The purpose of the study was to investigate the effects of lower extremity overuse injury proneness and task difficulty on joint kinetic variability. Ten subjects from each of two groups (healthy control and overuse injury prone) performed 10 step-off type landings from a platform (three heights:​ 50, 100, and 200% of the subject's maximum vertical jump, MVJ). Force platform (1000 Hz) and high speed video (200 Hz) information was collected and used to compute right lower extremity joint moment (ankle, knee, and hip), tibial compressive, and knee joint shear force values during the impact phase (0-100 ms post contact). Peak force values were selected and used to compute variability across 10 trials for each subject and height. Variability values were pooled by group and height classification and were used for group and single subject statistical analyses. A mixed two-factor Repeated Measures Multivariate Analysis of Variance (RM- MANOVA;​ group design) technique was used to evaluate potential differences between subject groups and among heights (p<0.05). Twenty one-factor MANOVAs were prepared to evaluate single subject responses among heights.

Group results showed the between subject main effect difference was not significant (p < 0.05), but the landing height effect and group by height interaction were significant. Results of planned comparisons for individual variables suggested a possible group difference for the ankle variable. Planned comparisons also suggested greater joint kinetic variability for higher heights, but with differential response patterns for each subject group.

Large correlations among dependant variables precluded the single subject MANOVA evaluations in favor of the planned univariate tests (planned comparisons). Single subject planned comparisons revealed differential height responses among subjects. The hip moment appeared to be the force variable most sensitive to height changes for the healthy group (6/10 subjects) and the three moment variables were equally sensitive for the injury prone group (5/10 subjects each). In conclusion, task difficulty increases might result in greater variability in some joint force variables. Similarly, persistently healthy subjects might be more variable for some joint force characteristics than their overuse injury prone counterparts suggesting a possible connection between variability and musculoskeletal health and injury.