Effect of a terminal constraint on control of balance during sit-to-stand

The speed at which sit-to-stand (STS) motions are performed and the subsequent terminal constraint upon upright stance can present subjects with contradictory goals. Previous findings suggested that subjects might adopt a strategy of limiting the peak horizontal momentum of the center of mass (CM), and perhaps of body segments as well, regardless of the speed of ascent. The primary purpose of this study was to test the hypothesis that the limitation in CM momentum is related to the constraint on upright stance at the termination of the task. The secondary purpose was to describe the contribution of the shank, thigh, and upper body (head-arm-trunk) to the peak horizontal momentum of the CM under each test condition. Nine healthy adult males rose from a seated position under the following three conditions: (a) at natural speeds (natural STS); (b) as fast as possible (fast STS); and (c) as fast as possible, followed by falling forward while keeping the feet fixed and using the arms on a support bar to stop the fall (fast STS+fall). The results showed that the peak horizontal momentum of the CM did not change substantially from the natural to fast STS, but increased significantly from the fast STS to the fast STS+fall. These findings are consistent with the hypothesis that limiting peak horizontal momentum of the CM may reflect a movement control strategy related to maintaining equilibrium at the termination of the voluntary task of rising from a chair. The momentum profile of the upper body, but not of the thigh or shank, remained constant across all experimental conditions, suggesting that the motion of the upper body may be tightly controlled in STS, regardless of the altered constraint on balance.