Hindlimb kinetics and neural control during slope walking in the cat: unexpected findings

Different forms of locomotion have been studied in the cat in an effort to understand the neural mechanisms involved in movement control. Recent studi es have focused on the roles of one- and two-joint muscles, the integration of central commands with sensory input, and the notion that the control sy stem may be organized around the mechanical actions of muscles and the numb er of joints they span. To investigate the load-sharing between the two-joi nt medial gastrocnemius and one-joint soleus muscles, a single cat was trai ned to walk in an instrumented Plexiglas enclosed walkway at slopes ranging +/-75%. Surgically implanted tendon force transducers monitored force outp ut from each muscle. Equations in Newtonian mechanics were used to calculat e joint kinetics. Results suggest that as slope angle decreased, the one-jo int soleus became the primary contributor to the plantar-flexor moment calc ulated during stance. Unexpectedly, as slope angle increased, force in the one-joint soleus decreased while force in the two-joint medial gastrocnemiu s increased in the presence of the increased plantar-flexor moment calculat ed during stance. One explanation is that activation and force in the two-j oint medial gastrocnemius should increase in the presence of a knee flexor and plantar-flexor moment. This was the case during upslope walking, as two -joint muscles increase their activation when they act as an agonist at bot h joints they cross. Additionally, a force-dependent inhibition of the sole us by the medial gastrocnemius has been described as part of a neural contr ol system organized around the mechanical actions of muscles and the number of joints they span. Hence, a decrease in one-joint soleus force might be expected under certain conditions in upslope walking.