Kinematics invariance in multi-directional complex movements in free space: effect of changing initial direction

We investigated in normal human subjects the effect of changing the initial direction on the kinematic properties of figure '8' movement performed as fast as possible by the right arm extended in free space. To this end, the motion of the index finger was monitored by the ELITE system. The figure '8 ' movement was characterized by a complex tangential velocity profile (Vt) presenting 5 bell-shaped components. It was found that the temporal segment ation following Vt was not significantly different, whatever the initial di rection of the movement. The decomposition of Vt into different velocity pr ofiles with respect to vertical (3 phases, Iy-IIIy) and horizontal (5 phase s, Iz-Vz) directions showed a significant relationship between the amplitud e and the maximal velocity for all the different phases (except the IIy pha se), which demonstrated a good conservation of the Isochrony Principle. How ever, we showed that the transition between the clockwise and counterclockw ise loop (inflection point) induced greater variability in the vertical vel ocity profile 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 significant changes depending on the initial direction. A highly significant positive correlation was observed between the instant aneous curvature and angular velocity. This was expressed by a power law si milar to that previously describe for other types of movement. Furthermore, it was found that this covariation between geometrical and kinematic prope rties of the trajectory is not dependent on the initial direction of moveme nt. In conclusion, these results support the idea that the fast execution i n different directions of a figure '8' movement is mainly controlled by two types of invariant commands. The first one is reflected in the 2/3 power l aw between angular velocity and curvature and the second one is represented by a segmented tangential velocity profile. (C) 1999 Elsevier Science Irel and Ltd. All rights reserved.