Stiffness control after fast goal-directed arm movements

Abstract

Mechanical parameters of the effector system directly after the termination of fast goal-directed arm movements were studied.

Subjects were asked to move their hand as fast as possible to a target the instant the target was presented. Only movements of the subjects' forearms were allowed. They were also instructed not to react actively to forces applied suddenly to their forearm after the movement. As a result of such a force pulse the arm moved to a new position. The apparent stiffness, i.e. the quotient of the applied force and the resultant change of position, was measured. This stiffness is a measure for the resistance of the forearm to externally applied mechanical disturbances.

It was found that after the arm has reached the target the apparent stiffness decreases as a function of time. This is an agreement with the declining amplitude of the electromyographic activity of the muscles that effect the movement.

Arguments are given to support the hypothesis that this apparent stiffness control is part of the motor programme for movements of the forearm, i.e. the stiffness is planned together with the movement.