Effect of Sex on Linear and Nonlinear Kinematic Variability during a Stop Jump

Abstract

Women are 4 to 6 times more likely to sustain a non-contact ACL injury compared to men. Sex differences in lower extremity landing mechanics are believed to be associated with this increased risk. However, no previous studies have examined sex-specific differences in the variability of lower extremity mechanics during a landing. Variability may provide unique information regarding movement control which pertains to injury risk. PURPOSE: To compare variability in hip and knee kinematics during a stop jump between men and women. METHODS: 22 male and 20 female healthy uninjured individuals completed seven bilateral stop jumps while lower extremity kinematics and kinetics were collected. Hip and knee joint 3D kinematics were calculated from initial contact to toe off of the first landing and time normalized using Visual 3D. Linear variability was quantified as the standard deviation of peak knee flexion and peak knee ab/adduction across the seven trials, then as the average standard deviation during ground contact. Joint couples were created between knee flexion and knee abduction, hip flexion and knee abduction, hip rotation and knee abduction, knee flexion and knee flexion velocity, and knee abduction and knee abduction velocity, all isolated to the landing phase. Vector coding variability and divergence of nearest neighboring trajectories was quantified for each couple and divergence was quantified for knee flexion and knee abduction angle time series. All variability outcome measures were compared between sex for the dominant limb only using independent t-tests. RESULTS: There were no sex-based differences when looking at any linear variability measures. Women had increased vector coding variability for the knee abduction/knee abduction velocity coupling (F: 36°±12°, M: 29°±7°, p=0.025). Women had increased trajectory divergence of knee abduction angles (F: 1.3°/s ±.4°/s, M: 1.0°/s ±.3°/s, p=0.009) and of the knee abduction/knee abduction velocity coupling (F: 1.4°/s±.4°/s, M: 1.1°/s ±.3°/s, p=0.013). No other sex-specific differences were observed. CONCLUSION: Women have increased knee kinematic variability during landing then men, particularly when looking at knee abduction. Linear methods of quantifying variability may be insensitive for identifying sex differences in landing variability.