IS A VIRTUAL TRAJECTORY NECESSARY IN REACHING MOVEMENTS

The mass-spring model of limb control was extended to two-joint arm mo vements in the horizontal plane and tested against kinematic data from human subjects. Two versions of the model were compared in order to t est the idea that the equilibrium position of the hand moves along a ' 'virtual'' trajectory as demonstrated in single-joint arm movements (B izzi et al. 1982, 1984; Latash and Gottlieb 1991). In the peripheral v ersion, the equilibrium position was shifted abruptly, while the torqu es generated at the joints are gated by rise-time functions. In the ce ntral version, the equilibrium position was updated gradually accordin g to a predefined trajectory. The paths and tangential velocity profil es of the hand generated by these two versions were compared to Morass o's (1981) experimental data. The central version generally performed better throughout the workspace except in certain special directions. Moreover, its paths exhibited more stability as the movement speed was varied.