Equilibrium Control Vectors Subserving Rapid Goal-Directed Arm Movements

Abstract

The composition of central commands underlying rapid goal-directed arm movements to visual targets was explored within the framework of the equilibrium point hypothesis. This hypothesis suggests that movements arise from shifts in equilibrium associated with the dynamic interaction of central commands, reflex mechanisms, muscle properties, and loads. Central commands control this process by regulating muscle threshold lengths (λs) for motoneuron recruitment. Subjects performed rapid arm movements to fixed and displaced targets (LEDs) located in a horizontal plane. The position of the first target and the onset and the position of the second target were varied. Experimental trajectories of the movement endpoint (e.g., the hand or wrist) were compared with simulated trajectories, based on the X model, generated with theoretical central commands. The findings support the hypothesis that multi-joint arm motions are planned in equilibrium coordinates corresponding to the position of the endpoint. Moreover, the results suggest that, in the absence of overriding constraints, the equilibrium position of the hand is shifted at a constant velocity towards the target. Finally, for rapid movements the rate of shift appears to be the same for movements of different amplitude.