The dynamic properties of the cat soleus muscle were studied in freely walking animal preparations. The force and EMG responses of the soleus following supramaximal, ins tants of the step cycle. The sensorimotor interactions of soleus with the medial head of the gastrocnemius (a functional agonist of the soleus at the ankle) and the tibialis anterior (a functional antagonist of soleus at the ankle) were studied by measuring their force and EMG responses following the artifical stimulation of the soleus nerve. Supramaximal nerve stimulation showed distinct increases in the soleus forces during the entire swing phase and the second part (after peak forces had been reached) of the stance phase. Soleus forces could only be increased slightly in the first part of stance (from paw contact to peak force). These results suggest that force production of the soleus is virtually maximal during the early phases of stance but is submaximal for the remainder of the step cycle. Forces and EMGs of the medial gastrocnemius muscle were affected by the soleus nerve stimulation only in the latter part of the swing phase. In these cases, the force and EMG of the medial gastrocnemius were reduced significantly for the step cycle following the perturbation. The active force production of soleus during late swing causes an inhibition of medial gastrocnemius activity and force. Forces and EMGs of the tibialis anterior muscle were always affected by the soleus nerve stimulation during the swing phase of the step cycle. In these case, the force EMG of the medial gastrocnemius were reduced significantly for the step cycle following the perturbation. The active force production of soleus during late swing causes an inhibition of medial gastrocnemius activity and force. Forces and EMGs of the tibialis anterior muscle were always affected by the soleus nerve stimulation during the swing phase of the step cycle. In these instances, forces and EMGs of the tibialis anterior were significantly increased compared to step cycles preceding or following the perturbation. Part of the force enhancement is caused by the stretch of the activated tibialis anterior by the soleus, and part of the enhancement is caused by reflex activation. No effects on forces or EMGs of the tibialis anterior were observed when the soleus nerve stimulation showed its effects during the stance phase of the step cycle. The results of theis study suggest that the magnitude and the quality of ensorimotor interactions of soleus with medial gastrocnemius and tibialis anterior depend on the phase of the step cycle. The strongest interactions appear to exist during the swing phase; no observable interactions were found during stance.
Mechanism for reflex reversal during walking in human tibialis anterior muscle revealed by single motor unit recording.