Roles of dynamic and stationary components of a motor command in establishing the equilibrium state at simplest targeted movements

We studied in humans interrelations between the kinematic characteristics of targeted movements of the arm and current levels of EMG of the muscles providing these movements; the movements were relatively slow, and the attained joint angle was held for a time. The EMG level was considered a correlate of the level of integral motor commands (efferent activity of the respective motoneuronal pools). Application of low-amplitude non-inertial loadings, directed against the forces developed by one or another muscle group, allowed us to provide realization of targeted movements exclusively by the activity of this muscle group, without Involvement of the antagonists. It was demonstrated that the target equilibrium joint angle is reached synchronously with the dynamic phase of EMG activity, before the latter reaches a stationary level. The structure of the dynamic EMG phase itself is complex; in most cases it is split into several components. The dependence between the joint angle and amplitude of the EMG stationary phase is rather complex and variable, and usually it is difficult to predict the characteristics of this phase based on simple biomechanical considerations. There are proofs that at the performance of the studied movements and maintaining a target position there are some components in the mechanical muscle activity, which are not controlled by the motor commands. Thus, the stationary level of a motor command represents only one of several factors responsible for attaining and maintaining a target equilibrium position. Establishing this position is provided, first of all, by interaction of dynamic components of the motor commands to different muscles. Our results show that the attempts to interpret the processes of control of targeted movements on the basis of modifications of the equilibrium point hypothesis are inadequate; these data are in better compliance with the concept of impulse-temporal control; at their interpretation it is also necessary to take more thoroughly into account nonlinear properties of the muscle reactions.