Impedance is modulated to meet accuracy demands during goal-directed arm movements.
The neuromuscular system is inherently noisy and joint impedance may serve to filter this noise. In the present experiment, we investigated whether individuals modulate joint impedance to meet spatial accuracy demands. Twelve subjects were instructed to make rapid, time constrained, elbow extensions to three differently sized targets. Some trials (20 out of 140 for each target, randomly assigned) were perturbed mechanically at 75% of movement amplitude. Inertia, damping and stiffness were estimated from the torque and angle deviation signal using a forward simulation and optimization routine. Increases in endpoint accuracy were not always reflected in a decrease in trajectory variability. Only in the final quarter of the trajectory the variability decreased as target width decreased. Stiffness estimates increased significantly with accuracy constraints. Damping estimates only increased for perturbations that were initially directed against the movement direction. We concluded that joint impedance modulation is one of the strategies used by the neuromuscular system to generate accurate movements, at least during the final part of the movement.
Selen
Luc P J
LP
Faculty of Human Movement Sciences, Institute for Fundamental and Clinical Human Movement Sciences, Vrije Universiteit, Van der Boechorststraat 9, 1081 BT Amsterdam, The Netherlands.
Beek
Peter J
PJ
van Dieën
Jaap H
JH
D000328:Adult; D001132:Arm / Q000502:physiology*; D004576:Electromyography / Q000379:methods; D005246:Feedback; D005260:Female; D006801:Humans; D008297:Male; D009068:Movement / Q000502:physiology*; D009142:Musculoskeletal Physiological Phenomena; D011597:Psychomotor Performance / Q000502:physiology*; D013037:Spatial Behavior / Q000502:physiology; D013997:Time Factors; D019415:Torque