The Machinery of Movement

Abstract

Skeletal movements occur around joints due to the selective contraction and relaxation of striated muscles. Since muscles can only pull, never push, they must work in opposing groups. Operationally, groups of muscles that contribute to a movement in a particular direction at a particular joint (i.e., pull together) are called agonists, and those that oppose the movement are called antagonists. Much of the time, however, appropriate movements are accomplished by synergistic contractions of muscles, even to the extent that antagonistic groups of muscles cooperate to produce the movements. Moreover, the movements that are made during practical behaviors occur around more than one joint at a time and involve several sets of agonist and antagonist muscles in a highly choreographed pattern of contractions and relaxations. To produce effective behaviors, the central systems that control skeletal movements must reflect the anatomical organization of the musculoskeletal apparatuses. Further, the computation of effective movement commands requires that the central control systems receive a continuous stream of sensory data about the spatial layout of the local environment, the position of the body and limbs in space, and the load changes (tension) on the muscles. Since the motor systems are arranged in parallel as well as hierarchically (see below), the sensory information delivered to each level must be formated appropriately for the contribution of that particular level to movement control (Fig. 14.1).