EFFECTS OF REDUCING PLANTAR SUPPORT ON ANTICIPATORY POSTURAL AND INTENTIONAL ACTIVITIES ASSOCIATED WITH FLEXION OF THE LOWER-LIMB

This study examines the anticipatory dynamics of the center of gravity and the EMG activity of ipsi- and contralateral (to the moving leg) soleus (SOL) and tibialis anterior (TA) in flexion of the lower limb executed from different plantar support surfaces. Subjects were standing initially on a plane surface or a surface reduced to two narrow rods under the moving or stance foot. Reduction of the plantar support surface under the moving limb induced a decrease in the anticipatory activity of both TA and in the intentional ipsilateral SOL EMG activity, but the anticipatory activity of SOL did not change. The lateral and vertical accelerations of the center of gravity decreased. When the reduction was under the stance limb, the biomechanical traces did not change markedly and the EMG activity was similar to that of the control situation. The differential effects in EMG activity, depending on whether the reduced plantar surface was under the moving or the stance side, suggest that tonic plantar stimulation induces complex reflex actions on muscles involved in the preparation and execution of voluntary movement. The functional demand could explain the gating from one motor organization to another.