A method for measuring endpoint sti!ness during multi-joint arm movements

Current methods for measuring sti!ness during human arm movements are either limited to one-joint motions, or lead to systematic errors. The technique presented here enables a simple, accurate and unbiased measurement of endpoint sti!ness during multi-joint movements. Using a computer-controlled mechanical interface, the hand is displaced relative to a prediction of the undisturbed trajectory. Sti!ness is then computed as the ratio of restoring force to displacement amplitude. Because of the accuracy of the prediction ( 1 cm error after 200 m s) and the quality of the implementation, the movement is not disrupted by the perturbation. This technique requires only 1 3 as many trials to identify sti!ness as the method of Gomi and Kawato (1997, Biological Cybernetics 76, 163}171) and may, therefore, be used to investigate the evolution of sti!ness during motor adaptation. ( 2000 Elsevier Science Ltd. All rights reserved.