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Landing ground reaction forces in figure skaters and non-skaters a
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Nathan W. Saunders , Nicholas Hanson , Panos Koutakis , Ajit M. Chaudhari & Steven T. b
Devor a
Department of Health, Athletic Training, Recreation, & Kinesiology, Longwood University, Farmville, USA b
Department of Human Sciences, Kinesiology Program, The Ohio State University, Columbus, USA c
Department of Surgery, University of Nebraska Medical Center, Omaha, USA
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Department of Orthopaedics and Sport Health & Performance Institute, The Ohio State University, Columbus, USA Published online: 30 Jan 2014.
To cite this article: Nathan W. Saunders, Nicholas Hanson, Panos Koutakis, Ajit M. Chaudhari & Steven T. Devor (2014) Landing ground reaction forces in figure skaters and non-skaters, Journal of Sports Sciences, 32:11, 1042-1049, DOI: 10.1080/02640414.2013.877593 To link to this article: http://dx.doi.org/10.1080/02640414.2013.877593
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Journal of Sports Sciences, 2014 Vol. 32, No. 11, 1042–1049, http://dx.doi.org/10.1080/02640414.2013.877593
Landing ground reaction forces in figure skaters and non-skaters NATHAN W. SAUNDERS1, NICHOLAS HANSON2, PANOS KOUTAKIS3, AJIT M. CHAUDHARI4, & STEVEN T. DEVOR2 1
Department of Health, Athletic Training, Recreation, & Kinesiology, Longwood University, Farmville, USA, 2Department of Human Sciences, Kinesiology Program, The Ohio State University, Columbus, USA, 3Department of Surgery, University of Nebraska Medical Center, Omaha, USA and 4Department of Orthopaedics and Sport Health & Performance Institute, The Ohio State University, Columbus, USA
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(Accepted 17 December 2013)
Abstract Researchers and clinicians have suggested that overuse injuries to the lower back and lower extremities of figure skaters may be associated with the repeated high impact forces sustained during jump landings. Our primary aim was to compare the vertical ground reaction forces (GRFs) in freestyle figure skaters (n = 26) and non-skaters (n = 18) for the same barefoot single leg landing on a force plate from a 20 cm platform. Compared with non-skaters, skaters exhibited a significantly greater normalised peak GRF (3.50 ± 0.47 × body weight for skaters vs. 3.13 ± 0.45 × body weight for non-skaters), significantly shorter time to peak GRF (81.21 ± 14.01 ms for skaters vs. 93.81 ± 16.49 ms for non-skaters), and significantly longer time to stabilisation (TTS) of the GRF (2.38 ± 0.07 s for skaters vs. 2.22 ± 0.07 s for non-skaters). Skaters also confined their centre of pressure (CoP) to a significantly smaller mediolateral (M–L) (25%) and anterior–posterior (A–P) (40%) range during the landing phase, with the position of the CoP located in the mid to forefoot region. The narrower and more forward position of the CoP in skaters may at least partially explain the greater peak GRF, shorter time to peak, and longer TTS. Training and/or equipment modification serve as potential targets to decrease peak GRF by distributing it over a longer time period. More comprehensive studies including electromyography and motion capture are needed to fully characterise the unique figure skater landing strategy. Keywords: ground reaction forces, time to stabilisation, postural control, force plates, jump biomechanics
Introduction The success of freestyle figure skaters in competition is heavily dependent on the success and quality of jump landings (USFS, 2012), and while landing technique is emphasised during training, the majority of figure skating injuries are overuse injuries to the landing foot and ankle (Dubravcic-Simunjak, Pecina, Kuipers, Moran, & Haspl, 2003; Fortin & Roberts, 2003; Lipetz & Kruse, 2000; Smith, 2000). Researchers and sports medicine clinicians have suggested many of these injuries may be associated with the repeated high ground reaction forces (GRFs) sustained during jump landings (DubravcicSimunjak et al., 2003; Fortin & Roberts, 2003). However, little research has focused on evaluating and reducing the jump landing GRF in figure skaters. There is reason to believe that figure skater landing GRFs increase in magnitude with increasing skill level (King, 2005; Lockwood & Gervais, 1997).
Specifically, Lockwood and Gervais (1997) found that on-ice jump landing impact forces increased with increasing jump revolutions, even though the jump height was the same for single, double, and triple revolution jumps. They concluded that the greater revolution jumps place skaters closer to the ice surface at the completion of the rotations, which decreases the time difference between fore and rear foot impact forces; there is less time to dissipate the GRF. Relatively large GRFs have also been attributed to the thick and rigid skating boots popular among competitive figure skaters of all ages (Bruening & Richards, 2006). The vertical GRF resulting from a backward figure skater landing from a 30 cm box were compared between figure skaters wearing traditional rigid skating boots and the same skaters wearing newly designed articulated boots permitting a greater ankle range of motion about the mediolateral (M–L) axis. Compared with
Correspondence: Steven T. Devor, The Ohio State University, Department of Human Sciences, Kinesiology Program, 305 W 17th Ave – PAES building, Columbus, 43210 USA. E-mail: [email protected] © 2014 Taylor & Francis
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Landing ground reaction forces in figure skaters and non-skaters traditional skating boots, figure skaters wearing articulated boots exhibited an 18% lower peak heel GRF. While no specific mechanism was identified to explain the reduced GRF, the articulated boot dissipated the GRF by distributing it over a longer period of time. How the number of jump revolutions and bootlimited ankle range of motion contribute to greater landing GRFs in figure skaters is informative and important, but our understanding of skater landings is incomplete. Figure skaters are likely to develop unique landing mechanics as a result of their skating equipment, landing surface, and the sport mandated need to be aesthetically pleasing in order to be scored highly by judges. One way to evaluate the unique landing mechanics of skaters is to compare the landings of skaters with untrained non-skaters. While no work like this has been done with figure skaters, interesting results have come from studies comparing drop landing GRFs in barefoot gymnasts and recreational athletes (Seegmiller & McCaw, 2003). Surprisingly, competitive gymnasts have been shown to exhibit greater GRFs than recreational athletes, though it is not clear why. Our aim was to compare the peak GRF, time to peak GRF, and time to stabilisation (TTS) of the GRF in figure skaters and non-skaters for the same barefoot single leg landing. We tested the hypothesis that compared with non-skaters, figure skaters would exhibit greater peak vertical GRF and shorter time to peak GRF for the same barefoot single leg landing. We also hypothesised that skaters may exhibit a longer TTS as they would be performing a barefoot landing without the ankle support normally provided by the boot.
Methods Participants Female freestyle figure skaters and non-skaters 10– 28 years of age were recruited from local ice rinks in Columbus, Ohio. Figure skaters were eligible if they had at least 1 year of competition experience and practiced on the ice an average of at least 2 h per week. Skaters were excluded if they recently missed any practice due to injury, if they had any physical or physiological deficiencies affecting balance (e.g. scoliosis, ear infection, etc.), or if they had a known pregnancy. Male skaters were excluded due to lack of a sufficient number. Non-skaters were excluded if they had recent experience (in the preceding 12 months) or 2 years total experience in the following activities: any form of dance, gymnastics, cheerleading, martial arts, or diving. These activities were selected as
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Table I. Uniform ranking system. Ice Skating Institute Freestyle Freestyle Freestyle Freestyle Freestyle Freestyle Freestyle Freestyle
1–3 4 5 6 7 8 9 10
US Figure Skating
Uniform Ranking System
Pre-preliminary Preliminary Pre-juvenile Juvenile Intermediate Novice Junior Senior
Level Level Level Level Level Level Level Level
1 2 3 4 5 6 7 8
Note: This system is based on testing requirements, not competitive skill sets.
Table II. Descriptive characteristics of participants; N = 44. Figure skaters (n = 26) Age (years) Height (cm) Body mass (kg) Skill level Competition experience (years) Practice time/week (h)
14.7 155.8 50.4 4.0 5.4
± ± ± ± ±
4.5 9.6 11.8 2.2 3.1
5.8 ± 4.0
Nonskaters (n = 18) 15.4 ± 5.3 157.9 ± 11.5 54.1 ± 14.7 N/A N/A N/A
Note: Values are expressed as mean ± SD.
exclusion criteria because many of their skill elements are similar to that of a figure skater. Given that participants were members of one or more skating organisations, each with its own distinct testing requirements and ranking system, we established a uniform ranking system based on similar testing requirements, wherein each skater was assigned a skill level, 1–8, with 8 being the most advanced (Table I). The Ohio State University Institutional Review Board approved informed written consent was obtained from all participants before participation in this study (parental/guardian consent was obtained for minors). In all, 44 participants volunteered for this study and completed the landing assessment, 26 singles figure skaters and 18 non-skaters (Table II). Testing protocol A cross-sectional design was utilised, and all interested participants meeting the inclusion criteria underwent a landing assessment. Participants were asked to stand on a 20 cm high platform that was positioned 10 cm in front of a force plate (Kovacs, Birmingham, Forwell, & Litchfield, 2004). Others have used a 30 cm platform height (Bruening & Richards, 2006), but a lower height was selected due to the inclusion of untrained non-skaters. Standing barefoot with
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Figure 1. A representative example of vertical TTS data from a single trial. The solid line of best fit represents the sequential average of the vertical GRF. The two horizontal dotted lines represent ±0.25 standard deviations from the overall mean. The vertical dashed line represents the point where the sequential average of the vertical GRF entered and remained within ±0.25 standard deviations of the overall 14 s mean. M–L and A–P TTS were calculated utilising the same method.
their heels at the edge of the platform and facing away from the plate they were instructed to drop backwards from two feet and land on the force plate in the typical skater landing position (arms extended to their sides, chest upright, free leg extended straight behind them, and supporting knee bent to the angle of their choosing). Skaters were instructed to use their normal landing leg, while non-skaters selected the leg they would naturally use to kick a ball. Each participant was asked to practice the drop landing three times to diminish any learning effect that might occur (Kovacs et al., 2004; Seegmiller & McCaw, 2003). Upon landing, participants were asked to remain as motionless as possible until 15 s of data acquisition were complete. A trial was considered successful if the supporting foot remained in contact with the plate and the non-supporting foot made no contact with the plate for the entire trial duration. An average of the three successful trials was taken for data analysis. Due to a technical issue, the final second of data for all trials was discarded. Instruments and data analysis To assess the vertical GRF of each participant we utilised a Bertec 4060–10 force plate (Bertec Corporation, Columbus, OH). Data were collected at a sampling rate of 1000 Hz utilising Vicon Nexus software (Oxford Metrics, Oxford, UK). All data processing and analyses were performed with custom made Matlab programs (Matlab R2009a, Natick, MA, USA). Spectral analysis identified most of the power of the signal below 10 Hz. We then applied a low-pass filter at 10 Hz with a fourth-order and zerolag Butterworth filter (Ross & Guskiewicz, 2003; Ross, Guskiewicz, Gross, & Yu, 2009). The normalised peak vertical GRF was calculated by taking an average of the greatest GRF magnitude for three successful trials and dividing by participant body weight. Time to peak GRF was simply the time
difference between the first non-zero vertical GRF and the peak vertical GRF. We adapted our calculation of TTS from an earlier study (Colby, Hintermeister, Torry, & Steadman, 1999). Briefly, an average of the vertical GRF for the entire trial duration (14 s) was taken. TTS was identified as the point where the sequential average of the respective GRF remained within ±0.25 standard deviations of the overall 14 s mean (Figure 1). Impulse was calculated as the area under the vertical GRF curve (body weight × s). To further describe differences between skater and non-skater landings we also calculated the centre of pressure (CoP) range for the time period between initial foot contact with the plate and the peak vertical GRF. CoP range was calculated as the sum of the absolute maximum and minimum excursions of the CoP for the M–L and A–P axes. Statistical analyses Statistical analyses were performed by STATA (Version 11, 2009; StataCorp LP, College Station, Texas). Having met the assumptions of normal distribution and equal variance, between-group comparisons of normalised peak vertical GRF, time to peak GRF, TTS, impulse, and CoP range (M–L and A–P) were determined with two-tailed two-sample t-tests. The significance level was set a priori at alpha = 0.05. Results While the loading rate (rate of increase in the vertical GRF) was similar for both groups during the first 50 ms of the landing (Figure 2), compared with nonskaters, figure skaters exhibited a significantly greater peak vertical GRF (3.50 ± 0.47 × body weight for skaters vs. 3.13 ± 0.45 × body weight for non-skaters, Figure 3A). The expected inverse relationship between peak GRF and time to peak GRF did exist for both
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Figure 2. Vertical GRF for all participants. Vertical GRF was averaged across all participants from each group to display the average group GRF position over time. Solid line indicates skaters. Dashed line indicates non-skaters. Shaded regions represent two standard deviations from the mean.
groups, with skaters having significantly shorter time to peak GRF (81.21 ± 14.01 ms for skaters vs. 93.81 ± 16.49 ms for non-skaters, Figure 3B). Figure 3C shows the vertical TTS for skaters and non-skaters. Skaters did exhibit marginally but significantly longer vertical TTS (2.38 ± 0.07 s) compared with non-skaters (2.22 ± 0.07 s). The impulse (Figure 3D) was similar for both skaters and non-skaters. Figure 4 shows the average horizontal sway path of the CoP for each group for the time period from initial contact with the plate until the average time to peak GRF for each group, respectively. In order to quantify differences between the sway path of the CoP of skaters and non-skaters we calculated CoP range for the time period from initial contact with the plate until the average time to peak GRF (Figure 5). Compared with non-skaters, skaters exhibited a significantly smaller M–L CoP range (26.43 ± 6.49 mm for skaters vs. 35.43 ± 13.16 mm for non-skaters) and A–P CoP range (109.97 ± 15.51 mm for skaters vs. 183.07 ± 51.56 mm for non-skaters).
Discussion Foot and ankle acute and overuse injuries are becoming more common among freestyle figure skaters, and repeated microtrauma to the foot and ankle from excessively forceful landings is likely to play a significant role (Pelham, Holt, & Stalker, 2001). Landing impact forces have been shown to increase with an increase in the number of jump revolutions (Lockwood & Gervais, 1997). A biomechanically poor skate boot design, one that limits the ankle range of motion about the M–L axis via stiff ankle support and a raised heel, is also thought to contribute to greater landing impact forces (Anderson, Weber, Steinbach, & Ballmer, 2004; Bradley, 2006; Brown, Varney, & Micheli, 2000; Bruening & Richards, 2006; Ehrenman, 2004; Haguenauer, Legreneur, & Monteil, 2006; Pelham et al., 2001; Smith, 2000; Smith & Ludington, 1989). The purpose of the present study was to identify unique landing mechanics in freestyle figure skaters that may further contribute to the excessively forceful landings. We did this by comparing the vertical
Figure 3. (A) Peak vertical GRF normalised to body weight (BW), (B) time to peak vertical GRF, (C) TTS, and (D) Impulse. Error bars represent one standard deviation from the mean. * indicates a significant difference between skaters and non-skaters (P < 0.05).
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Peak GRF
Figure 4. CoP sway path representing the time period between initial contact and peak GRF. Horizontal CoP (x, y) coordinates were averaged across all participants from each group to display the average group CoP position over time. The CoP was calculated based on the force plate local coordinate system and we did not use an absolute reference frame to normalise for foot location. The average CoP sway path for each group is superimposed over a force plate with the dimensions 400 × 600 mm. The CoP for all participants began in the forefoot and travelled posteriorly towards the heel. Solid line indicates skaters. Dotted line indicates nonskaters.
Figure 5. M–L and A–P CoP range representing the time period between initial contact and peak GRF. Error bars represent one standard deviation from the mean. * indicates a significant difference between skaters and non-skaters (P < 0.05).
Our first hypothesis was supported as compared with non-skaters, figure skaters exhibited significantly greater peak vertical GRF, even though this was a customary task for the skaters, but quite novel for the non-skaters. This result is in agreement with others (Seegmiller & McCaw, 2003) who studied gymnasts and recreational athletes dropping from heights of 30, 60, and 90 cm. Compared with recreational athletes, gymnasts experienced 21%, 33%, and 33% greater vertical GRF for the 30, 60, and 90 cm heights, respectively. Largely due to a small sample size (10 participants per group) the group difference for the 30 cm height was not significant. They suggested the lower height was not sufficiently challenging to discriminate between the two groups. In the present study, a 20 cm height was capable of discriminating between skaters and non-skaters, with skater exhibiting an 11% greater GRF. However, our participants were required to drop backwards and land on one foot, while the gymnasts and recreational athletes performed a forward drop with a two-foot landing. The increased degree of difficulty appears to have compensated for a lower platform height. In fact, the 20 cm height in the present study elicited similar peak GRF for skaters (3.5 × body weight) to that of the 60 cm height for gymnasts (4.1 × body weight). Nevertheless, based on the results of Seegmiller and McCaw (2003) we would expect the disparity between skater and nonskater GRF to increase with an increased platform height. We have also identified some important relationships between our findings and a study comparing normal skate boots to a newly developed skate boot with an articulated ankle that permits a greater ankle range of motion about the M–L axis (Bruening & Richards, 2006). Dropping from a 30 cm platform onto a force plate, compared with the normal skate boots, the articulated skate boots resulted in an 18% lower peak heel GRF, but no difference in peak toe GRF. Though no conclusive evidence of heel contact exists due to the backward barefoot landing protocol in the present study, the difference between skaters and non-skaters in vertical GRF was only evident after 50 ms (Figure 2), which may suggest that the difference in peak vertical GRF partially resulted from heel contact in the non-skaters (but not in the skaters) later in the landing phase. Interestingly, the magnitude of the peak GRF in the articulated boot (4.1 × body weight) was only 15% greater than the barefoot skaters in our study (3.5 × body weight), a
Landing ground reaction forces in figure skaters and non-skaters difference which could simply be the result of the platform height disparity. Though not directly measured or studied, it is important to note that there were no observable technical differences between skaters and non-skaters in how they dropped from the box. This is supported by the absence of a significant group difference in impulse (Figure 3D).
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Time to peak ground reaction force There is typically an inverse correlation between peak GRF and time to peak GRF (Seegmiller & McCaw, 2003), and our results support this relationship. While figure skaters exhibited greater peak vertical GRF than non-skaters, the time to peak GRF of the skaters was significantly shorter. Even though this was expected based on previous investigations, this is still an alarming result. A biomechanically optimal model for figure skater landings has been developed (Lockwood, Gervais, & McCreary, 2006). One of the main features of this optimal landing was the ability to dissipate the impact forces over a long period of time. Yet, the skaters in the present study were less able to dissipate these forces than their untrained counterparts. Just as the articulated boot of Bruening and Richards (2006) reduced the peak GRF, it decreased the loading rate by 31%. They argued that the improved landings were a result of greater range of motion at the ankle. However, in their on-ice comparison of regular versus the articulated boots there were no differences in landing forces or loading rates, possibly because many skaters, especially the more advanced skaters, did not make use of the greater potential range of motion in the articulated boots. Likewise, even though our participants were barefoot giving them the potential for full range of motion, the skaters, out of habit, may have resisted ankle plantarflexion and/or dorsiflexion. Time to stabilisation The use of TTS has previously demonstrated a sensitivity capable of discriminating between limbs with compromised anterior cruciate ligaments and uninjured limbs (Colby et al., 1999), and between functionally stable and unstable ankles (Ross, Guskiewicz, & Yu, 2005). In these studies, lower TTS values has been associated with better lower limb health. Using this rational we would hope for lower TTS in figure skaters compared with nonskaters. That said, we suggested skaters in the present study may take longer to stabilise the vertical GRF as they were performing a barefoot landing without the ankle support normally provided by the boot. As
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hypothesised, compared with non-skaters, the skaters had significantly greater TTS. Though not shown here, this result did not change even when we normalised TTS to peak GRF. There are several plausible explanations. First, the extra firm skating boots common among skaters may lead to muscle weakness. While not likely the case in the present study population, due to a broad range of skill levels, most of the weight bearing hours for highly competitive figure skaters is spent in a rigid skating boot, which may actually weaken and shorten ankle muscles as if they were in an immobilisation cast (Smith, 2000). Another endorsement for the muscle weakness hypothesis is that most ankle injuries to figure skaters occur during off-ice conditioning when the protective boots are absent (Porter, Young, Niedfeldt, & Gottschlich, 2007). Importantly, we did not test for muscle weakness, and therefore cannot confirm this hypothesis. Second, the greater TTS in skaters may be related to training habits rather than muscle weakness. In other words, skaters may become somewhat reliant on the extra boot support even if it is not needed. Finally, TTS is influenced by the whole-body landing mechanics, and only a comprehensive kinematic examination would be able to describe this relationship. Centre of pressure range One of the more intriguing results of this study came from the analysis of the CoP range. When standing, as a person goes into more dorsiflexion, the CoP moves anteriorly. However, when landing backward with the supporting ankle partially plantarflexed and the non-supporting leg traveling posteriorly, the CoP moves posteriorly as the ankle is dorsiflexed. Relative to non-skaters, skaters exhibited a 25% smaller M–L range and a 40% smaller A–P range during the landing phase. We suggest that this smaller range may be a function of training with a raised heel. Because skaters are unable to achieve full dorsiflexion of the ankle in their skates, they may have been resistant to complete dorsiflexion even when barefoot. If this were the case, the CoP would have been maintained towards the forefoot, thereby reducing the A–P CoP range. The CoP tracing during the landing phase (Figure 4) could support the speculation of a forefoot dominance in our skaters and may partially explain the greater peak GRF, shorter time to peak GRF, and longer TTS seen in figure skaters. Study limitations While we were able to contribute significantly to the present literature on landing GRF in figure skaters, this study was limited in a number of ways. First, the
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participants were diverse both in age and skating experience, and thus few participants represented each skill level and age group. Second, no electromyography or muscle strength data were collected. This prevented us from testing several of the hypothetical explanations regarding TTS. Third, the lack of motion capture data prevented us from analysing whole-body kinematics, which would have certainly contributed to a better explanation of the greater peak GRF, shorter time to peak GRF, and longer TTS seen in these skaters. Finally, and possibly most significant, the inclusion of non-skaters necessitated a laboratory environment for our tests. Laboratory tests often do not translate well to a natural environment (Bruening & Richards, 2006). We recommend that these results be used cautiously.
Conclusion We are the first to report on differences between skaters and non-skaters in landing GRF and CoP placement. In agreement with our hypotheses, compared with non-skaters, figure skaters exhibited significantly greater peak GRF, significantly shorter time to peak GRF, and significantly longer TTS for the same barefoot drop landing. Skaters also confined their CoP to a significantly smaller M–L and A–P range, with the position of the CoP located in the mid to forefoot region. The narrower and more forward position of the CoP in skaters may at least partially explain the greater peak GRF. Considering the possible implications of excessive landing forces on chronic and acute injuries in figure skaters, continued work in this area is warranted. Specifically, it is important to evaluate why skaters maintain a narrower and more forward CoP position. Training and/or equipment modification serve as potential targets to decrease peak GRF by distributing the forces over a longer time period. More comprehensive studies including electromyography and motion capture are needed to fully characterise the unique figure skater landing strategy. References Anderson, S. E., Weber, M., Steinbach, L. S., & Ballmer, F. T. (2004). Shoe rim and shoe buckle pseudotumor of the ankle in elite and professional figure skaters and snowboarders: MR imaging findings. Skeletal Radiology, 33(6), 325–329. doi:10.1007/s00256-004-0778-6 Bradley, M. A. (2006). Prevention and treatment of foot and ankle injuries in figure skaters. Current Sports Medicine Reports, 5(5), 258–261. Retrieved from www.ncbi.nlm.nih.gov/pubmed/ 16934208 Brown, T. D., Varney, T. E., & Micheli, L. J. (2000). Malleolar bursitis in figure skaters. The American Journal of Sports
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Smith, A. D., & Ludington, R. (1989). Injuries in elite pair skaters and ice dancers. The American Journal of Sports Medicine, 17(4), 482–488. Retrieved from http://www.ncbi.nlm.nih.gov/ pubmed/2782532 USFS (Producer). (2012, 2013). The 2013 official figure skating rule book. Retrieved from: http://www.usfsa.org/content/201213%20Rulebook%20for%20eReaders.pdf
Journal of Sports Sciences Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/rjsp20
Landing ground reaction forces in figure skaters and non-skaters a
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c
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Nathan W. Saunders , Nicholas Hanson , Panos Koutakis , Ajit M. Chaudhari & Steven T. b
Devor a
Department of Health, Athletic Training, Recreation, & Kinesiology, Longwood University, Farmville, USA b
Department of Human Sciences, Kinesiology Program, The Ohio State University, Columbus, USA c
Department of Surgery, University of Nebraska Medical Center, Omaha, USA
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Department of Orthopaedics and Sport Health & Performance Institute, The Ohio State University, Columbus, USA Published online: 30 Jan 2014.
To cite this article: Nathan W. Saunders, Nicholas Hanson, Panos Koutakis, Ajit M. Chaudhari & Steven T. Devor (2014) Landing ground reaction forces in figure skaters and non-skaters, Journal of Sports Sciences, 32:11, 1042-1049, DOI: 10.1080/02640414.2013.877593 To link to this article: http://dx.doi.org/10.1080/02640414.2013.877593
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Journal of Sports Sciences, 2014 Vol. 32, No. 11, 1042–1049, http://dx.doi.org/10.1080/02640414.2013.877593
Landing ground reaction forces in figure skaters and non-skaters NATHAN W. SAUNDERS1, NICHOLAS HANSON2, PANOS KOUTAKIS3, AJIT M. CHAUDHARI4, & STEVEN T. DEVOR2 1
Department of Health, Athletic Training, Recreation, & Kinesiology, Longwood University, Farmville, USA, 2Department of Human Sciences, Kinesiology Program, The Ohio State University, Columbus, USA, 3Department of Surgery, University of Nebraska Medical Center, Omaha, USA and 4Department of Orthopaedics and Sport Health & Performance Institute, The Ohio State University, Columbus, USA
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(Accepted 17 December 2013)
Abstract Researchers and clinicians have suggested that overuse injuries to the lower back and lower extremities of figure skaters may be associated with the repeated high impact forces sustained during jump landings. Our primary aim was to compare the vertical ground reaction forces (GRFs) in freestyle figure skaters (n = 26) and non-skaters (n = 18) for the same barefoot single leg landing on a force plate from a 20 cm platform. Compared with non-skaters, skaters exhibited a significantly greater normalised peak GRF (3.50 ± 0.47 × body weight for skaters vs. 3.13 ± 0.45 × body weight for non-skaters), significantly shorter time to peak GRF (81.21 ± 14.01 ms for skaters vs. 93.81 ± 16.49 ms for non-skaters), and significantly longer time to stabilisation (TTS) of the GRF (2.38 ± 0.07 s for skaters vs. 2.22 ± 0.07 s for non-skaters). Skaters also confined their centre of pressure (CoP) to a significantly smaller mediolateral (M–L) (25%) and anterior–posterior (A–P) (40%) range during the landing phase, with the position of the CoP located in the mid to forefoot region. The narrower and more forward position of the CoP in skaters may at least partially explain the greater peak GRF, shorter time to peak, and longer TTS. Training and/or equipment modification serve as potential targets to decrease peak GRF by distributing it over a longer time period. More comprehensive studies including electromyography and motion capture are needed to fully characterise the unique figure skater landing strategy. Keywords: ground reaction forces, time to stabilisation, postural control, force plates, jump biomechanics
Introduction The success of freestyle figure skaters in competition is heavily dependent on the success and quality of jump landings (USFS, 2012), and while landing technique is emphasised during training, the majority of figure skating injuries are overuse injuries to the landing foot and ankle (Dubravcic-Simunjak, Pecina, Kuipers, Moran, & Haspl, 2003; Fortin & Roberts, 2003; Lipetz & Kruse, 2000; Smith, 2000). Researchers and sports medicine clinicians have suggested many of these injuries may be associated with the repeated high ground reaction forces (GRFs) sustained during jump landings (DubravcicSimunjak et al., 2003; Fortin & Roberts, 2003). However, little research has focused on evaluating and reducing the jump landing GRF in figure skaters. There is reason to believe that figure skater landing GRFs increase in magnitude with increasing skill level (King, 2005; Lockwood & Gervais, 1997).
Specifically, Lockwood and Gervais (1997) found that on-ice jump landing impact forces increased with increasing jump revolutions, even though the jump height was the same for single, double, and triple revolution jumps. They concluded that the greater revolution jumps place skaters closer to the ice surface at the completion of the rotations, which decreases the time difference between fore and rear foot impact forces; there is less time to dissipate the GRF. Relatively large GRFs have also been attributed to the thick and rigid skating boots popular among competitive figure skaters of all ages (Bruening & Richards, 2006). The vertical GRF resulting from a backward figure skater landing from a 30 cm box were compared between figure skaters wearing traditional rigid skating boots and the same skaters wearing newly designed articulated boots permitting a greater ankle range of motion about the mediolateral (M–L) axis. Compared with
Correspondence: Steven T. Devor, The Ohio State University, Department of Human Sciences, Kinesiology Program, 305 W 17th Ave – PAES building, Columbus, 43210 USA. E-mail: [email protected] © 2014 Taylor & Francis
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Landing ground reaction forces in figure skaters and non-skaters traditional skating boots, figure skaters wearing articulated boots exhibited an 18% lower peak heel GRF. While no specific mechanism was identified to explain the reduced GRF, the articulated boot dissipated the GRF by distributing it over a longer period of time. How the number of jump revolutions and bootlimited ankle range of motion contribute to greater landing GRFs in figure skaters is informative and important, but our understanding of skater landings is incomplete. Figure skaters are likely to develop unique landing mechanics as a result of their skating equipment, landing surface, and the sport mandated need to be aesthetically pleasing in order to be scored highly by judges. One way to evaluate the unique landing mechanics of skaters is to compare the landings of skaters with untrained non-skaters. While no work like this has been done with figure skaters, interesting results have come from studies comparing drop landing GRFs in barefoot gymnasts and recreational athletes (Seegmiller & McCaw, 2003). Surprisingly, competitive gymnasts have been shown to exhibit greater GRFs than recreational athletes, though it is not clear why. Our aim was to compare the peak GRF, time to peak GRF, and time to stabilisation (TTS) of the GRF in figure skaters and non-skaters for the same barefoot single leg landing. We tested the hypothesis that compared with non-skaters, figure skaters would exhibit greater peak vertical GRF and shorter time to peak GRF for the same barefoot single leg landing. We also hypothesised that skaters may exhibit a longer TTS as they would be performing a barefoot landing without the ankle support normally provided by the boot.
Methods Participants Female freestyle figure skaters and non-skaters 10– 28 years of age were recruited from local ice rinks in Columbus, Ohio. Figure skaters were eligible if they had at least 1 year of competition experience and practiced on the ice an average of at least 2 h per week. Skaters were excluded if they recently missed any practice due to injury, if they had any physical or physiological deficiencies affecting balance (e.g. scoliosis, ear infection, etc.), or if they had a known pregnancy. Male skaters were excluded due to lack of a sufficient number. Non-skaters were excluded if they had recent experience (in the preceding 12 months) or 2 years total experience in the following activities: any form of dance, gymnastics, cheerleading, martial arts, or diving. These activities were selected as
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Table I. Uniform ranking system. Ice Skating Institute Freestyle Freestyle Freestyle Freestyle Freestyle Freestyle Freestyle Freestyle
1–3 4 5 6 7 8 9 10
US Figure Skating
Uniform Ranking System
Pre-preliminary Preliminary Pre-juvenile Juvenile Intermediate Novice Junior Senior
Level Level Level Level Level Level Level Level
1 2 3 4 5 6 7 8
Note: This system is based on testing requirements, not competitive skill sets.
Table II. Descriptive characteristics of participants; N = 44. Figure skaters (n = 26) Age (years) Height (cm) Body mass (kg) Skill level Competition experience (years) Practice time/week (h)
14.7 155.8 50.4 4.0 5.4
± ± ± ± ±
4.5 9.6 11.8 2.2 3.1
5.8 ± 4.0
Nonskaters (n = 18) 15.4 ± 5.3 157.9 ± 11.5 54.1 ± 14.7 N/A N/A N/A
Note: Values are expressed as mean ± SD.
exclusion criteria because many of their skill elements are similar to that of a figure skater. Given that participants were members of one or more skating organisations, each with its own distinct testing requirements and ranking system, we established a uniform ranking system based on similar testing requirements, wherein each skater was assigned a skill level, 1–8, with 8 being the most advanced (Table I). The Ohio State University Institutional Review Board approved informed written consent was obtained from all participants before participation in this study (parental/guardian consent was obtained for minors). In all, 44 participants volunteered for this study and completed the landing assessment, 26 singles figure skaters and 18 non-skaters (Table II). Testing protocol A cross-sectional design was utilised, and all interested participants meeting the inclusion criteria underwent a landing assessment. Participants were asked to stand on a 20 cm high platform that was positioned 10 cm in front of a force plate (Kovacs, Birmingham, Forwell, & Litchfield, 2004). Others have used a 30 cm platform height (Bruening & Richards, 2006), but a lower height was selected due to the inclusion of untrained non-skaters. Standing barefoot with
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Figure 1. A representative example of vertical TTS data from a single trial. The solid line of best fit represents the sequential average of the vertical GRF. The two horizontal dotted lines represent ±0.25 standard deviations from the overall mean. The vertical dashed line represents the point where the sequential average of the vertical GRF entered and remained within ±0.25 standard deviations of the overall 14 s mean. M–L and A–P TTS were calculated utilising the same method.
their heels at the edge of the platform and facing away from the plate they were instructed to drop backwards from two feet and land on the force plate in the typical skater landing position (arms extended to their sides, chest upright, free leg extended straight behind them, and supporting knee bent to the angle of their choosing). Skaters were instructed to use their normal landing leg, while non-skaters selected the leg they would naturally use to kick a ball. Each participant was asked to practice the drop landing three times to diminish any learning effect that might occur (Kovacs et al., 2004; Seegmiller & McCaw, 2003). Upon landing, participants were asked to remain as motionless as possible until 15 s of data acquisition were complete. A trial was considered successful if the supporting foot remained in contact with the plate and the non-supporting foot made no contact with the plate for the entire trial duration. An average of the three successful trials was taken for data analysis. Due to a technical issue, the final second of data for all trials was discarded. Instruments and data analysis To assess the vertical GRF of each participant we utilised a Bertec 4060–10 force plate (Bertec Corporation, Columbus, OH). Data were collected at a sampling rate of 1000 Hz utilising Vicon Nexus software (Oxford Metrics, Oxford, UK). All data processing and analyses were performed with custom made Matlab programs (Matlab R2009a, Natick, MA, USA). Spectral analysis identified most of the power of the signal below 10 Hz. We then applied a low-pass filter at 10 Hz with a fourth-order and zerolag Butterworth filter (Ross & Guskiewicz, 2003; Ross, Guskiewicz, Gross, & Yu, 2009). The normalised peak vertical GRF was calculated by taking an average of the greatest GRF magnitude for three successful trials and dividing by participant body weight. Time to peak GRF was simply the time
difference between the first non-zero vertical GRF and the peak vertical GRF. We adapted our calculation of TTS from an earlier study (Colby, Hintermeister, Torry, & Steadman, 1999). Briefly, an average of the vertical GRF for the entire trial duration (14 s) was taken. TTS was identified as the point where the sequential average of the respective GRF remained within ±0.25 standard deviations of the overall 14 s mean (Figure 1). Impulse was calculated as the area under the vertical GRF curve (body weight × s). To further describe differences between skater and non-skater landings we also calculated the centre of pressure (CoP) range for the time period between initial foot contact with the plate and the peak vertical GRF. CoP range was calculated as the sum of the absolute maximum and minimum excursions of the CoP for the M–L and A–P axes. Statistical analyses Statistical analyses were performed by STATA (Version 11, 2009; StataCorp LP, College Station, Texas). Having met the assumptions of normal distribution and equal variance, between-group comparisons of normalised peak vertical GRF, time to peak GRF, TTS, impulse, and CoP range (M–L and A–P) were determined with two-tailed two-sample t-tests. The significance level was set a priori at alpha = 0.05. Results While the loading rate (rate of increase in the vertical GRF) was similar for both groups during the first 50 ms of the landing (Figure 2), compared with nonskaters, figure skaters exhibited a significantly greater peak vertical GRF (3.50 ± 0.47 × body weight for skaters vs. 3.13 ± 0.45 × body weight for non-skaters, Figure 3A). The expected inverse relationship between peak GRF and time to peak GRF did exist for both
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Figure 2. Vertical GRF for all participants. Vertical GRF was averaged across all participants from each group to display the average group GRF position over time. Solid line indicates skaters. Dashed line indicates non-skaters. Shaded regions represent two standard deviations from the mean.
groups, with skaters having significantly shorter time to peak GRF (81.21 ± 14.01 ms for skaters vs. 93.81 ± 16.49 ms for non-skaters, Figure 3B). Figure 3C shows the vertical TTS for skaters and non-skaters. Skaters did exhibit marginally but significantly longer vertical TTS (2.38 ± 0.07 s) compared with non-skaters (2.22 ± 0.07 s). The impulse (Figure 3D) was similar for both skaters and non-skaters. Figure 4 shows the average horizontal sway path of the CoP for each group for the time period from initial contact with the plate until the average time to peak GRF for each group, respectively. In order to quantify differences between the sway path of the CoP of skaters and non-skaters we calculated CoP range for the time period from initial contact with the plate until the average time to peak GRF (Figure 5). Compared with non-skaters, skaters exhibited a significantly smaller M–L CoP range (26.43 ± 6.49 mm for skaters vs. 35.43 ± 13.16 mm for non-skaters) and A–P CoP range (109.97 ± 15.51 mm for skaters vs. 183.07 ± 51.56 mm for non-skaters).
Discussion Foot and ankle acute and overuse injuries are becoming more common among freestyle figure skaters, and repeated microtrauma to the foot and ankle from excessively forceful landings is likely to play a significant role (Pelham, Holt, & Stalker, 2001). Landing impact forces have been shown to increase with an increase in the number of jump revolutions (Lockwood & Gervais, 1997). A biomechanically poor skate boot design, one that limits the ankle range of motion about the M–L axis via stiff ankle support and a raised heel, is also thought to contribute to greater landing impact forces (Anderson, Weber, Steinbach, & Ballmer, 2004; Bradley, 2006; Brown, Varney, & Micheli, 2000; Bruening & Richards, 2006; Ehrenman, 2004; Haguenauer, Legreneur, & Monteil, 2006; Pelham et al., 2001; Smith, 2000; Smith & Ludington, 1989). The purpose of the present study was to identify unique landing mechanics in freestyle figure skaters that may further contribute to the excessively forceful landings. We did this by comparing the vertical
Figure 3. (A) Peak vertical GRF normalised to body weight (BW), (B) time to peak vertical GRF, (C) TTS, and (D) Impulse. Error bars represent one standard deviation from the mean. * indicates a significant difference between skaters and non-skaters (P < 0.05).
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Peak GRF
Figure 4. CoP sway path representing the time period between initial contact and peak GRF. Horizontal CoP (x, y) coordinates were averaged across all participants from each group to display the average group CoP position over time. The CoP was calculated based on the force plate local coordinate system and we did not use an absolute reference frame to normalise for foot location. The average CoP sway path for each group is superimposed over a force plate with the dimensions 400 × 600 mm. The CoP for all participants began in the forefoot and travelled posteriorly towards the heel. Solid line indicates skaters. Dotted line indicates nonskaters.
Figure 5. M–L and A–P CoP range representing the time period between initial contact and peak GRF. Error bars represent one standard deviation from the mean. * indicates a significant difference between skaters and non-skaters (P < 0.05).
Our first hypothesis was supported as compared with non-skaters, figure skaters exhibited significantly greater peak vertical GRF, even though this was a customary task for the skaters, but quite novel for the non-skaters. This result is in agreement with others (Seegmiller & McCaw, 2003) who studied gymnasts and recreational athletes dropping from heights of 30, 60, and 90 cm. Compared with recreational athletes, gymnasts experienced 21%, 33%, and 33% greater vertical GRF for the 30, 60, and 90 cm heights, respectively. Largely due to a small sample size (10 participants per group) the group difference for the 30 cm height was not significant. They suggested the lower height was not sufficiently challenging to discriminate between the two groups. In the present study, a 20 cm height was capable of discriminating between skaters and non-skaters, with skater exhibiting an 11% greater GRF. However, our participants were required to drop backwards and land on one foot, while the gymnasts and recreational athletes performed a forward drop with a two-foot landing. The increased degree of difficulty appears to have compensated for a lower platform height. In fact, the 20 cm height in the present study elicited similar peak GRF for skaters (3.5 × body weight) to that of the 60 cm height for gymnasts (4.1 × body weight). Nevertheless, based on the results of Seegmiller and McCaw (2003) we would expect the disparity between skater and nonskater GRF to increase with an increased platform height. We have also identified some important relationships between our findings and a study comparing normal skate boots to a newly developed skate boot with an articulated ankle that permits a greater ankle range of motion about the M–L axis (Bruening & Richards, 2006). Dropping from a 30 cm platform onto a force plate, compared with the normal skate boots, the articulated skate boots resulted in an 18% lower peak heel GRF, but no difference in peak toe GRF. Though no conclusive evidence of heel contact exists due to the backward barefoot landing protocol in the present study, the difference between skaters and non-skaters in vertical GRF was only evident after 50 ms (Figure 2), which may suggest that the difference in peak vertical GRF partially resulted from heel contact in the non-skaters (but not in the skaters) later in the landing phase. Interestingly, the magnitude of the peak GRF in the articulated boot (4.1 × body weight) was only 15% greater than the barefoot skaters in our study (3.5 × body weight), a
Landing ground reaction forces in figure skaters and non-skaters difference which could simply be the result of the platform height disparity. Though not directly measured or studied, it is important to note that there were no observable technical differences between skaters and non-skaters in how they dropped from the box. This is supported by the absence of a significant group difference in impulse (Figure 3D).
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Time to peak ground reaction force There is typically an inverse correlation between peak GRF and time to peak GRF (Seegmiller & McCaw, 2003), and our results support this relationship. While figure skaters exhibited greater peak vertical GRF than non-skaters, the time to peak GRF of the skaters was significantly shorter. Even though this was expected based on previous investigations, this is still an alarming result. A biomechanically optimal model for figure skater landings has been developed (Lockwood, Gervais, & McCreary, 2006). One of the main features of this optimal landing was the ability to dissipate the impact forces over a long period of time. Yet, the skaters in the present study were less able to dissipate these forces than their untrained counterparts. Just as the articulated boot of Bruening and Richards (2006) reduced the peak GRF, it decreased the loading rate by 31%. They argued that the improved landings were a result of greater range of motion at the ankle. However, in their on-ice comparison of regular versus the articulated boots there were no differences in landing forces or loading rates, possibly because many skaters, especially the more advanced skaters, did not make use of the greater potential range of motion in the articulated boots. Likewise, even though our participants were barefoot giving them the potential for full range of motion, the skaters, out of habit, may have resisted ankle plantarflexion and/or dorsiflexion. Time to stabilisation The use of TTS has previously demonstrated a sensitivity capable of discriminating between limbs with compromised anterior cruciate ligaments and uninjured limbs (Colby et al., 1999), and between functionally stable and unstable ankles (Ross, Guskiewicz, & Yu, 2005). In these studies, lower TTS values has been associated with better lower limb health. Using this rational we would hope for lower TTS in figure skaters compared with nonskaters. That said, we suggested skaters in the present study may take longer to stabilise the vertical GRF as they were performing a barefoot landing without the ankle support normally provided by the boot. As
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hypothesised, compared with non-skaters, the skaters had significantly greater TTS. Though not shown here, this result did not change even when we normalised TTS to peak GRF. There are several plausible explanations. First, the extra firm skating boots common among skaters may lead to muscle weakness. While not likely the case in the present study population, due to a broad range of skill levels, most of the weight bearing hours for highly competitive figure skaters is spent in a rigid skating boot, which may actually weaken and shorten ankle muscles as if they were in an immobilisation cast (Smith, 2000). Another endorsement for the muscle weakness hypothesis is that most ankle injuries to figure skaters occur during off-ice conditioning when the protective boots are absent (Porter, Young, Niedfeldt, & Gottschlich, 2007). Importantly, we did not test for muscle weakness, and therefore cannot confirm this hypothesis. Second, the greater TTS in skaters may be related to training habits rather than muscle weakness. In other words, skaters may become somewhat reliant on the extra boot support even if it is not needed. Finally, TTS is influenced by the whole-body landing mechanics, and only a comprehensive kinematic examination would be able to describe this relationship. Centre of pressure range One of the more intriguing results of this study came from the analysis of the CoP range. When standing, as a person goes into more dorsiflexion, the CoP moves anteriorly. However, when landing backward with the supporting ankle partially plantarflexed and the non-supporting leg traveling posteriorly, the CoP moves posteriorly as the ankle is dorsiflexed. Relative to non-skaters, skaters exhibited a 25% smaller M–L range and a 40% smaller A–P range during the landing phase. We suggest that this smaller range may be a function of training with a raised heel. Because skaters are unable to achieve full dorsiflexion of the ankle in their skates, they may have been resistant to complete dorsiflexion even when barefoot. If this were the case, the CoP would have been maintained towards the forefoot, thereby reducing the A–P CoP range. The CoP tracing during the landing phase (Figure 4) could support the speculation of a forefoot dominance in our skaters and may partially explain the greater peak GRF, shorter time to peak GRF, and longer TTS seen in figure skaters. Study limitations While we were able to contribute significantly to the present literature on landing GRF in figure skaters, this study was limited in a number of ways. First, the
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participants were diverse both in age and skating experience, and thus few participants represented each skill level and age group. Second, no electromyography or muscle strength data were collected. This prevented us from testing several of the hypothetical explanations regarding TTS. Third, the lack of motion capture data prevented us from analysing whole-body kinematics, which would have certainly contributed to a better explanation of the greater peak GRF, shorter time to peak GRF, and longer TTS seen in these skaters. Finally, and possibly most significant, the inclusion of non-skaters necessitated a laboratory environment for our tests. Laboratory tests often do not translate well to a natural environment (Bruening & Richards, 2006). We recommend that these results be used cautiously.
Conclusion We are the first to report on differences between skaters and non-skaters in landing GRF and CoP placement. In agreement with our hypotheses, compared with non-skaters, figure skaters exhibited significantly greater peak GRF, significantly shorter time to peak GRF, and significantly longer TTS for the same barefoot drop landing. Skaters also confined their CoP to a significantly smaller M–L and A–P range, with the position of the CoP located in the mid to forefoot region. The narrower and more forward position of the CoP in skaters may at least partially explain the greater peak GRF. Considering the possible implications of excessive landing forces on chronic and acute injuries in figure skaters, continued work in this area is warranted. Specifically, it is important to evaluate why skaters maintain a narrower and more forward CoP position. Training and/or equipment modification serve as potential targets to decrease peak GRF by distributing the forces over a longer time period. More comprehensive studies including electromyography and motion capture are needed to fully characterise the unique figure skater landing strategy. References Anderson, S. E., Weber, M., Steinbach, L. S., & Ballmer, F. T. (2004). Shoe rim and shoe buckle pseudotumor of the ankle in elite and professional figure skaters and snowboarders: MR imaging findings. Skeletal Radiology, 33(6), 325–329. doi:10.1007/s00256-004-0778-6 Bradley, M. A. (2006). Prevention and treatment of foot and ankle injuries in figure skaters. Current Sports Medicine Reports, 5(5), 258–261. Retrieved from www.ncbi.nlm.nih.gov/pubmed/ 16934208 Brown, T. D., Varney, T. E., & Micheli, L. J. (2000). Malleolar bursitis in figure skaters. The American Journal of Sports
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Landing ground reaction forces in figure skaters and non-skaters
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Smith, A. D., & Ludington, R. (1989). Injuries in elite pair skaters and ice dancers. The American Journal of Sports Medicine, 17(4), 482–488. Retrieved from http://www.ncbi.nlm.nih.gov/ pubmed/2782532 USFS (Producer). (2012, 2013). The 2013 official figure skating rule book. Retrieved from: http://www.usfsa.org/content/201213%20Rulebook%20for%20eReaders.pdf
