Posture control and trajectory formation during arm movement

TLDR

A hypothesis for generating spatially oriented arm movements posits that desired joint position is set by the ratio of agonist and antagonist muscle tensions, and that transitions between equilibrium states depend only on motor unit contraction time and arm mechanical properties. The authors tested the hypothesis in intact and deafferented rhesus monkeys by holding the forearm, measuring the accelerative transient after release, and directly recording the torque increase time course during movement. Both measurement methods found that peak torque was reached after about 400 ms in a 700‑ms movement, yet perturbing the arm trajectory caused it to settle at intermediate points rather than the expected equilibrium positions, indicating that trajectory formation involves more complex processes than a simple switch between equilibria.

Abstract

One hypothesis for the generation of spatially oriented arm movements by the central nervous system is that a desired joint position is determined by the ratio of the tensions of agonist and antagonist muscles. According to this hypothesis, the transition between equilibrium states should be solely a function of the contraction time of the motor units and the mechanical properties of the arm. We tested this hypothesis in intact and deafferented rhesus monkeys by holding the forearm and measuring the accelerative transient after release of the forearm and by directly measuring the time course of the increase in torque during the movement. Both methods indicated an average time of 400 msec for attaining peak torque in a movement with a duration of 700 msec. In addition, by displacing the arm from its normal trajectory during the movement, we observed that the arm returned neither to the initial nor to the final equilibrium positions, but to points intermediate between them. We conclude that the processes underlying trajectory formation must be more complex than a simple switch between one equilibrium position and another.