Abstract:

The behaviour of linked body segments during sit-to-stand was the subject of this study which investigated the relationship between the trunk and lower limb segments by varying the initial position of the trunk. Six subjects were videotaped as they stood up with feet on a forceplate from three initial positions: erect sitting, trunk flexed forward 30 deg, and 60 deg. When subjects actively flexed the trunk in the pre-extension phase, the order in which lower limb joints extended was knee, hip, ankle. However, when there was no active flexion, the order of onsets changed, the hip extending first followed by the knee and ankle. An extensor support moment (SM), a summation of extensor moments at hip, knee and ankle, occurred throughout the extension phase. The mean peak value of SM remained invariant in all three conditions despite variability in individual hip, knee and ankle moments. When active trunk flexion was absent, the duration of the extension phase was longer and a high value of SM was sustained for a longer proportion of the phase, indicating that more muscle force was required. The findings support the view that biomechanical characteristics emerge naturally from a functional coupling between segments, according to the demands of the action.