The purpose of this paper is three-fold: (a) to summarize available data on coordination of major two- and one-joint muscles in multijoint tasks and identify basic features of muscle coordination, (b) to demonstrate that there may exist an optimization criterion that predicts essential features of electromyographic activity of individual muscles in a variety of tasks, and (c) to address the functional consequences of the observed muscle coordination and underlying mechanisms of its control. The analysis of the literature revealed that basic features of muscle coordination are similar among different voluntary motor tasks and reflex responses. It is demonstrated that these basic features of coordination of one- and two-joint muscles in two-dimensional tasks are qualitatively predicted by minimizing the sum of muscle stresses cubed. Functional consequences of the observed coordination of one- and two-joint muscles are (a) reduction of muscle force as well as stress, mechanical and metabolic energy expenditure, muscle fatigue, and perceived effort; (b) a spring-like behavior of a multi-joint limb during maintenance of an equilibrium posture; and (c) energy transfer between joints via two-joint muscles. A conceptual scheme of connections between motoneuron pools of one- and two-joint muscles, which accounts for the observed muscle coordination, is proposed. An important part of this scheme is the force-dependent inhibition and excitation from two-joint to one-joint synergists and antagonists, respectively.
Robert J. Gregor, Judith L. Smith, Dagan W. Smith, Alanna Oliver and Boris I. Prilutsky
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.