Weight distribution influences the time required to lift the leg even under normal standing condition

The purpose of the present study is to demonstrate whether the time taken to initiate a lift-one-leg task from a quiet standing position is determined by the initial weight distribution (WD), irrespective of whether the initial posture is controlled spontaneously or by using visual feedback. Seven healthy young subjects performed a lift-one-leg task in response to visual stimulation provided by an LED. The experiment included the normal (N) condition in which the participants stand on both legs symmetrically, as subjectively estimated, and the three feedback (FL, FC and FR) conditions in which the participants regulate the WD at −20% (left-weighted by 6:4), 0% (center-weighted by 5:5) and 20% (right-weighted by 4:6), respectively, through the use of visual feedback. In the N condition, the inter-trial variation of WD was larger than that of the FC condition, which was enough to significantly correlate with the time required to lift the leg. The slope, intercept, and standard error of estimation of the regression equation between the WD and the duration of anticipatory postural adjustment were consistent between conditions. The quantification of the effect of WD on anticipatory postural adjustment indicated that the time taken to lift the leg was delayed by 2.81 ms as the center of mass shifted toward the lifting leg by 1 mm. Our results demonstrate that asymmetric loading is a simple biomechanical factor that determines the time needed to initiate a subsequent lift-one-leg task.