EMG amplitude relationships between the rat soleus and medial gastrocnemius during various motor tasks.

The amplitude and temporal interrelationships of the EMG signals from the rat soleus (Sol) and medial gastrocnemius (MG) muscles during standing, locomoting on a treadmill at various speeds and inclines and swimming were studied. Joint probability density distributions demonstrated a high level of excitation of the Sol relative to the MG during standing. With increasing treadmill speed and/or incline, there was a greater probability of an enhancement in amplitude of the MG than the Sol. Further, there was a greater probability of higher EMG amplitudes of the deep, mixed fiber type region than the superficial, homogeneously fast fiber type region of the MG as speed and incline of locomotion increased. Based on the plotting of sequential points for the joint probability density distributions of EMG amplitudes of the two synergists, the data demonstrated a much greater amplitude modulation of the Sol than the MG during the initial phases of extensor activity. These plots also showed that the Sol activity terminated earlier than that of the MG. Consequently, the recruitment and derecruitment patterns of these two synergists showed a hysteresis effect, indicating that the motor pools of these two muscles do not function as a single entity. Further, differences in the EMG amplitude relationships of these synergists during a non-weight-bearing activity, i.e. swimming, in comparison to weight-supporting treadmill locomotion suggest that there is a strong peripheral influence on the neural control mechanisms involved.