The Nature of Voluntary Control of Motor Actions

Natural laws express the relationships between certain variables called state variables. Constrained by natural laws these variables cannot be specified directly by the nervous system, as illustrated by the failure of the force control theory that relies on the idea of direct programming of kinematics and muscle torques. Natural laws include parameters, some of which are not conditioned by these laws but define essential characteristics of the system’s behavior under the action of these laws. This implies that the neural control of motor actions involves changes in parameters of the system. This strategy allows the nervous system to take advantage of natural laws in producing the desired motor output without actually knowing these laws or imitating them in the form of internal models. A well established form of parametric control—threshold control—is briefly reviewed with a major focus on how it helps to solve several motor problems, in particular, the problem of the relationship between posture and movement and redundancy problems in the control of multiple muscles and joints.