Sports Biomechanics, 2015 Vol. 14, No. 1, 45–56, http://dx.doi.org/10.1080/14763141.2015.1029514
Greater lower limb flexion in gymnastic landings is associated with reduced landing force: a repeated measures study
ALLANA SLATER, AMITY CAMPBELL, ANNE SMITH, & LEON STRAKER School of Physiotherapy and Exercise Science, Curtin University, Perth, Australia (Received 4 September 2014; accepted 4 March 2015)
Abstract High impact forces during gymnastic landings are thought to contribute to the high rate of injuries. Lower limb joint flexion is currently limited within gymnastic rules, yet might be an avenue for reduced force absorption. This study investigated whether lower limb flexion during three gymnastic landings was related to force. Differences between landings were also explored. Twenty-one elite women’s artistic gymnasts performed three common gymnastic techniques: drop landing (DL), front and back somersaults. Ankle, knee, and hip angles, and vertical ground reaction force [(vGRF) magnitude and time to peak], were measured using three-dimensional motion analysis and force platform. The DL had significantly smaller peak vGRF, greater time to peak vGRF and larger lower limb flexion ranges than landing from either somersault. Peak vGRF and time to peak vGRF were inversely related. Peak vGRF was significantly reduced in gymnasts who landed with greater hip flexion, and time to peak was significantly increased with increasing ankle, knee, and hip flexion. Increased range of lower limb flexion should be encouraged during gymnastic landings to increase time to peak vGRF and reduce high impact force. For this purpose, judging criteria limitations on lower limb flexion should be reconsidered.
Keywords: Lower limb injuries, injury prevention, pre-adolescent athletes, gymnastics judging criteria
Introduction Gymnastics is a popular international sport, with 50 million participants worldwide recorded in 2011 (Federation International Gymnastics, 2011) and 126,736 currently registered Australian athletes (Gymnastics Australia, 2010). In the USA, of the more than five million participants, 74.5% are women’s artistic gymnasts (USA Gymnastics, 2011). Gymnastics has one of the highest rates of injury of any adolescent girls sport (Caine et al., 2003). For example, one population of elite Australian gymnasts all reported an injury between 1996 and 1997, with 96% reporting three or more (Kolt & Kirkby, 1999). The lower limbs appear to account for the greatest number of injuries, with the ankle the most commonly injured joint (Seegmiller & McCaw, 2003). These injuries typically result in significant amounts of missed training/competition, with one study estimating that gymnasts missed an average of 29% of one competitive season due to injury (Daly, Bass, & Finch, 2001). Correspondence: Amity Campbell, School of Physiotherapy and Exercise Science, Curtin University, GPO Box U1987, Perth, WA 6845, Australia, E-mail: [email protected] q 2015 Taylor & Francis
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This high rate of injuries with significant consequences warrants further research into potential gymnastic injury mechanisms. Injuries have anecdotally been linked to training intensity and load, with landing increasingly recognised as a potentially important activity (Seegmiller & McCaw, 2003). Indeed, several studies have confirmed a link between the excessive load gymnasts must absorb during landings and the high rate of acute lower limb injuries gymnasts report (Daly et al., 2001; Kirialanis, Malliou, Beneka, & Giannakopoulos, 2003; Marshall, Covassin, Dick, Nassar, & Agel, 2007; McAuley et al., 1987; Mills, Pain, & Yeadon, 2008, 2009; Seegmiller & McCaw, 2003; Wade, Campbell, Smith, Norcott, & O’Sullivan, 2012). Further, given that landing is a judged component of all gymnastic events, it is focused on throughout training and competition. This repetition, combined with the large forces, has also been linked with the high rate of overuse lower limb injuries seen in gymnasts (Mills et al., 2008, 2009; Sands, Shultz, & Newman, 1993). While there is little argument that the large ground reaction forces that gymnasts absorb during landing are likely to be linked to high rates of injury, little research has explored potential methods of reducing this force. While one study utilised model simulation to demonstrate that it is possible for gymnasts to reduced their landing ground reaction force magnitude through modified landing strategies (Mills et al., 2009), this is yet to be confirmed in a non-simulation environment. The only study to demonstrate that increased sequential lower limb joint flexion will increase the time to peak vertical ground reaction force (vGRF) and reduce the peak vGRF magnitude, did not utilise a gymnastics population (Daly et al., 2001). Further, while increasing the time to peak vGRF will theoretically reduce the peak vGRF, one study failed to find this relationship when both gymnasts and non-gymnasts performed drop landings (DLs) (Seegmiller & McCaw, 2003). This study did, however, demonstrate that gymnasts produced 33% greater peak vGRF than other athletes when performing a drop land off a 60 cm box (Seegmiller & McCaw, 2003). The authors suggested that the higher forces were a reflection of the gymnasts assuming an observed ‘stiffer’ posture; however, they did not record lower limb kinematics to assess this hypothesis (Dufek, Schot, & Bates, 1990; Seegmiller & McCaw, 2003). ‘Stiff’ lower limb landing techniques may be due to the judging criteria, as points are currently deducted for knee and hip flexion greater than 90 degrees. A better understanding of the relationship between lower limb flexion during landing and vGRF may inform injury prevention through modification of desirable landing posture and related judging criteria (FIG Population, 2011 [cited 2011 17 September]). In summary, there is some evidence that gymnasts land with a greater vGRF due to reduced lower limb flexion (Seegmiller & McCaw, 2003); however, this relationship has not been formally clarified. Therefore, this study aimed to investigate the relationship between vGRF and lower limb flexion during gymnastic landings. More specifically, it was hypothesised that there would be a relationship between peak vGRF and lower limb flexion, time to peak vGRF and lower limb flexion, and peak vGRF and the time to peak vGRF. It was also hypothesised that there would be differences in vGRF and lower limb flexion between the DL, backsault (BS), and frontsault (FS) landings. Methods Twenty-one elite women’s artistic gymnasts, mean (standard deviation) age 13 (3) years, mass 39.8 (8.9) kg and height 148.1(10.5) cm, participated in this study. Gymnasts were recruited from a regional gymnastics club in Perth, Western Australia and the Western Australian Institute of Sport (WAIS). All were trained in excess of 20 h per week and were excluded if they had musculoskeletal injuries causing training modification for the six weeks
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prior to testing, were unable to perform all three techniques required or were declared unfit by the WAIS physiotherapist at the time of data collection. Ethical approval was granted from the Curtin University Human Research Ethics Committee (PT0134). Participants and guardians provided written informed assent/consent. Data collection occurred at the School of Physiotherapy Motion Analysis Laboratory, Curtin University, using a 10-camera Vicon MX infrared passive marker motion analysis system (Oxford Metrics, Inc., UK) and an AMTI force plate embedded in the floor of the laboratory. The cameras and force plate were sampled at 250 and 1000 Hz, respectively, to ensure adequate detection of the moment of impact. Upon arrival, participants’ mass and height were measured, followed by the application of 15 mm retro-reflective markers to their feet, legs, and pelvis to support an established cluster-based model (Besier, Sturnieks, Alderson, & Lloyd, 2003) that followed the International Society of Biomechanics recommendations (Wu et al., 2002). Subsequent to warm-up, each participant performed the three landing techniques, DL, BS, and FS, in a randomised order as previously outlined (Wade et al., 2012) and used in similar gymnastics investigations (McNitt-Gray, 1993; McNitt-Gray, Yokoi, & Millward, 1994). The DL was performed from a 1 m high box onto the force plate. Both somersault techniques were performed from an air-filled launch mat, with the BS from standing and the FS from the 1 m box to the air-filled launch mat then into the somersault. For safety, 5 cm thick gymnastics matting was secured on the force plate and the surrounding floor using high strength hook and loop tape. The matting over the force plate will have dampened the magnitude of the vGRF collection; however, it has been reported that less than 5% difference in reaction forces can be expected when mats up to 12 cm thick are secured to a force plate (McNitt-Gray, Hester, Mathiyakom, & Munkasy, 2001). Three to five trials for each participant’s technique were recorded, with trials deemed successful if landed without taking a step or falling. To avoid the consequences of fatigue, a maximum of 10 trials for each technique were attempted by each participant. For the analysis, one trial of each technique for each participant was randomly chosen, using Microsoft Excel (Microsoft Corporation, New Mexico, USA) random number generator. The data from the elite gymnasts were found to be repeatable (mean standard error within each participant’s 3 – 5 trials was: 0.4 N/kg, 1.4 ms and ranged from 2.38 to 5.98 for the lower limb angles). Data analysis Kinematic data processing was conducted using specialised Vicon motion analysis software (Nexus; Oxford Metric, Inc., UK). Trajectories containing ‘breaks’ were interpolated, with no breaks greater than 10 frames, prior to running a residual analysis to determine the optimal cut-off frequency (10 Hz) for the low pass Butterworth filtering that was subsequently performed on each trajectory, the force plate data were filtered using a low pass cut-off of 50 Hz. Finally, a customised three-dimensional mathematical model calculated lower limb kinematics (Besier et al., 2003). The data for peak vGRF were normalised for the body mass of each gymnast. A customised LabVIEW program (National Instruments, Inc., Austin, USA) was developed to output left and right ankle, knee and hip angle, as well as peak vGRF magnitude and time from ground contact to peak vGRF. The range of motion (ROM) at each joint angle from the instant of ground contact until peak vGRF (Figure 1(a)– (c)) and until the force plate output indicated that the gymnast had returned to a stationary position (i.e. the output was equivalent to their body weight) which was calculated in Excel (Microsoft Excel for Mac
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Figure 1. (a) Vertical ground reaction force (vGRF) and lower limb flexion throughout an example drop landing (DL), (b) backsault (BS), and (c) frontsault (FS). The ankle, knee, and hip ROM were output form the time from initial foot contact (indicated by the first vertical line) until the instant of the force plate indicated that body weight had returned to stability, i.e. static conditions.
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2008), and averaged across right and left limbs. A more negative ROM represents a greater magnitude of flexion movement. Statistical analysis was carried out using Stata/IC for Windows (StataCorp LP, College Station, Texas USA). All data were visually inspected for quality and normality using scatter graphs. For all tests, the statistical significance was set at p , 0.05. All data were normally distributed. Repeated measures analyses of variance with pairwise contrast tests were utilised to assess if there were any significant differences in joint flexion ROM (from contact to body weight stability), the magnitude of vGRF and time from ground contact to peak vGRF between the three techniques (DL, BS, and FS). Separate linear regression analyses, using generalised estimating equations to account for correlations across repeated task measures, were used to test the association between ankle, knee or hip joint flexion and the two dependent variables, (1) the magnitude of vGRF and (2) the time from ground contact to peak vGRF over the three techniques (DL, BS, and FS), whilst estimating and accounting for differences in the value of the dependent variable across techniques. An unstructured correlation was assumed (i.e. allowing the within-person correlation to vary by technique). Differences in the degree of association between joint flexion and the dependent variable were estimated by evaluating the Wald statistic for a joint flexion £ technique interaction term, with separate estimations made by technique if a , ¼ 0.100 for the interaction term. Models were evaluated for linearity, homogeneity of residual variance and absence of influential outliers. In addition, for significant associations, correlation coefficients were also calculated from linear regression models adjusted for technique where necessary. Correlation magnitudes were described as very small (0.0 – 0.1), small (0.1 –0.3), moderate (0.3 –0.5), large (0.5 –0.7), very large (0.7 – 0.9), and nearly perfect relationship (0.9 –1).
Results Differences between three landing techniques The descriptive statistics by technique generally demonstrated a large number of differences between the DL and the two somersaults, with fewer differences between the two somersaults (Table I). The DL was associated with significantly smaller peak vGRF than the FS (mean difference: 5.1 N/kg, 95% Confidence Interval (CI) 4.0 2 6.0, p , 0.001) and BS
Table I. Normalised (body mass) peak vGRF magnitude, time from contact to peak vGRF, and joint ranges of motion (ROM) over time to peak vGRF, for the three gymnastic techniques.
Drop landing Backsault Frontsault
Peak vGRF (N/kg)
Time to Peak vGRF (ms)
Ankle ROM (8)
Knee ROM (8)
Hip ROM (8)
Mean (SD) min, max
Mean (SD) min, max
Mean (SD) min, max
Mean (SD) min, max
Mean (SD) min, max
6.8 (1.3)a (4.6, 9.9) 9.9 (1.7)b (7.1, 13.2) 11.9 (2.5)c (7.6, 15.8)
43.3 (6.9)a (27.0, 59.0) 21.8 (6.8)b (11.0, 30.0) 22.5 (4.7)b (11.0, 30.0)
-20.2 (10.4)a -30.5, -0.8 -15.7 (9.5)b -35.7, 1.3 -17.5 (8.7)ab -28.6, 2.1
-21.0 (9.4)a -34.1, -3.0 -11.1 (7.2)b -21.7, 2.2 -16.1 (9.4)b -30.1, 2.2
-19.5 (5.3)a -27.6, -8.7 -7.7 (7.4)b -21.7, 1.6) 1.2 (2.1)c -2.7, 4.4
Note: a, b, cdifferent letters represent significantly different values ( p , 0.05).
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(mean difference: -3.1, 95% CI -2.1 to -4.1, p , 0.001), and significantly larger time to peak vGRF (BS: mean difference: 21.6 ms, 95% CI: 17.6 – 25.6, p , 0.001; FS: mean difference: 20.8, 95% CI: 16.7 –24.8, p , 0.001). The DL had significantly greater ankle (mean difference: 4.58, 95% CI 0.1 –8.3, p ¼ 0.049), knee (mean difference: 9.88, 95% CI 5.1 – 14.2, p , 0.001) and hip ROM (mean difference: 11.78, 95% CI 8.7 –14.8, p , 0.001) than the BS, and knee (mean difference: 5.08, 95% CI 1.0 –9.6, p ¼ 0.016) and hip ROM (mean difference: 20.68, 95% CI 17.8 –23.5, p , 0.001) than the FS. There were only two differences between the somersaults, with the FS associated with a significantly greater peak vGRF than the BS (mean difference: 1.9 N/kg, 95% CI 0.8 – 3.0, p ¼ 0.001), and smaller hip ROM (mean difference: 8.98, 95% CI 5.7 – 12.1, p , 0.001).
Relationships between vGRF magnitude and time to peak vGRF Considering all three techniques together and adjusting for technique, the results of the linear regression analysis indicated that peak vGRF was significantly associated with time to peak vGRF (Figure 2(a)), and that there was a significant interaction between technique and time to peak vGRF, meaning that the association between peak vGRF and time to peak vGRF differed significantly across techniques (Table II). It was estimated that each one millisecond increase in time to peak vGRF was associated with a peak vGRF decrease of -1.64 N/kg (95%CI: -1.29 to -1.98) for the DL and -2.59 N/kg (95%CI: -0.50 to -3.98) for the FS, with the corresponding Pearson’s correlation coefficient being -0.929 for the DL and -0.504 for the FS. There was no evidence of an association between the two variables for the BS (Table II). Relationship between lower limb flexion and force measures Ankle range of motion. Ankle ROM was not significantly associated with peak vGRF and there was no evidence of variation in this association by technique (Table II). Ankle ROM was significantly associated with time to peak vGRF (Figure 2(b)) and it was estimated that a one degree increase in ankle flexion was associated with an increase in time to peak vGRF of 0.32 ms (95%CI: 0.15 – 0.47), corresponding to a correlation of -0.530.There was no statistical evidence that this association varied by technique. Knee range of motion. Knee ROM was not significantly associated with peak vGRF and there was no evidence of variation in this association by technique (Table II). Knee ROM was significantly associated with time to peak vGRF and there was statistical evidence that this association varied by technique (Figure 2(c)). A one degree increase in knee flexion was estimated to be associated with an increase in time to peak vGRF of 0.40 ms (95%CI: 0.03 – 0.84) for DL, 0.63 ms (95%CI: 0.38 – 0.88) for BS, and 0.13 ms (95%CI: 0.16 – 0.41) for FS, corresponding to correlation coefficients of -0.530, -0.800, and -0.247, respectively. Hip range of motion. Hip ROM was associated with peak vGRF across techniques but there was no evidence of variation in this association by technique (Figure 2(d); Table II). It was estimated that a one degree increase in hip flexion was associated with a reduction in peak vGRF of 1.04 N/kg (95%CI: 0.23 –1.84), corresponding to a correlation of 0.373.Hip ROM was also associated with time to peak vGRF across techniques, but again there was no evidence of variation in this association by technique (Figure 2(e); Table II). It was estimated that a one degree increase in hip flexion was associated with an increase in time to peak vGRF of 0.70 ms (95%CI: 0.44 – 0.97), corresponding to a correlation of 0.603 (Figure 2(e)).
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Figure 2. Scattergrams of: (a) the relationship between normalised peak vGRF and time to peak vGRF, (b) the relationship between normalised peak vGRF and ankle ROM, (c) the relationship between normalised time to peak vGRF and knee ROM, (d) the relationship between normalised time to peak vGRF and hip ROM, and (e) the relationship between normalised time to peak vGRF and hip ROM, for combined data of all three techniques.
Discussion and implications This is the first study that has investigated relationships between lower limb flexion and peak vGRF magnitude and time to peak vGRF during three different gymnastic landing techniques in female gymnasts. The results identified kinetic and kinematic differences between landings, confirmed a relationship between peak vGRF and time to peak vGRF as well as detailed the relationship between lower limb flexion and vGRF. The results also showed that the relationships varied with the measure of force and landing technique being performed.
b
a
-1.19a (21) (-1.90, -0.48) -1.64 (19) (-1.98, -1.29) -0.06 (14) (-1.32, 1.20) -2.59 (14) (-4.68, -0.50) 0.015
.924
,.001
.001
p
-0.32 (20) (-0.47, -0.15)
-0.06 (20) (-0.54, 0.42)
bb (n) (95%CI)
Ankle
,.001
.803
p
-0.36a (21) (-0.52, -0.18) -0.40 (19) (-0.84, 0.03) -0.63 (13) (-0.88, -0.38) -0.13 (14) (-0.41, 0.16)
0.06 (21) (-0.53, 0.64)
bb (n) (95%CI)
Knee
.380
,.001
.069
,.001
.848
p
Hip
-0.70 (21) (-0.97, -0.44)
1.04 (21) (0.23, 1.84)
bb (n) (95%CI)
Indicates a , ¼ 0.100 for the vGRF or time to vGRF £ technique interaction term, therefore estimates provided separately by technique. b Indicates the beta coefficient and represents the estimated change in the dependent variable for a 1-unit increase in the independent variable.
Frontsault
Backsault
Drop Landing
Time to Peak vGRF(ms) Combined
Frontsault
Backsault
Drop Landing
Peak vGRF (N/kg) Combined
bb (n) (95%CI)
Time to Peak vGRF (ms)
ROM (8)
,.001
.012
p
Table II. Associations between normalised peak vGRF magnitude, time from contact to peak vGRF, and joint ranges of motion (ROM) over time to peak vGRF, for the three gymnastic techniques.
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Differences were detected for all variables between DL, BS, and FS, which is consistent with a previous investigation in male gymnasts that reported differences in kinematics and reaction forces between these three landing techniques (McNitt-Gray et al., 2001). The results suggest that the DL may be a less injurious landing than that following a somersault, with a smaller peak vGRF and greater time to peak vGRF and greater magnitudes of lower limb flexion. The FS might present a greater risk of injuries than the BS, given its larger peak vGRF and smaller hip flexion. These differences are not all that surprising given the different acceleration that would have been required in order to perform the somersaults, in comparison to the simple drop jump. The vGRF magnitudes ranged between 5 and 13 times the body mass in the current investigation and are in accordance with previously reported values in gymnasts performing similar landings (Harringe, Renstrom, & Werner, 2007; McNitt-Gray, 1993; McNitt-Gray et al., 1994). These magnitudes are also much higher than typical vGRF magnitudes in other common female sports, such as basketball, volleyball, and running, which range between 2 and 5 times body weights (DeVita & Skelly, 1992). Given the known relationship between increased force and injury (Kirialanis et al., 2003; Kolt & Kirkby, 1999; Seegmiller & McCaw, 2003), it is of little surprise that a large number of injuries occur during gymnastic landing (Seegmiller & McCaw, 2003) and that the lower limbs account for the greatest proportion of gymnastic injuries (Caine et al., 2003; Dixon & Fricker, 1991; Kolt & Kirkby, 1999; Sands et al., 1993). Compounding this, is the notion that more dynamic landings than those recorded in this investigation, generating potentially far greater vGRF, are frequently executed by gymnasts. Thus, understanding the factors influencing forces during gymnastic landings to develop injury risk reduction strategies is a clear need. The results confirmed a relationship between peak vGRF and time to peak vGRF in the combined data after being adjusted for technique. Reduced peak vGRF was linked with a greater time to peak vGRF in accordance with general expectations, in spite of a previous investigation that failed to find this relationship in gymnasts (Seegmiller & McCaw, 2003). This implies that methods of increasing landing time will be associated with reduced peak vGRF and in turn reduced injury risk. The results detailed the relationship between the magnitude of lower limb flexion during landing and vGRF magnitude and time to peak vGRF. The magnitude of peak vGRF was only reduced with increased hip flexion, with a 18 increase in hip flexion during landing linked with a 1.04 N/kg reduction in force. In contrast, increased ROM at all three joints was demonstrated to increase the time to peak vGRF. This supports previous research suggesting that sequential lower limb joint flexion increases time for force absorption and decreases force magnitude (DeVita & Skelly, 1992). Given that knee flexion had been specifically linked with anterior cruciate ligament injuries in other populations of female athletes (Hewett et al., 2005), future intervention studies aimed at reducing vGRF and lower limb injuries in gymnasts should compare strategies focussed on increased flexion at just hip, just knee or all three lower limb joints. These findings have important implications for gymnastics worldwide. The high annual rate of lower limb injuries (52 –63%) (Kolt & Kirkby, 1999; Sands et al., 1993) and the impact of these on athletes’ ability to train and compete (Caine, Cochrane, Caine, & Zemper, 1989) suggest that risk-reduction strategies are required. Whilst this study provides the first strong evidence of the importance of lower limb flexion for force absorption during gymnastics landings, physicians, and coaches have previously understood this as a general principle (DeVita & Skelly, 1992; Dufek et al., 1990). There is also evidence linking both increased GRF and reduced knee flexion to specific lower limb injuries in other sports. For example, one prospective study demonstrated that female
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athletes who reported an anterior cruciate ligament injury landed from a vertical drop jump with 20% greater vGRF, than the athletes who did not get injured (Hewett et al., 2005). Reduced knee flexion during landing has also been linked with sustaining an anterior cruciate ligament injury in female basketball and hand ball athletes (Koga et al., 2010; Krosshaug et al., 2007). Given the high forces associated with gymnastic landings as found in this and prior studies and previously demonstrated links between reduced knee flexion and increased vGRF during landing and lower limb injuries (Harringe, Lindblad, Werner, 2004; Koga et al., 2010; Krosshaug et al., 2007; McNitt-Gray, 1993), encouraging athletes to flex their lower limbs to dissipate forces on landings is a feasible injury prevention strategy. However, this understanding has not yet translated into gymnastics judging criteria, as deductions are currently incurred for 90 degrees or more of knee or hip flexion during landing. Given that gymnasts were reported to land with a larger GRF and observed ‘stiffer’ manner than non-gymnasts even in a very simple DL where hip flexion values are known not to approach 90 degrees (Seegmiller & McCaw, 2003), it might be that deductions associated with increased flexion result in gymnasts always attempting to land as erect as possible. Encouraging landing strategies that have lower vGRF is likely to reduce the risk of injury (DeVita & Skelly, 1992), particularly to the lower limbs. Therefore, in order to encourage increased lower limb flexion during landing, the current Federation of International Gymnastics judging criteria regarding lower limb flexion should be revised to allow for more biomechanically appropriate landing techniques (FIG Population., 2011 [cited 2011 17 September]). A limitation of this study was the ecological validity of the laboratory environment. However, this was minimised by the use of standard gymnastics equipment and gymnasts being accustomed to competing and training in different environments. This investigation was limited to a population of elite female gymnasts within Western Australia. Future studies should include male gymnasts and other landing techniques. Only one trial was used to represent each gymnast’s performance of each landing technique. However, it should be noted that these were straightforward ‘warm-up’ tasks for this population, and were therefore likely highly repeatable (Wade et al., 2012). Intervention research should be conducted to assess the effects of gymnasts attempting to use different lower limb flexion strategies during landing. Conclusion In summary, the results of this investigation support that gymnastics landings are associated with large vGRFs that might explain the high rate of lower limb injuries, although future research including prospective studies is required in order to confirm this link. Gymnasts who land with increased lower limb flexion were demonstrated to have reduced peak vGRF and greater time to peak vGRF. Therefore, landing with increased flexion should be encouraged in this population and rules relating to thresholds of lower limb flexion revised. Acknowledgements The authors would like to thank all of the gymnasts and their families, Western Australian gymnastics support staff: Liz Chetkovich and Joanne Norcott, and Curtin colleagues Peter O’Sullivan, Paul Davey, and Melanie Wade, for their participation and contribution to this study.
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Conflicting interests There are no conflicts of interest for any of the authors involved in this study. References Besier, T. F., Sturnieks, D. L., Alderson, J. A., & Lloyd, D. G. (2003). Repeatability of gait data using a functional hip joint centre and a mean helical knee axis. Journal of Biomechanics, 36, 1159– 1168. doi:10.1016/S0021-9290 (03)00087-3 Caine, D., Cochrane, B., Caine, C., & Zemper, E. (1989). An epidemiologic investigation of injuries affecting young competitive female gymnasts. American Journal of Sports Medicine, 17, 811 – 820. doi:10.1177/ 036354658901700616 Caine, D., Knutzen, K., Howe, W., Keeler, L., Sheppard, L., Henrichs, D., & Fast, J. (2003). A three-year epidemiological study of injuries affecting young female gymnasts. Physical Therapy in Sport, 4, 10–23. doi:10. 1016/S1466-853X(02)00070-6 Daly, R. M., Bass, S. L., & Finch, C. F. (2001). Balancing the risk of injury to gymnasts: how effective are the counter measures? British Journal of Sports Medicine, 35, 8–19. doi:10.1136/bjsm.35.1.8 DeVita, P., & Skelly, W. A. (1992). Effect of landing stiffness on joint kinetics and energetics in the lower extremity. Medicine and Science in Sports and Exercise, 24, 108–115. Dixon, M., & Fricker, P. (1991). Injuries to elite gymnasts over 10 years. Medicine and Science in Sports and Exercise, 25, 1322–1329. Dufek, J. S., Schot, P. K., & Bates, B. T. (1990). 102 Lower extremity moments of force during landings. Medicine and Science in Sports and Exercise, 22, S17–377. doi:10.1249/00005768-199004000-00102 Federation International Gymnastics. (2011). Fig Population. Retrieved October 10, 2011, from http://www.figgymnastics.com/vsite/vcontent/page/custom/0,8510,5187-204413-221636-49055-313082-custom-item,00. html FIG Population. (2011 [cited 2011 17 September]). Federation International Gymnastics. (pp. 9–11). Gymnastics Australia. (2010). Gymnastics Australia 2010 annual report (pp. 14–15). Melbourne, VIC: Author. Harringe, M. L., Lindblad, S., & Werner, S. (2004). Do team gymnasts compete in spite of symptoms from an injury? British Journal of Sports Medicine, 38, 398–401. doi:10.1136/bjsm.2002.001990 Harringe, M. L., Renstrom, P., & Werner, S. (2007). Injury incidence, mechanism and diagnosis in top-level teamgym: a prospective study conducted over one season. Scandinavian Journal of Medicine and Science in Sports, 17, 115– 119. Hewett, T. E., Myer, G. D., Ford, K. R., Heidt, Jr, R. S., Colosimo, A. J., McLean, S. G., . . . Succop, P. (2005). Biomechanical measures of neuromuscular control and valgus loading of the knee predict anterior cruciate ligament injury risk in female athletes: a prospective study. American Journal of Sports Medicine, 33, 492–501. doi:10.1177/0363546504269591 Kirialanis, P., Malliou, P., Beneka, A., & Giannakopoulos, K. (2003). Occurrence of acute lower limb injuries in artistic gymnasts in relation to event and exercise phase. British Journal of Sports Medicine, 37, 137– 139. doi:10. 1136/bjsm.37.2.137 Koga, H., Nakamae, A., Shima, Y., Iwasa, J., Myklebust, G., Engebretsen, L., . . . Krosshaug, T. (2010). Mechanisms for noncontact anterior cruciate ligament injuries: Knee joint kinematics in 10 injury situations from female team handball and basketball. American journal of sports medicine, 38, 2218–2225. doi:10.1177/ 0363546510373570 Kolt, G. S., & Kirkby, R. J. (1999). Epidemiology of injury in elite and subelite female gymnasts: a comparison of retrospective and prospective findings. British Journal of Sports Medicine, 33, 312– 318. doi:10.1136/bjsm.33.5. 312 Krosshaug, T., Nakamae, A., Boden, B. P., Engebretsen, L., Smith, G., Slauterbeck, J. R., . . . Bahr, R. (2007). Mechanisms of anterior cruciate ligament injury in basketball: video analysis of 39 cases. American Journal of Sports Medicine, 35, 359– 367. doi:10.1177/0363546506293899 Marshall, S. W., Covassin, T., Dick, R., Nassar, L. G., & Agel, J. (2007). Descriptive epidemiology of collegiate women’s gymnastics injuries: National collegiate athletic association injury surveillance system 1988-1989 through 2003–2004. Journal of Athletic Training, 42, 234– 240. McAuley, E., Hudash, G., Shields, K., Albright, J. P., Garrick, J., Requa, R., & Wallace, R. K. (1987). Injuries in women’s gymnastics: The state of the art. American Journal of Sports Medicine, 15, 558–565. doi:10.1177/ 036354658701500607 McNitt-Gray, J. L. (1993). Kinetics of the lower extremities during drop landings from three heights. Journal of Biomechanics, 26, 1037–1046. doi:10.1016/S0021-9290(05)80003-X
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McNitt-Gray, J. L., Hester, D. M. E., Mathiyakom, W., & Munkasy, B. A. (2001). Mechanical demand and multijoint control during landing depend on orientation of the body segments relative to the reaction force. Journal of Biomechanics, 34, 1471–1482. doi:10.1016/S0021-9290(01)00110-5 McNitt-Gray, J. L., Yokoi, T., & Millward, C. (1994). Landing strategies used by gymnasts on DiMerent Surfaces. Journal of Applied Biomechanics, 10, 237–252. Mills, C., Pain, M. T., & Yeadon, M. R. (2008). The influence of simulation model complexity on the estimation of internal loading in gymnastics landings. Journal of Biomechanics, 41, 620–628. doi:10.1016/j.jbiomech.2007.10. 001 Mills, C., Pain, M. T., & Yeadon, M. R. (2009). Reducing ground reaction forces in gymnastics landings may increase internal loading. Journal of Biomechanics, 42, 671– 678. doi:10.1016/j.jbiomech.2009.01.019 Sands, W. A., Shultz, B. B., & Newman, A. P. (1993). Women’s gymnastics injuries: A 5-year study. American Journal of Sports Medicine, 21, 271–276. doi:10.1177/036354659302100218 Seegmiller, J. G., & McCaw, S. T. (2003). Ground reaction forces among gymnasts and recreational athletes in drop landings. Journal of Athletic Training, 38, 311–314. USA Gymnastics. (2011). Membership information. Retrieved September 17, 2011, from http://www.usagymnastics.org/PDFs/About%20USA%20Gymnastics/Statistics/MembershipInfo.pdf Wade, M., Campbell, A., Smith, A., Norcott, J., & O’Sullivan, P. (2012). Investigation of spinal posture signatures and ground reaction forces during landing in elite female gymnasts. Journal of Applied Biomechanics, 28, 677–686. Wu, G., Siegler, S., Allard, P., Kirtley, C., Leardini, A., Rosenbaum, D., . . . Stokes, I. (2002). ISB recommendation on definitions of joint coordinate system of various joints for the reporting of human joint motion - part I: ankle, hip and spine. Journal of Biomechanics, 35, 543–548.
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Greater lower limb flexion in gymnastic landings is associated with reduced landing force: a repeated measures study
ALLANA SLATER, AMITY CAMPBELL, ANNE SMITH, & LEON STRAKER School of Physiotherapy and Exercise Science, Curtin University, Perth, Australia (Received 4 September 2014; accepted 4 March 2015)
Abstract High impact forces during gymnastic landings are thought to contribute to the high rate of injuries. Lower limb joint flexion is currently limited within gymnastic rules, yet might be an avenue for reduced force absorption. This study investigated whether lower limb flexion during three gymnastic landings was related to force. Differences between landings were also explored. Twenty-one elite women’s artistic gymnasts performed three common gymnastic techniques: drop landing (DL), front and back somersaults. Ankle, knee, and hip angles, and vertical ground reaction force [(vGRF) magnitude and time to peak], were measured using three-dimensional motion analysis and force platform. The DL had significantly smaller peak vGRF, greater time to peak vGRF and larger lower limb flexion ranges than landing from either somersault. Peak vGRF and time to peak vGRF were inversely related. Peak vGRF was significantly reduced in gymnasts who landed with greater hip flexion, and time to peak was significantly increased with increasing ankle, knee, and hip flexion. Increased range of lower limb flexion should be encouraged during gymnastic landings to increase time to peak vGRF and reduce high impact force. For this purpose, judging criteria limitations on lower limb flexion should be reconsidered.
Keywords: Lower limb injuries, injury prevention, pre-adolescent athletes, gymnastics judging criteria
Introduction Gymnastics is a popular international sport, with 50 million participants worldwide recorded in 2011 (Federation International Gymnastics, 2011) and 126,736 currently registered Australian athletes (Gymnastics Australia, 2010). In the USA, of the more than five million participants, 74.5% are women’s artistic gymnasts (USA Gymnastics, 2011). Gymnastics has one of the highest rates of injury of any adolescent girls sport (Caine et al., 2003). For example, one population of elite Australian gymnasts all reported an injury between 1996 and 1997, with 96% reporting three or more (Kolt & Kirkby, 1999). The lower limbs appear to account for the greatest number of injuries, with the ankle the most commonly injured joint (Seegmiller & McCaw, 2003). These injuries typically result in significant amounts of missed training/competition, with one study estimating that gymnasts missed an average of 29% of one competitive season due to injury (Daly, Bass, & Finch, 2001). Correspondence: Amity Campbell, School of Physiotherapy and Exercise Science, Curtin University, GPO Box U1987, Perth, WA 6845, Australia, E-mail: [email protected] q 2015 Taylor & Francis
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This high rate of injuries with significant consequences warrants further research into potential gymnastic injury mechanisms. Injuries have anecdotally been linked to training intensity and load, with landing increasingly recognised as a potentially important activity (Seegmiller & McCaw, 2003). Indeed, several studies have confirmed a link between the excessive load gymnasts must absorb during landings and the high rate of acute lower limb injuries gymnasts report (Daly et al., 2001; Kirialanis, Malliou, Beneka, & Giannakopoulos, 2003; Marshall, Covassin, Dick, Nassar, & Agel, 2007; McAuley et al., 1987; Mills, Pain, & Yeadon, 2008, 2009; Seegmiller & McCaw, 2003; Wade, Campbell, Smith, Norcott, & O’Sullivan, 2012). Further, given that landing is a judged component of all gymnastic events, it is focused on throughout training and competition. This repetition, combined with the large forces, has also been linked with the high rate of overuse lower limb injuries seen in gymnasts (Mills et al., 2008, 2009; Sands, Shultz, & Newman, 1993). While there is little argument that the large ground reaction forces that gymnasts absorb during landing are likely to be linked to high rates of injury, little research has explored potential methods of reducing this force. While one study utilised model simulation to demonstrate that it is possible for gymnasts to reduced their landing ground reaction force magnitude through modified landing strategies (Mills et al., 2009), this is yet to be confirmed in a non-simulation environment. The only study to demonstrate that increased sequential lower limb joint flexion will increase the time to peak vertical ground reaction force (vGRF) and reduce the peak vGRF magnitude, did not utilise a gymnastics population (Daly et al., 2001). Further, while increasing the time to peak vGRF will theoretically reduce the peak vGRF, one study failed to find this relationship when both gymnasts and non-gymnasts performed drop landings (DLs) (Seegmiller & McCaw, 2003). This study did, however, demonstrate that gymnasts produced 33% greater peak vGRF than other athletes when performing a drop land off a 60 cm box (Seegmiller & McCaw, 2003). The authors suggested that the higher forces were a reflection of the gymnasts assuming an observed ‘stiffer’ posture; however, they did not record lower limb kinematics to assess this hypothesis (Dufek, Schot, & Bates, 1990; Seegmiller & McCaw, 2003). ‘Stiff’ lower limb landing techniques may be due to the judging criteria, as points are currently deducted for knee and hip flexion greater than 90 degrees. A better understanding of the relationship between lower limb flexion during landing and vGRF may inform injury prevention through modification of desirable landing posture and related judging criteria (FIG Population, 2011 [cited 2011 17 September]). In summary, there is some evidence that gymnasts land with a greater vGRF due to reduced lower limb flexion (Seegmiller & McCaw, 2003); however, this relationship has not been formally clarified. Therefore, this study aimed to investigate the relationship between vGRF and lower limb flexion during gymnastic landings. More specifically, it was hypothesised that there would be a relationship between peak vGRF and lower limb flexion, time to peak vGRF and lower limb flexion, and peak vGRF and the time to peak vGRF. It was also hypothesised that there would be differences in vGRF and lower limb flexion between the DL, backsault (BS), and frontsault (FS) landings. Methods Twenty-one elite women’s artistic gymnasts, mean (standard deviation) age 13 (3) years, mass 39.8 (8.9) kg and height 148.1(10.5) cm, participated in this study. Gymnasts were recruited from a regional gymnastics club in Perth, Western Australia and the Western Australian Institute of Sport (WAIS). All were trained in excess of 20 h per week and were excluded if they had musculoskeletal injuries causing training modification for the six weeks
Increased lower limb flexion for reduced landing force
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prior to testing, were unable to perform all three techniques required or were declared unfit by the WAIS physiotherapist at the time of data collection. Ethical approval was granted from the Curtin University Human Research Ethics Committee (PT0134). Participants and guardians provided written informed assent/consent. Data collection occurred at the School of Physiotherapy Motion Analysis Laboratory, Curtin University, using a 10-camera Vicon MX infrared passive marker motion analysis system (Oxford Metrics, Inc., UK) and an AMTI force plate embedded in the floor of the laboratory. The cameras and force plate were sampled at 250 and 1000 Hz, respectively, to ensure adequate detection of the moment of impact. Upon arrival, participants’ mass and height were measured, followed by the application of 15 mm retro-reflective markers to their feet, legs, and pelvis to support an established cluster-based model (Besier, Sturnieks, Alderson, & Lloyd, 2003) that followed the International Society of Biomechanics recommendations (Wu et al., 2002). Subsequent to warm-up, each participant performed the three landing techniques, DL, BS, and FS, in a randomised order as previously outlined (Wade et al., 2012) and used in similar gymnastics investigations (McNitt-Gray, 1993; McNitt-Gray, Yokoi, & Millward, 1994). The DL was performed from a 1 m high box onto the force plate. Both somersault techniques were performed from an air-filled launch mat, with the BS from standing and the FS from the 1 m box to the air-filled launch mat then into the somersault. For safety, 5 cm thick gymnastics matting was secured on the force plate and the surrounding floor using high strength hook and loop tape. The matting over the force plate will have dampened the magnitude of the vGRF collection; however, it has been reported that less than 5% difference in reaction forces can be expected when mats up to 12 cm thick are secured to a force plate (McNitt-Gray, Hester, Mathiyakom, & Munkasy, 2001). Three to five trials for each participant’s technique were recorded, with trials deemed successful if landed without taking a step or falling. To avoid the consequences of fatigue, a maximum of 10 trials for each technique were attempted by each participant. For the analysis, one trial of each technique for each participant was randomly chosen, using Microsoft Excel (Microsoft Corporation, New Mexico, USA) random number generator. The data from the elite gymnasts were found to be repeatable (mean standard error within each participant’s 3 – 5 trials was: 0.4 N/kg, 1.4 ms and ranged from 2.38 to 5.98 for the lower limb angles). Data analysis Kinematic data processing was conducted using specialised Vicon motion analysis software (Nexus; Oxford Metric, Inc., UK). Trajectories containing ‘breaks’ were interpolated, with no breaks greater than 10 frames, prior to running a residual analysis to determine the optimal cut-off frequency (10 Hz) for the low pass Butterworth filtering that was subsequently performed on each trajectory, the force plate data were filtered using a low pass cut-off of 50 Hz. Finally, a customised three-dimensional mathematical model calculated lower limb kinematics (Besier et al., 2003). The data for peak vGRF were normalised for the body mass of each gymnast. A customised LabVIEW program (National Instruments, Inc., Austin, USA) was developed to output left and right ankle, knee and hip angle, as well as peak vGRF magnitude and time from ground contact to peak vGRF. The range of motion (ROM) at each joint angle from the instant of ground contact until peak vGRF (Figure 1(a)– (c)) and until the force plate output indicated that the gymnast had returned to a stationary position (i.e. the output was equivalent to their body weight) which was calculated in Excel (Microsoft Excel for Mac
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Figure 1. (a) Vertical ground reaction force (vGRF) and lower limb flexion throughout an example drop landing (DL), (b) backsault (BS), and (c) frontsault (FS). The ankle, knee, and hip ROM were output form the time from initial foot contact (indicated by the first vertical line) until the instant of the force plate indicated that body weight had returned to stability, i.e. static conditions.
Increased lower limb flexion for reduced landing force
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2008), and averaged across right and left limbs. A more negative ROM represents a greater magnitude of flexion movement. Statistical analysis was carried out using Stata/IC for Windows (StataCorp LP, College Station, Texas USA). All data were visually inspected for quality and normality using scatter graphs. For all tests, the statistical significance was set at p , 0.05. All data were normally distributed. Repeated measures analyses of variance with pairwise contrast tests were utilised to assess if there were any significant differences in joint flexion ROM (from contact to body weight stability), the magnitude of vGRF and time from ground contact to peak vGRF between the three techniques (DL, BS, and FS). Separate linear regression analyses, using generalised estimating equations to account for correlations across repeated task measures, were used to test the association between ankle, knee or hip joint flexion and the two dependent variables, (1) the magnitude of vGRF and (2) the time from ground contact to peak vGRF over the three techniques (DL, BS, and FS), whilst estimating and accounting for differences in the value of the dependent variable across techniques. An unstructured correlation was assumed (i.e. allowing the within-person correlation to vary by technique). Differences in the degree of association between joint flexion and the dependent variable were estimated by evaluating the Wald statistic for a joint flexion £ technique interaction term, with separate estimations made by technique if a , ¼ 0.100 for the interaction term. Models were evaluated for linearity, homogeneity of residual variance and absence of influential outliers. In addition, for significant associations, correlation coefficients were also calculated from linear regression models adjusted for technique where necessary. Correlation magnitudes were described as very small (0.0 – 0.1), small (0.1 –0.3), moderate (0.3 –0.5), large (0.5 –0.7), very large (0.7 – 0.9), and nearly perfect relationship (0.9 –1).
Results Differences between three landing techniques The descriptive statistics by technique generally demonstrated a large number of differences between the DL and the two somersaults, with fewer differences between the two somersaults (Table I). The DL was associated with significantly smaller peak vGRF than the FS (mean difference: 5.1 N/kg, 95% Confidence Interval (CI) 4.0 2 6.0, p , 0.001) and BS
Table I. Normalised (body mass) peak vGRF magnitude, time from contact to peak vGRF, and joint ranges of motion (ROM) over time to peak vGRF, for the three gymnastic techniques.
Drop landing Backsault Frontsault
Peak vGRF (N/kg)
Time to Peak vGRF (ms)
Ankle ROM (8)
Knee ROM (8)
Hip ROM (8)
Mean (SD) min, max
Mean (SD) min, max
Mean (SD) min, max
Mean (SD) min, max
Mean (SD) min, max
6.8 (1.3)a (4.6, 9.9) 9.9 (1.7)b (7.1, 13.2) 11.9 (2.5)c (7.6, 15.8)
43.3 (6.9)a (27.0, 59.0) 21.8 (6.8)b (11.0, 30.0) 22.5 (4.7)b (11.0, 30.0)
-20.2 (10.4)a -30.5, -0.8 -15.7 (9.5)b -35.7, 1.3 -17.5 (8.7)ab -28.6, 2.1
-21.0 (9.4)a -34.1, -3.0 -11.1 (7.2)b -21.7, 2.2 -16.1 (9.4)b -30.1, 2.2
-19.5 (5.3)a -27.6, -8.7 -7.7 (7.4)b -21.7, 1.6) 1.2 (2.1)c -2.7, 4.4
Note: a, b, cdifferent letters represent significantly different values ( p , 0.05).
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(mean difference: -3.1, 95% CI -2.1 to -4.1, p , 0.001), and significantly larger time to peak vGRF (BS: mean difference: 21.6 ms, 95% CI: 17.6 – 25.6, p , 0.001; FS: mean difference: 20.8, 95% CI: 16.7 –24.8, p , 0.001). The DL had significantly greater ankle (mean difference: 4.58, 95% CI 0.1 –8.3, p ¼ 0.049), knee (mean difference: 9.88, 95% CI 5.1 – 14.2, p , 0.001) and hip ROM (mean difference: 11.78, 95% CI 8.7 –14.8, p , 0.001) than the BS, and knee (mean difference: 5.08, 95% CI 1.0 –9.6, p ¼ 0.016) and hip ROM (mean difference: 20.68, 95% CI 17.8 –23.5, p , 0.001) than the FS. There were only two differences between the somersaults, with the FS associated with a significantly greater peak vGRF than the BS (mean difference: 1.9 N/kg, 95% CI 0.8 – 3.0, p ¼ 0.001), and smaller hip ROM (mean difference: 8.98, 95% CI 5.7 – 12.1, p , 0.001).
Relationships between vGRF magnitude and time to peak vGRF Considering all three techniques together and adjusting for technique, the results of the linear regression analysis indicated that peak vGRF was significantly associated with time to peak vGRF (Figure 2(a)), and that there was a significant interaction between technique and time to peak vGRF, meaning that the association between peak vGRF and time to peak vGRF differed significantly across techniques (Table II). It was estimated that each one millisecond increase in time to peak vGRF was associated with a peak vGRF decrease of -1.64 N/kg (95%CI: -1.29 to -1.98) for the DL and -2.59 N/kg (95%CI: -0.50 to -3.98) for the FS, with the corresponding Pearson’s correlation coefficient being -0.929 for the DL and -0.504 for the FS. There was no evidence of an association between the two variables for the BS (Table II). Relationship between lower limb flexion and force measures Ankle range of motion. Ankle ROM was not significantly associated with peak vGRF and there was no evidence of variation in this association by technique (Table II). Ankle ROM was significantly associated with time to peak vGRF (Figure 2(b)) and it was estimated that a one degree increase in ankle flexion was associated with an increase in time to peak vGRF of 0.32 ms (95%CI: 0.15 – 0.47), corresponding to a correlation of -0.530.There was no statistical evidence that this association varied by technique. Knee range of motion. Knee ROM was not significantly associated with peak vGRF and there was no evidence of variation in this association by technique (Table II). Knee ROM was significantly associated with time to peak vGRF and there was statistical evidence that this association varied by technique (Figure 2(c)). A one degree increase in knee flexion was estimated to be associated with an increase in time to peak vGRF of 0.40 ms (95%CI: 0.03 – 0.84) for DL, 0.63 ms (95%CI: 0.38 – 0.88) for BS, and 0.13 ms (95%CI: 0.16 – 0.41) for FS, corresponding to correlation coefficients of -0.530, -0.800, and -0.247, respectively. Hip range of motion. Hip ROM was associated with peak vGRF across techniques but there was no evidence of variation in this association by technique (Figure 2(d); Table II). It was estimated that a one degree increase in hip flexion was associated with a reduction in peak vGRF of 1.04 N/kg (95%CI: 0.23 –1.84), corresponding to a correlation of 0.373.Hip ROM was also associated with time to peak vGRF across techniques, but again there was no evidence of variation in this association by technique (Figure 2(e); Table II). It was estimated that a one degree increase in hip flexion was associated with an increase in time to peak vGRF of 0.70 ms (95%CI: 0.44 – 0.97), corresponding to a correlation of 0.603 (Figure 2(e)).
Increased lower limb flexion for reduced landing force
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Figure 2. Scattergrams of: (a) the relationship between normalised peak vGRF and time to peak vGRF, (b) the relationship between normalised peak vGRF and ankle ROM, (c) the relationship between normalised time to peak vGRF and knee ROM, (d) the relationship between normalised time to peak vGRF and hip ROM, and (e) the relationship between normalised time to peak vGRF and hip ROM, for combined data of all three techniques.
Discussion and implications This is the first study that has investigated relationships between lower limb flexion and peak vGRF magnitude and time to peak vGRF during three different gymnastic landing techniques in female gymnasts. The results identified kinetic and kinematic differences between landings, confirmed a relationship between peak vGRF and time to peak vGRF as well as detailed the relationship between lower limb flexion and vGRF. The results also showed that the relationships varied with the measure of force and landing technique being performed.
b
a
-1.19a (21) (-1.90, -0.48) -1.64 (19) (-1.98, -1.29) -0.06 (14) (-1.32, 1.20) -2.59 (14) (-4.68, -0.50) 0.015
.924
,.001
.001
p
-0.32 (20) (-0.47, -0.15)
-0.06 (20) (-0.54, 0.42)
bb (n) (95%CI)
Ankle
,.001
.803
p
-0.36a (21) (-0.52, -0.18) -0.40 (19) (-0.84, 0.03) -0.63 (13) (-0.88, -0.38) -0.13 (14) (-0.41, 0.16)
0.06 (21) (-0.53, 0.64)
bb (n) (95%CI)
Knee
.380
,.001
.069
,.001
.848
p
Hip
-0.70 (21) (-0.97, -0.44)
1.04 (21) (0.23, 1.84)
bb (n) (95%CI)
Indicates a , ¼ 0.100 for the vGRF or time to vGRF £ technique interaction term, therefore estimates provided separately by technique. b Indicates the beta coefficient and represents the estimated change in the dependent variable for a 1-unit increase in the independent variable.
Frontsault
Backsault
Drop Landing
Time to Peak vGRF(ms) Combined
Frontsault
Backsault
Drop Landing
Peak vGRF (N/kg) Combined
bb (n) (95%CI)
Time to Peak vGRF (ms)
ROM (8)
,.001
.012
p
Table II. Associations between normalised peak vGRF magnitude, time from contact to peak vGRF, and joint ranges of motion (ROM) over time to peak vGRF, for the three gymnastic techniques.
52 A. Slater et al.
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Differences were detected for all variables between DL, BS, and FS, which is consistent with a previous investigation in male gymnasts that reported differences in kinematics and reaction forces between these three landing techniques (McNitt-Gray et al., 2001). The results suggest that the DL may be a less injurious landing than that following a somersault, with a smaller peak vGRF and greater time to peak vGRF and greater magnitudes of lower limb flexion. The FS might present a greater risk of injuries than the BS, given its larger peak vGRF and smaller hip flexion. These differences are not all that surprising given the different acceleration that would have been required in order to perform the somersaults, in comparison to the simple drop jump. The vGRF magnitudes ranged between 5 and 13 times the body mass in the current investigation and are in accordance with previously reported values in gymnasts performing similar landings (Harringe, Renstrom, & Werner, 2007; McNitt-Gray, 1993; McNitt-Gray et al., 1994). These magnitudes are also much higher than typical vGRF magnitudes in other common female sports, such as basketball, volleyball, and running, which range between 2 and 5 times body weights (DeVita & Skelly, 1992). Given the known relationship between increased force and injury (Kirialanis et al., 2003; Kolt & Kirkby, 1999; Seegmiller & McCaw, 2003), it is of little surprise that a large number of injuries occur during gymnastic landing (Seegmiller & McCaw, 2003) and that the lower limbs account for the greatest proportion of gymnastic injuries (Caine et al., 2003; Dixon & Fricker, 1991; Kolt & Kirkby, 1999; Sands et al., 1993). Compounding this, is the notion that more dynamic landings than those recorded in this investigation, generating potentially far greater vGRF, are frequently executed by gymnasts. Thus, understanding the factors influencing forces during gymnastic landings to develop injury risk reduction strategies is a clear need. The results confirmed a relationship between peak vGRF and time to peak vGRF in the combined data after being adjusted for technique. Reduced peak vGRF was linked with a greater time to peak vGRF in accordance with general expectations, in spite of a previous investigation that failed to find this relationship in gymnasts (Seegmiller & McCaw, 2003). This implies that methods of increasing landing time will be associated with reduced peak vGRF and in turn reduced injury risk. The results detailed the relationship between the magnitude of lower limb flexion during landing and vGRF magnitude and time to peak vGRF. The magnitude of peak vGRF was only reduced with increased hip flexion, with a 18 increase in hip flexion during landing linked with a 1.04 N/kg reduction in force. In contrast, increased ROM at all three joints was demonstrated to increase the time to peak vGRF. This supports previous research suggesting that sequential lower limb joint flexion increases time for force absorption and decreases force magnitude (DeVita & Skelly, 1992). Given that knee flexion had been specifically linked with anterior cruciate ligament injuries in other populations of female athletes (Hewett et al., 2005), future intervention studies aimed at reducing vGRF and lower limb injuries in gymnasts should compare strategies focussed on increased flexion at just hip, just knee or all three lower limb joints. These findings have important implications for gymnastics worldwide. The high annual rate of lower limb injuries (52 –63%) (Kolt & Kirkby, 1999; Sands et al., 1993) and the impact of these on athletes’ ability to train and compete (Caine, Cochrane, Caine, & Zemper, 1989) suggest that risk-reduction strategies are required. Whilst this study provides the first strong evidence of the importance of lower limb flexion for force absorption during gymnastics landings, physicians, and coaches have previously understood this as a general principle (DeVita & Skelly, 1992; Dufek et al., 1990). There is also evidence linking both increased GRF and reduced knee flexion to specific lower limb injuries in other sports. For example, one prospective study demonstrated that female
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athletes who reported an anterior cruciate ligament injury landed from a vertical drop jump with 20% greater vGRF, than the athletes who did not get injured (Hewett et al., 2005). Reduced knee flexion during landing has also been linked with sustaining an anterior cruciate ligament injury in female basketball and hand ball athletes (Koga et al., 2010; Krosshaug et al., 2007). Given the high forces associated with gymnastic landings as found in this and prior studies and previously demonstrated links between reduced knee flexion and increased vGRF during landing and lower limb injuries (Harringe, Lindblad, Werner, 2004; Koga et al., 2010; Krosshaug et al., 2007; McNitt-Gray, 1993), encouraging athletes to flex their lower limbs to dissipate forces on landings is a feasible injury prevention strategy. However, this understanding has not yet translated into gymnastics judging criteria, as deductions are currently incurred for 90 degrees or more of knee or hip flexion during landing. Given that gymnasts were reported to land with a larger GRF and observed ‘stiffer’ manner than non-gymnasts even in a very simple DL where hip flexion values are known not to approach 90 degrees (Seegmiller & McCaw, 2003), it might be that deductions associated with increased flexion result in gymnasts always attempting to land as erect as possible. Encouraging landing strategies that have lower vGRF is likely to reduce the risk of injury (DeVita & Skelly, 1992), particularly to the lower limbs. Therefore, in order to encourage increased lower limb flexion during landing, the current Federation of International Gymnastics judging criteria regarding lower limb flexion should be revised to allow for more biomechanically appropriate landing techniques (FIG Population., 2011 [cited 2011 17 September]). A limitation of this study was the ecological validity of the laboratory environment. However, this was minimised by the use of standard gymnastics equipment and gymnasts being accustomed to competing and training in different environments. This investigation was limited to a population of elite female gymnasts within Western Australia. Future studies should include male gymnasts and other landing techniques. Only one trial was used to represent each gymnast’s performance of each landing technique. However, it should be noted that these were straightforward ‘warm-up’ tasks for this population, and were therefore likely highly repeatable (Wade et al., 2012). Intervention research should be conducted to assess the effects of gymnasts attempting to use different lower limb flexion strategies during landing. Conclusion In summary, the results of this investigation support that gymnastics landings are associated with large vGRFs that might explain the high rate of lower limb injuries, although future research including prospective studies is required in order to confirm this link. Gymnasts who land with increased lower limb flexion were demonstrated to have reduced peak vGRF and greater time to peak vGRF. Therefore, landing with increased flexion should be encouraged in this population and rules relating to thresholds of lower limb flexion revised. Acknowledgements The authors would like to thank all of the gymnasts and their families, Western Australian gymnastics support staff: Liz Chetkovich and Joanne Norcott, and Curtin colleagues Peter O’Sullivan, Paul Davey, and Melanie Wade, for their participation and contribution to this study.
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