The analysis of lower extremity function during running has predominately been conducted on a unilateral basis. Symmetrical limb function is either implied or specifically stated, although the research presents equivocal results. The purpose of the study was to evaluate within-subject lower extremity symmetry during the support phase of running. The interday reliability of the right-left measurements was evaluated by comparing data from separate days.

Five volunteer subjects were tested at two sessions. At each session, subjects were filmed (100 Hz) while running (training pace + 15 s/km) across a force platform (500 Hz). Fifteen trials were collected for each leg. Leg testing order was counterbalanced across subjects and across days. Functional kinetics of the lower extremity were calculated using an inverse dynamics analysis technique.

Data processing consisted of the evaluation of within day/leg condition variability and the assessment of bilateral symmetry for the ground reaction forces and the joint positions, moments of force and powers. Symmetry was defined as the absence of measurable bilateral differences based upon the observed within day/leg variability. The correlation coefficient between paired curves was computed as a measure of temporal similarity. Magnitude similarity was assessed using the coefficient of variation and mean standard deviation, and by evaluating 15-trial mean differences.

All measures for each subject indicated stable within day/leg condition results. Less variability was present in the GRF and joint position data than in the joint moment and joint power data. The joint moment and power data exhibited increased variability from the ankle to the hip joint.

Bilateral temporal symmetry was present for the GRF and joint position data but not the joint moment and power data. Magnitude asymmetries were identified for all measures. There was no evidence in the data of any subject to suggest that one leg was consistently exposed to a greater level of stress. Interday variability in the direction of the asymmetries was especially apparent among the joint moment and joint power data.

The results indicate that asymmetry is present in lower extremity function during the support phase of running. Further research is necessary to determine the source of the observed asymmetry.