On the assumption of bilateral lower extremity joint moment symmetry during the sit-to-stand task

This study examined the validity of the assumption of bilateral lower extremity joint moment symmetry during the sit-to-stand motion for a group of young (n = 7) and a group of elderly (n = 7) female subjects. Two force plates and a motion analysis system were used to determine peak joint moments at the ankles, knees, and hips following liftoff from a chair. Statistically, bilateral asymmetries in peak joint moments were found at the knee joint in the young group [a right to left difference of 0.43 %BW × BH (body weight × body height)] and at the hip joint in both subject groups (differences of 0.20 %BW × BH and 1.09 %BW × BH for the young and elderly subjects, respectively). Subsequent data analysis, using an algorithm that assumed bilateral ground reaction force (GRF) symmetry, was performed to determine whether the bilateral differences were a result of kinematic or GRF asymmetry. It was concluded from these results that both the kinematic and GRF data account for the bilateral asymmetry. The results of the subsequent analysis also showed that the method which assumed bilateral GRF symmetry underestimated the peak joint moments at the ankles, knees, and hips, with the greatest difference between methods being 0.10 %BW × BH for the ankle joint. The results of this study suggest that the assumption of bilateral symmetry of lower extremity joint moments during the sit-to-stand is not valid. However, the biomechanical significance of the errors associated with assuming symmetry must also be taken into account. Studies involving the sit-to-stand task should consider the effects of asymmetry on the interpretation and application of their results.