The Three-Dimensional Curvature of Straight-Ahead Movements.

Three-dimensional curvature of point-to-point hand movements in the forward direction was examined. Subjects (N = 4) moved their hand from a position above the start point to a forward position above targets of different size and distance. Paths were curved as a result of an initial lateral and downward movement that was compensated for in the second half of the movement. The downward component of motion had a bell-shaped velocity profile and was temporally coupled to the forward motion. Curvature was greater for movements to near targets. Examination of the relation between kinematics and geometry revealed that velocity was related to radius of curvature by a power law with an exponent of 0.59. Simulations of the component of motion in the vertical plane reproduced the qualitative behavior of curvature and fit a power law relationship between velocity and radius of curvature