Variations in force-time histories of cat gastrocnemius, soleus and plantaris muscles for consecutive walking steps

Force-sharing among muscles during locomotion has been studied experimentally using 'representative' or 'average' step cycles. Mathematical approaches aimed at predicting individual muscle forces during locomotion are based on the assumption that force-sharing among muscles occurs in a consistent and unique way. In this study, we quantify normal variations in muscular force-time histories for step cycles executed at a given nominal speed, so that we can appreciate what it means to analyze 'representative' or 'average' step cycles and can evaluate whether these normal variations in muscular force-time histories are random or may be associated with variations in the kinematics of consecutive step cycles. Forces in gastrocnemius, soleus and plantaris muscles were measured for step cycles performed at a constant nominal speed in freely moving cats. Gastrocnemius forces were always larger than peak plantaris or soleus forces. Also, peak gastrocnemius forces typically occurred first after paw contact, followed by peak soleus and then peak plantaris forces. Furthermore, it was found that variations in muscular force-time histories were substantial and were systematically related to step-cycle durations. The results of this study suggest that findings based on 'representative' or 'average' step cycles for a given nominal speed of locomotion should be viewed cautiously and that variations in force-sharing among muscles are systematically related to variations in locomotor kinematics.