Central Processes Involved in Arm Movement Control

Abstract

The mechanism whereby the central nervous system (CNS) controls, simultaneously, the large number of degrees of freedom of the multijoint musculoskeletal system has long been one of the major questions in motor physiology. This incredibly complex task must be dependent upon “rules” which permit original and creative motor solutions to problems such as reaching for objects in a structured space. As a step toward understanding these rules, a number of investigators have studied relatively simple motions involving a limited number of joints (Asatryan & Feldman, 1965; Bizzi, Polit, & Morasso, 1976; Cooke, 1979; Kelso & Holt, 1980). The results have indicated some of the processes that subserve these movements and have given us a picture of the elementary building blocks from which more complex movements may be constructed. Several points have emerged from these investigations. First, it has become apparent that the forces that control the muscles result from “commands” that are, to a great extent, precomputed in some part of the CNS. These observations are based on studies made in deafferented animals that have demonstrated open-loop reaching (Bizzi et al., 1976; Polit & Bizzi, 1979; Taub, Goldberg, & Taub, 1975). Second, recent investigations have convincingly demonstrated that muscles, to a first approximation, can be thought of as behaving like tunable springs (Rack & Westbury, 1974).