This proposal addresses the fundamental issue of how movements are controlled. It is hypothesized that there exists a finite set of spinal neural networks, each of which generates forces for a particular posture, i.e., motor output is modularized. In addition, the force output of these modules follows a principle of vector summation. In this way, the finite set of force patterns may be viewed as representing an elementary alphabet from which, through superimposition, a vast number of actions could be fashioned by impulses conveyed by supraspinal pathways. Three sets of experiments will address this hypothesis of spinal motor modules. First, the organization of the spinal neural circuitry will be defined anatomically and electrophysiologically. By combining anatomical localization with physiological recordings in the in vitro frog spinal cord, the connectivity patterns of neurons comprising the spinal networks will be distinguished and related to particular postures. Second, the utilization of spinal motor modules by descending systems will be investigated. The probability that descending systems access spinal modules will be evaluated by comparing the force output generated by stimulation of different descending pathways to simulations of force output generated by model muscles and model spinal force fields. Third, theoretical and experimental studies will explore the control of movements by combinations of the outputs of motor modules. A computer model of the frog hindlimb will be developed to answer three questions: (1) What is the temporal course of the fields generated by the control modules in the motor system? (2) What are the dynamical effects of vector field summation? (3) How does kinematic redundancy affect the control of dynamic behavior? These findings should provide guidance for the development of new technologies or strategies for the rehabilitation of individuals suffering spinal cord injuries or stroke.