Stability and movement of a one-link neuromusculoskeletal sagittal arm.

This paper's focus is the stability, point-to-point, and rhythmic movements of a one-link sagittal arm. The system is highly nonlinear in all its physical and physiological attributes. The major physiological characteristics of this system are simultaneous activation of a pair of nonlinear muscle-like actuators for control purposes, existence of nonlinear spindle-like sensors, and actions of gravity and loading. Transmission delays are included in the afferent and efferent neural paths to account for a more accurate representation of the reflex loops. An algorithm for computation of the actuator forces and the feedback signals in the system is provided. It automatically renders positive forces and positive neural signals. The positiveness of the forces represents the unidirectional character of muscular forces, i.e., natural muscles can only pull in the direction of shortening. The positiveness of the neural signals implies these signals can correspond to firing rates. The stability of the system is analyzed. The role of the nonlinearities in the dynamics, actuators, and feedback signals, and the delays in the feedback loops in destabilizing the system, and the consequently undesirable oscillations are studied by simulation. The system is designed to perform stable point-to-point movement. The effects of the presetting of the input signals, gain of the feedback loops, and the duration of delays in generating undesirable tremor-like oscillations at the end of the movement are studied and demonstrated by digital computer simulations.