This chapter discusses the control strategies adopted by the central nervous system (CNS) to execute movements and maintain posture. It is necessary to look first at the mechanical properties of the musculo-skeletal apparatus. Such an approach is based on the assumption that the functional properties of the motor system have been developed by the need not only to control, but also to take advantage of the mechanical properties of the musculo-skeletal apparatus. If muscles have spring-like properties, then a limb's posture is maintained when the forces exerted by the agonist and antagonist muscle groups are equal and opposite. This implies that when a force is applied, the limb is displaced by an amount proportional to both the external force, and the stiffness of the muscles. When the external force is removed, the limb should return to the original position. This prediction is nothing else than a restatement of Hooke's Law, but in a biological context. This chapter discusses the evidence supporting the idea that muscles in vivo indeed have spring-like properties, and discusses the implication of these findings for trajectory control.
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