The effects of age and sex on biomechanical asymmetries of the lower limbs during a walk, run and side-cut task
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INTRODUCTION: The anterior cruciate ligament (ACL) is an important contributor to knee joint stability during athletic maneuvers such as a side-cut. ACL rupture is associated with short- and long-term consequences that place a heavy burden on the health care system. 70-80% of all ACL injuries are non-contact in nature and are 2-8X more prevalent in females. The concept of limb lateralization may explain differences in dynamic control between the lower limbs and may be evidence for the unconscious preference toward greater loading of one limb versus the other, thus resulting in asymmetry of lower limbs. The purpose of this study was threefold: i) to identify if asymmetry existed beyond a clinically accepted 10% threshold for peak hip and knee joint flexion and abduction moments as well as peak hip and knee flexion angles and flexion angles at initial contact (IC) in an athletic population across age and sex, as well as during walk, run, and side-cut tasks, ii) identify the proportion of each population that experienced greater than 10% (>10%) asymmetry for each of the biomechanical variables of interest, and iii) to identify if differences in asymmetry exist across age and sex to further understand if asymmetry may function as an etiological risk factor for ACL injury. METHODS: Bilateral data was collected for 122 healthy high-performance cutting sport athletes. Four populations were identified based on age and sex (pre-pubescent males/females; post-pubescent males/females). Mean peak hip and knee internal joint flexion and abduction moments and mean peak hip and knee flexion angles and flexion angles at initial contact were calculated for stance phase of over-ground walking, running and side-cut tasks. Right and left limbs were reclassified as greater or lesser to prevent obscuring absolute asymmetry. Calculated asymmetry measures were subject to a 2x2 ANOVA to detect statistically significant differences among groups. The proportions of participants experiencing >10% asymmetry were calculated for each population and differences between populations was tested using a Chi-Square Test. Confidence intervals for the proportion of subjects with >10% asymmetry between limbs were estimated based on the binomial distribution. RESULTS: The percentage of asymmetry for peak extension and peak abduction moments as well as flexion angles at instant of contact during all tasks were greater than expected for all populations. At least 27% of the total population had >10% asymmetry across all variables and across all tasks. Age effects were noted for peak hip flexion and hip flexion angles at initial contact for all tasks, peak knee flexion angle (pKFA) and knee flexion angle at initial contact (KFA_IC) during the cut task, and peak knee extension moment (pKEM) during the walking task. In all cases, pre-pubescent athletes displayed greater asymmetry than post-pubescent athletes. Main effects of sex were noted for KFA_IC during the walk task and pKEM during the running task. In both cases, males displayed a greater asymmetry than females. No interaction effects were found. Differences in proportions of participants experiencing >10% asymmetry were found for pKFA during the walk and cutting tasks. Differences in the proportion of athletes exhibiting >10% asymmetry were found for pKFA during walk and cut tasks. CONCLUSION: Findings of this study may have important implications on gait evaluations, particularly in clinical and research settings where asymmetry is used as an outcome. The high proportion of the healthy population exhibiting >10% asymmetry suggests additional research is required to determine acceptable levels of lower limb kinematic and kinetic asymmetry in a healthy population as well as for return to play criteria. High variance for each variable among groups may have been a limiting factor for identifying age and sex effects.