Reflex response and control of the human soleus and gastrocnemius muscles during walking and running at increasing velocity.

We measured the soleus and the gastrocnemius H-reflex modulation in seven subjects during walking at 4.5 km/h and during running at 8, 12 and 15 km/h. The recordings in the medial gastrocnemius were corrected for cross-talk from the soleus muscle. The gastrocnemius H-reflex was in general lower than the soleus H-reflex. In both muscles the H-reflex increased significantly from walking to running but also with increasing running speed. The peak of EMG activity increased in both muscles with increasing speed. The V-wave of both muscles was absent or rather low during walking, but it increased significantly from walking to running with increasing running speed in the soleus but not in the medial gastrocnemius. In both muscles the V-wave was highest just prior to heel strike. It is suggested that this was due to a high firing frequency of the motoneurones in this phase of the movement. It is concluded that a shift towards the faster gastrocnemius at higher running speeds on behalf of the soleus muscle did not occur. The fact that the physiological cross-sectional area of the soleus is much larger than that of the lumped gastrocnemii is most probably the reason why the soleus is important also at higher running velocities.