Introduction: Landing asymmetry is an injury risk factor that may have antecedents at takeoff. The goals of this study were to (1) characterize normal takeoff and landing asymmetry and (2) examine how landing asymmetry is related to takeoff asymmetry for both force and kinematic variables during a back tucked salto. Since a takeoff force asymmetry likely generates asymmetric changes in momentum, any unwanted rotation may result in corresponding alterations at landing. It was hypothesized that takeoff force asymmetry would elicit changes in takeoff and landing force, impulse, and kinematic variables, unique to each takeoff condition.
Methods: Twelve male and four female injury free competitive gymnasts participated in this study. Participants completed a short questionnaire and then were tested for their maximal knee extension strength using a handheld dynamometer. The gymnasts completed their saltos from 30 cm high platforms onto standard gymnastics mats. Takeoff and landing forces under each foot were recorded using four force platforms (AMTI, Watertown, MA) and whole body kinematics were recorded using inertial measurement units (XSens Awinda, Enschede, NL). The gymnasts completed five normal baseline trials and three trials with a 60% manipulated vertical force takeoff asymmetry to the right (60R), and three to the left (60L). Takeoff asymmetry was achieved by having the participants lean slightly, aided by visual feedback of the vertical ground reaction force. Kinetic outcome variables included takeoff and landing vertical force and impulse in the vertical, anterior/posterior (AP), and medial/lateral (ML) directions. Kinematic variables included hip and knee joint angles, ranges of motion (ROM), angular momentum vector orientation, and center of gravity (CG) height. Asymmetry was measured using a standard symmetry index (SI). One sample t-tests assessed baseline results to zero. Repeated measures correlations tested the relationship between select takeoff, flight phase, and landing variables, and separate repeated measures ANOVA’s were used to examine the effects of manipulated takeoff conditions. Significance was set at p<0.05.
Results: Due to the COVID-19 pandemic restrictions, the number of female gymnasts assessed was too small to interpret in depth and their data were not combined or compared directly with males. Male and female baseline takeoff force and impulse SI were not significantly different than zero (p>0.05). Male landing baseline vertical force and impulse results indicated significant rightward SI (p<0.05). Male takeoff force and impulses showed expected differences between all conditions (p<0.05) where 60R had a rightward SI and 60L had a leftward SI compared to baseline. Both takeoff vertical force SI and vertical impulse SI were significantly correlated with landing vertical force SI and vertical impulse SI. In females, all conditions for takeoff vertical force, vertical impulse, and AP impulse displayed a rightward SI. The baseline total vertical force at landing for males was 13.0±3.5 times body weight (BW) and 17.1±3.4 times BW for females. For males at landing, 60R had greater rightward SI than 60L for vertical force and vertical impulse. Male gymnasts’ right limbs were significantly stronger than their left (t=3.066, p=0.011). Male flight phase frontal plane angular momentum angle was significantly different between all conditions where 60R and 60L showed a right and left angle deviation from baseline (p<0.05). Frontal plane angular momentum angle was also significantly correlated with takeoff and landing vertical force SI and vertical impulse SI.
Conclusion: Takeoff vertical force asymmetry is related to both force and kinematic factors of the takeoff, flight phase, and landing of a back tucked salto. Rightward landing asymmetry was present at baseline which may indicate a right limb dominance which is supported by the extensor strength data. Landing asymmetry levels were altered based on takeoff conditions. It is possible that certain kinematic variables could help coaches identify force asymmetries at landing, but the evidence found in this study was inconclusive. The female gymnasts’ data showed greater peak forces and more prevalent rightward SIs providing motivation for further investigation into landing asymmetry in female gymnasts. Although it is possible that landing asymmetry could cause injury, it is more likely that asymmetry may exacerbate injury risk through other injury mechanisms and could be the reason no empirical evidence on incidence of asymmetry injury exists currently.