Forces produced by medial gastrocnemius and soleus muscles during locomotion in freely moving cats.

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Abstract
Force transducers designed for chronic implantation were placed on the individual tendons of the medial gastrocnemius (MG) and soleus (SOL) muscles in the same hindlimb of adult cats. Electromyographic (EMG) and kinesiological data were recorded. Force, EMG and movement data obtained from intact, freely moving cats illustrate the division of labor between the mixed MG and its slow-twitch synergist SOL during posture, treadmill locomotion and jumping. The inherent stiffness of active muscle during the step cycle is apparently an important factor in the control of force output from hindlimb extensor muscles in locomotion. The division of labor between MG and SOL and the absolute force levels required from the MG during the full range of hindlimb movements in posture, locomotion and jumping appear to be precisely matched to the very different characteristics of the motor-unit populations composing these synergistic muscles.