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 su bsequent terminal constraint on upright stance can present subjects wi th contradictory goals. Previous findings suggested that subjects migh t adopt a strategy of limiting the peak horizontal momentum of the cen ter 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 const raint on upright stance at the termination of die task. The secondary purpose was to describe the contribution of the shank, thigh, and uppe r body (head-arm-trunk) to die peak horizontal momentum of the CM unde r each test condition. Nine healthy adults 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 th e 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 subs tantially from the natural to fast STS, but increased significantly fr om the fast STS to the fast STS+fall. These findings are consistent wi th the hypothesis that limiting peak horizontal momentum of the CM may reflect a movement control strategy related to maintaining equilibriu m 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, re mained constant across all experimental conditions, suggesting that th e motion of the upper body may be tightly controlled in STS, regardles s of the altered constraint on balance.