Soleus forces and soleus force potential during unrestrained cat locomotion

Abstract

It has been suggested that the soleus muscle in the cat is maximally activated during slow walking (e.g. Walmsley et al., Am. J. Physiol. 41, 1203-1216, 1978); however, this opinion is not shared throughout the scientific community (e.g. Hodgson, J. Physiol. 337, 553-562, 1983). Whether or not the soleus is maximally activated for low-demand movements is critical for understanding the interaction and force-sharing of the soleus with the remaining ankle extensor muscles. The purpose of this study was to test experimentally whether the soleus muscle was maximally activated during low-demand, unrestrained movements. In order to achieve this purpose, the soleus was stimulated supramaximally at different instants of the step cycle during walking using a chronically implanted nerve cuff electrode. Forces of the soleus during normal step cycles and during step cycles following supramaximal nerve stimulation were measured using a standard E-shaped tendon force transducer. For slow (0.4 m s-1) and intermediate (0.8 m s-1) speeds of walking, soleus forces could be substantially increased through supramaximal nerve stimulation during the swing and mid-stance phases of the step cycle; for fast walking (1.2 m s-1), such increases in force could be produced only during the swing phase. For all speeds of walking, the soleus forces could not be increased in the early (i.e. from paw contact to about peak force occurrence) or the late (i.e. the last 50-100 ms) parts of the stance phase. Specifically, peak forces and the rate of increase in force immediately following paw contact could not be increased substantially through supramaximal nerve stimulations at the appropriate instant in time for any of the conditions tested here. The results of this study indicate that force production of the soleus is maximal for fast walking (1.2 m s-1) throughout the stance phase, and is maximal for large portions of the stance phase for walking at slow (0.4 m s-1) and intermediate (0.8 m s-1) speeds. These findings have direct implications for explaining the possible mechanism of force-sharing among the cat ankle extensor muscles, and the in vivo mechanical properties of the soleus.