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 experim entally using 'representative' or 'average' step cycles. Mathematical approaches aimed at predicting individual muscle forces during locomot ion are based on the assumption that force-sharing among muscles occur s in a consistent and unique way. In this study, we quantify normal va riations in muscular force-time histories for step cycles executed at a given nominal speed, so that we can appreciate what it means to anal yze 'representative' or 'average' step cycles and can evaluate whether these normal variations in muscular force-time histories are random o r may be associated with variations in the kinematics of consecutive s tep cycles. Forces in gastrocnemius, soleus and plantaris muscles were measured for step cycles performed at a constant nominal speed in fre ely moving cats. Gastrocnemius forces were always larger than peak pla ntaris or soleus forces. Also, peak gastrocnemius forces typically occ urred first after paw contact, followed by peak soleus and then peak p lantaris 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 findi ngs based on 'representative' or 'average' step cycles for a given nom inal speed of locomotion should be viewed cautiously and that variatio ns in force-sharing among muscles are systematically related to variat ions in locomotor kinematics.