Different forms of locomotion have been studied in the cat in an effort to understand the neural mechanisms involved in movement control. Recent studies 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 system may be organized around the mechanical actions of muscles and the number of joints they span. To investigate the load-sharing between the two-joint medial gastrocnemius and one-joint soleus muscles, a single cat was trained to walk in an instrumented Plexiglas enclosed walkway at slopes ranging ±75%. Surgically implanted tendon force transducers monitored force output from each muscle. Equations in Newtonian mechanics were used to calculate joint kinetics. Results suggest that as slope angle decreased, the one-joint soleus became the primary contributor to the plantar-flexor moment calculated during stance. Unexpectedly, as slope angle increased, force in the one-joint soleus decreased while force in the two-joint medial gastrocnemius increased in the presence of the increased plantar-flexor moment calculated during stance. One explanation is that activation and force in the two-joint 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 both joints they cross. Additionally, a force-dependent inhibition of the soleus by the medial gastrocnemius has been described as part of a neural control 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.
The authors (other than J.L. Smith) are with the Center for Human Movement Studies, Dept. of Health & Performance Sciences, Georgia Tech, Atlanta, GA 30332-0110; J.L. Smith is with the Dept. of Physiological Science, UCLA, Los Angeles, CA 90095-1568.