Effects of changing the initial horizontal location of the center of mass on the anticipatory postural adjustments and task performance associated with step initiation.

The aim of this study was to elucidate whether and how the duration and/or amplitude parameters of anticipatory postural adjustments (APAs) affected the task performance in a single step forward from differences in the initial horizontal location of CoM. Ten male subjects performed initiation of a single step forward with the right swing leg with strict regulation of the step length at three initial body positions (Sw, step initiation from a 50% position of the center of pressure (CoP) shift in the maximum lateral right side; N, step initiation from the upright position at rest; St, step initiation from a 50% position of the CoP shift in the maximum lateral left side). The duration required for stepping from the heel-off to foot-contact of the swing leg (step time) in the Sw condition was significantly shorter than of the other two conditions. The APAs durations in the Sw condition was significantly longer than in the other two conditions. In the Sw condition, the durations of the anticipatory electromyographic (EMG) activities of the tibialis anteriors (TA) of the swing leg correlated significantly with mechanical parameters (the displacement of the CoM, velocity of the CoM, and propulsive force) in the anticipatory phase, while the mean amplitudes of the anticipatory EMG activities of the TA of the swing leg did not correlate significantly with mechanical parameters in the anticipatory phase. The present results suggest that the duration parameters of the APAs associated with single step forward motion are dependent on the displacement of the CoM estimated by the initial cutaneous messages from the swing leg prior to initiation of the single step, and that the APAs may be directly involved not only in the increase in propulsive force towards the supporting leg, but also increasing the forward propulsive force at heel-off.