 14% faster than the distance-specialised skiers, which were related to their ability to reach substantially higher cycle rates. This is in agreement to our hypothesis and in accordance with Zory et al. (2005) who demonstrated a positive correlation between cycle rate and diagonal sprint velocity. The present study demonstrates non-linear, concave upward and downward curves for cycle rate and cycle length, respectively, in agreement with the velocity adaptation in running (Hay, 2002). Moreover, Hay (2002) demonstrated that the maximal cycle length was reached at a velocity corresponding to approximately 85% of the maximal running velocity. The same relationship was observed here, whereby the fastest sprint-specialised skiers, when compared to distance-specialised skiers, generated longer cycle lengths at higher sub-maximal velocities before overcompensating by increasing cycle rate, and decreasing cycle length, up to maximal velocity (Figure 5). Regarding the comparison between sprint-specialised and distance-specialised skiers, there was no significant difference for peak pole forces, whereas the peak leg forces of the sprint-specialised skiers were 11% higher at the maximal velocity. Moreover, the sprintspecialised skiers demonstrated a 40% lower minimum force (i.e., greater unloading) preceding the leg thrust and an 18% higher rate of leg force development. The greater unloading in the sprint-specialised group reveals, indirectly, a higher position of the centre of mass at pole plant together with a rapid lowering of the centre of mass after the pole plant, which can be considered as a technical strategy to increase the effectiveness of the following kick phase (Norman, Caldwell, & Komi, 1985; Sto¨ggl et al., 2011). In conclusion, at maximal velocity, the sprint-specialised skiers exhibited greater and more rapid development of leg forces that were applied over shorter time periods. This might be related, at least in part, to larger vertical oscillations of the centre of mass, as indicated by the lower minimum force during the unloading phase, which previously has been verified as beneficial for leg force generation (Sto¨ggl et al., 2011).
Limitations In the present investigation, the leg forces analysed were the ground reaction forces directed perpendicular to the surface of the insole, while pole forces were measured along the pole. Therefore, it was not possible to extract vector components in vertical and forward directions nor possible to determine the relative contributions of poles and skis to propulsion at the different velocities. This is information that would have provided important additional insight. Another potential limitation is the insole bending during the kick phase, which could have altered the force measurements. However, since this bending of the ski boot occurred approximately halfway through the kick phase when no loading on the rear-foot was detected (Figure 4A), this effect is probably negligible. Moreover, since the pre-loading and kick phases were determined from video and force data collected at relatively low sampling frequencies (50 and 100 Hz, respectively), the values obtained are not exact. Although, the data obtained in the present study have shortcomings, they do provide a unique insight into the performance of diagonal skiing on snow by elite skiers.
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Conclusions
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Cycle length and cycle rate increased from moderate to high velocity, and maximal velocity was attained by substantially elevating cycle rate while shortening cycle length. Kick times were considerably shorter than poling times at all velocities and the rate of force development increased with a higher velocity for both the poling and the ski-loading phases. These findings reveal that the development of leg force was considerably faster during diagonal skiing on snow than for roller-skiing. Therefore, when training on roller-skis athletes should focus on generating high vertical leg forces early during the kick in order to avoid a technique adaptation that might result in ‘slipping’ when skiing on snow. Altogether, these findings emphasise the importance of rapid leg force generation and highly coordinated motor skills for fast diagonal skiing.
Acknowledgements The authors would like to thank the participants and their coach (Per-Øyvind Torvik) for the cooperation and participation. The authors would also like to thank Mikael Sware´n, Jonas Danvind, and Espen Emanuelsen for assistance in collecting the data. Funding This study was supported financially by the Swedish National Centre for Research in Sports [grant number 2011:23] and the Swedish Olympic Committee [grant number 2011-111].
References Ainegren, M., Carlsson, P., Laaksonen, M. S., & Tinnsten, M. (2012). The influence of grip on oxygen consumption and leg forces when using classical style roller skis. Scandinavian Journal of Medicine & Science in Sports, 24, 301–310. doi:10.1111/sms.12006 Bilodeau, B., Rundell, K. W., Roy, B., & Boulay, M. R. (1996). Kinematics of cross-country ski racing. Medicine & Science in Sports & Exercise, 28, 128–138. Bjo¨rklund, G., Sto¨ggl, T., & Holmberg, H.-C. (2010). Biomechanically influenced differences in O2 extraction in diagonal skiing: Arm versus leg. Medicine & Science in Sports & Exercise, 42, 1899–1908. Frederick, E. C. (1992). Mechanical constraints on Nordic ski performance. Medicine & Science in Sports & Exercise, 24, 1010–1014. Go¨pfert, C., Holmberg, H.-C., Sto¨ggl, T., Mu¨ller, E., & Lindinger, S. J. (2013). Biomechanical characteristics and speed adaptation during kick double poling on roller skis in elite crosscountry skiers. Sports Biomechanics, 12, 154– 174. Hay, J. (2002). Cycle rate, length, and speed of progression in human locomotion. Journal of Applied Biomechanics, 18, 257– 270. Hoffman, M. D., Clifford, P. S., & Bender, F. (1995). Effect of velocity on cycle rate and length for three roller skiing techniques. Journal of Applied Biomechanics, 20, 465–479. Holmberg, H.-C., Lindinger, S., Sto¨ggl, T., Eitzlmair, E., & Mu¨ller, E. (2005). Biomechanical analysis of double poling in elite cross-country skiers. Medicine & Science in Sports & Exercise, 37, 807 –818. Komi, P. V. (1985). Ground reaction forces in cross-country skiing. In D. Winter, R. Norman, R. Wells, K. Hayes, & A. Patla (Eds.), Biomechanics IX-B (pp. 185–190). Champaign, IL: Human Kinetics. Komi, P. V. (1987). Force measurements during cross-country skiing. International Journal of Sport Biomechanics, 3, 370– 381. Komi, P. V., & Norman, R. (1987). Preloading of the thrust phase in cross-country skiing. International Journal of Sports Medicine, 8, 48–54. Kram, R., & Taylor, C. R. (1990). Energetics of running: A new perspective. Nature, 346, 265– 267. Lindinger, S. J., Go¨pfert, C., Sto¨ggl, T., Mu¨ller, E., & Holmberg, H.-C. (2009). Biomechanical pole and leg characteristics during uphill diagonal roller skiing. Sports Biomechanics, 8, 318 –333.
Downloaded by [Selcuk Universitesi] at 09:43 02 February 2015
284
E. Andersson et al.
Nilsson, J., Tveit, P., & Eikrehagen, O. (2004). Effects of speed on temporal patterns in classical style and freestyle cross-country skiing. Sports Biomechanics, 3, 85– 107. Norman, R. W. (1989). Mechanical power output and estimated metabolic rates of Nordic skiers during Olympic competition. Journal of Applied Biomechanics, 5, 169–184. Norman, R. W., Caldwell, G., & Komi, P. V. (1985). Differences in body segment energy utilization between world class and recreational cross-country skiers. International Journal of Sport Biomechanics, 1, 253–262. Norman, R. W., & Komi, P. (1987). Mechanical energetics of world class cross-country skiing. Journal of Applied Biomechanics, 3, 353–369. Smith, G. A. (2002). Biomechanics of cross country skiing. In H. Rusko (Ed.), Cross country skiing: Olympic handbook of sports medicine and science (pp. 32–61). Malden, MA: Blackwell. Sto¨ggl, T., & Holmberg, H.-C. (2011). Force interaction and 3D pole movement in double poling. Scandinavian Journal of Medicine & Science in Sports, 21, e393–e404. Sto¨ggl, T., Mu¨ller, E., Ainegren, M., & Holmberg, H.-C. (2011). General strength and kinetics: Fundamental to sprinting faster in cross country skiing? Scandinavian Journal of Medicine & Science in Sports, 21, 791–803. Sto¨ggl, T. L., & Mu¨ller, E. (2009). Kinematic determinants and physiological response of cross-country skiing at maximal speed. Medicine & Science in Sports & Exercise, 41, 1476–1487. Va¨ha¨so¨yrinki, P., Komi, P. V., Seppala, S., Ishikawa, M., Kolehmainen, V., Salmi, J. A., & Linnamo, V. (2008). Effect of skiing speed on ski and pole forces in cross-country skiing. Medicine & Science in Sports & Exercise, 40, 1111–1116. Weyand, P. G., Sternlight, D. B., Bellizzi, M. J., & Wright, S. (2000). Faster top running speeds are achieved with greater ground forces not more rapid leg movements. Journal of Applied Physiology, 89, 1991–1999. Zory, R., Barberis, M., & Rouard, A. (2005). Kinematics of sprint cross-country skiing. Acta of Bioengineering and Biomechanics, 7, 87– 96.
