Effects of injury proneness and task difficulty on joint kinetic variability.

PURPOSE The purpose was to investigate the effects of lower extremity overuse injury proneness and landing height on the variability of selected joint moment characteristics. METHODS Ten subjects from each of two groups (healthy and injury prone) performed 10 landings from a platform (heights: 50, 100, and 200% of maximum vertical jump). Force platform (1000 Hz) and video (200 Hz) information were collected and used to compute ankle, knee, and hip joint moment values during impact (0-100 ms post contact). Moment peak, time to peak, and impulse values were used to determine variability across 10 trials for each subject and height. MANOVAs and univariate follow-up tests were used to assess variability differences between groups and among heights. RESULTS Results revealed ankle moment variability differences (P < or = 0.05; Tukey) between groups for peak and time to peak variables at the 100% and 50% heights, respectively. The injury-prone group exhibited greater variability for the peak variable, whereas the healthy group showed greater variability for the temporal variable. Groups also differed on the impulse variables (P < or = 0.02; MANOVA), but follow-up tests failed to determine specific joints or directions of differences. Both groups exhibited tendencies for greater variability (P < or = 0.05; Tukey) with increases in height up to 100% MVJ and decreases in variability with further height increases. CONCLUSIONS Results suggest that groups differed in joint moment variability, possibly indicating a relationship between variability and overuse injuries, although the variables and directions of differences were inconsistent. For some variables, variability increased and then decreased with height increases for both groups, suggesting a range of heights within which the neuromuscular system adapted.