OPTIMAL-CONTROL OF ANTAGONISTIC MUSCLE-STIFFNESS DURING VOLUNTARY MOVEMENTS

This paper presents a study on the control of antagonist muscle stiffn ess during single-joint arm movements by optimal control theory with a minimal effort criterion. A hierarchical model is developed based on the physiology of the neuromuscular control system and the equilibrium point hypothesis. For point-to-point movements, the model provides pr edictions on (1) movement trajectory, (2) equilibrium trajectory, (3) muscle control inputs, and (4) antagonist muscle stiffness, as well as other variables. We compared these model. predictions to the behavior observed in normal human subjects. The optimal movements capture the major invariant characteristics of voluntary movements, such as a sigm oidal movement trajectory with a bell-shaped velocity profile, an 'N'- shaped equilibrium trajectory, a triphasic burst pattern of muscle con trol inputs, and a dynamically modulated joint stiffness. The joint st iffness is found to increase in the middle of the movement as a conseq uence of the triphasic muscle activities. We have also investigated th e effects of changes in model parameters on movement control. We found that the movement kinematics and muscle control inputs are strongly i nfluenced by the upper bound of the descending excitation signal that activates motoneuron pools in the spinal cord. Furthermore, a class of movements with scaled velocity profiles can be achieved by tuning the amplitude and duration of this excitation signal. These model predict ions agree with a wide body of experimental data obtained from normal human subjects. The results suggest that the control of fast arm movem ents involves explicit planning for both the equilibrium trajectory an d joint stiffness, and that the minimal effort criterion best characte rizes the objective of movement planning and control.