Is the stabilization of quiet upright stance in humans driven by synchronized modulations of the activity of medial and lateral gastrocnemius muscles?

A matrix of 120 electromyogram (EMG) electrodes (8 rows and 15 columns) was used to investigate individual activation patterns of the medial (MG) and lateral gastrocnemius (LG) muscles during forward sways of the body in human quiet stance. This matrix was positioned on the right calf of eight subjects after identification of the MG and LG contours with ultrasound scanning. Gray-scale images were generated with the maxima and minima of the cross-correlation function between the envelope of each EMG signal and the body center of pressure (CoP) for individual forward sways. These images were automatically segmented to reduce the data set into representative and local values of EMG-CoP cross-correlation for each muscle. On average, modulations in EMG amplitude preceded the onset of forward sways with a variable timing, with both gastrocnemius muscles being similarly and synchronously modulated in 193 out of 236 sways. Variations in the timing of activation between muscles were less frequent, although consistent across subjects and significantly correlated with changes in the direction and velocity of body sways. Interestingly, the time shift between EMG and CoP traces sometimes varied consistently along different channels of the same column of electrodes, either in proximal-to-distal or distal-to-proximal direction. The variable EMG-CoP cross-correlation delay was not congruent with the delay expected for the propagation of surface potentials along muscle fibers. Comparison of surface EMGs with intramuscular EMGs recorded from six subjects demonstrated that surface potentials provide high spatial selectivity, thus supporting the notion of selective activation of motor units during quiet standing. Hence, the stabilization of the quiet standing posture likely relies on flexible rather than stereotyped mechanisms of control.