Pii: S1388-2457(99)00012-7

We investigated in normal human subjects the effect of changing the initial direction on the kinematic properties of ®gure `8' movement performed as fast as possible by the right arm extended in free space. To this end, the motion of the index ®nger was monitored by the ELITE system. The ®gure `8' movement was characterized by a complex tangential velocity pro®le (Vt) presenting 5 bell-shaped components. It was found that the temporal segmentation following Vt was not signi®cantly different, whatever the initial direction of the movement. The decomposition of Vt into different velocity pro®les with respect to vertical (3 phases, Iy±IIIy) and horizontal (5 phases, Iz±Vz) directions showed a signi®cant relationship between the amplitude and the maximal velocity for all the different phases (except the IIy phase), which demonstrated a good conservation of the Isochrony Principle. However, we showed that the transition between the clockwise and counterclockwise loop (in ̄ection point) induced greater variability in the vertical velocity pro®le than in the horizontal one. Moreover, some parameters such as the maximal velocity of Iy and the movement amplitude of the last phases (IIIy and Vz) showed signi®cant changes depending on the initial direction. A highly signi®cant positive correlation was observed between the instantaneous curvature and angular velocity. This was expressed by a power law similar to that previously describe for other types of movement. Furthermore, it was found that this covariation between geometrical and kinematic properties of the trajectory is not dependent on the initial direction of movement. In conclusion, these results support the idea that the fast execution in different directions of a ®gure `8' movement is mainly controlled by two types of invariant commands. The ®rst one is re ̄ected in the 2/3 power law between angular velocity and curvature and the second one is represented by a segmented tangential velocity pro®le. q 1999 Elsevier Science Ireland Ltd. All rights reserved.