A common assumption, either implicit or explicit, in biomechanical research is that bilateral lower extremity function is symmetrical. Symmetry investigations have, however, produced equivocal results based primarily upon cyclic diagonal activities. The purpose of this study was to assess bilateral lower extremity functional symmetry during the impact phase of vertical drop landings. Vertical ground reaction forces, joint kinematics and joint kinetics of the left and right legs were simultaneously recorded during each landing performance and bilateral comparisons were made for each trial.

Ten volunteer subjects (5 male, 5 female) were tested on three consecutive days. Each subject performed 25 vertical drop landings per test session from a height of 60 cm during which bilateral performance measures were collected (75 trials/subject). Landings occurred on dual force platforms (1000 Hz) which allowed for the acquisition of simultaneous independent left/right ground reaction force records. A dual camera video system (200 Hz) was employed to record concurrent left/right sagittal plane motion.

Bilateral comparisons of the vertical ground reaction force, joint kinematic (joint position and velocity) and joint kinetic (joint moment and power) records were made for the impact phase of landing. Algebraic left/right comparisons of selected magnitude and temporal parameters were calculated for each performance. Two criteria were applied to classify discrete performance measures as symmetrical or asymmetrical:​ 1) the left/right mean differences had to be statistically different from the theoretical symmetry value of zero and 2) the percentage of left-right directional differences had to be equal to or greater than 80%. Temporal and magnitude similarities of the parameter time histories were also evaluated.

The results indicated that some individuals exhibited bilateral functional asymmetry during the apparently synchronized lower extremity motions during the impact phase of drop landings. Bilateral functional relationships varied widely across individuals and parameter types. Selected asymmetries were demonstrated by all subjects, individually ranging from 18 to 58% for all parameter comparisons indicating considerable bilateral differences. The proportion of asymmetries identified within each data type was 47, 41 and 21% for vertical ground reaction force, joint kinematic and joint kinetic variables, respectively.

Evaluation of the modulation of asymmetry over blocks of trials and across test sessions indicated that the individual asymmetrical or symmetrical bilateral functional relationships were consistent.