A.G. Fel'dman is with the Dept. of Physiology, Université de Montréal, and the Research Center of the Institute de Réhabilitation de Montréal, 6300 Ave. Darlington, Montréal, PQ, Canada BBS 2J4. O.G. Meijer is with the Faculty of Human Movement Sciences, Vrije Universiteit, van der Boechorststraat 9, 1081 BT, Amsterdam, The Netherlands.