Muscular synergies during different conditions of postural activity

With the view to studying the muscular synergies in maintaining of upright posture EMG activity of soleus (SOL), tibialis anterior (TA), peroneus longus (PL), biceps femoris (BF), vastus lateralis (VL) and gluteus medius (GM) muscles during easy standing, voluntary leaning and induced body oscillations in frontal and sagital plane were recorded. The data showed that during easy standing and during small external disturbances the vertical posture was maintained mainly by the activity of SOL (triceps surae muscle) which was modulated by the modification of the external force field. TA remained silent. PL muscle was slightly active as a synergist of SOL. The pattern of muscular synergies during fast voluntary leaning and induced body oscillations differed as compared to the muscular synergies at easy standing. The activity of all muscles studied increased and became continuous with exception of that of the TA. The latter was silent but was involved earlier when gravity center passed behind its projection. VL being a synergist of TA during slow sway became continuously active as a synergist of SOL during the fast sway. Typical changes in muscular synergies appeared when induced body oscillations were imposed during horizontal and rotational movements of the platform. The various patterns of muscular synergies during pronation and supination of the foot depended on the knee joint angle. The amplitude of body leaning forward and backward and its variability as well as the plane of the lateral sway depended on the biomechanical variables and related motor programs for postural control.