ARM POSITION CONSTRAINTS DURING POINTING AND REACHING IN 3-D SPACE

Arm movements in 3-D space were studied to investigate the reduction i n the number of rotational degrees of freedom in the shoulder and elbo w during pointing movements with the fully extended arm and during poi nting movements to targets in various directions and at various distan ces relative to the shoulder, requiring flexion/extension in the elbow . The postures of both the upper arm and forearm can be described by r otation vectors, which represent these postures as a rotation from a r eference position to the current position. The rotation vectors descri bing the posture of the upper arm and forearm were found to lie in a 2 -D (curved) surface both for pointing with the fully extended arm and for pointing with elbow flexion. This result generalizes on previous r esults on the reduction of the number of degrees of freedom from three to two in the shoulder for the fully extended arm to a similar reduct ion in the number of degrees of freedom for the upper arm and forearm for normal arm movements involving also elbow flexion and extension. T he orientation of the 2-D surface fitted to the rotation vectors descr ibing the position of the upper arm and forearm was the same for point ing with the extended arm and for movements with flexion/extension of the elbow. The scatter in torsion of the rotation vectors describing t he position of the upper arm and forearm relative to the 2-D surface w as typically 3-4 degrees, which is small considering the range of simi lar to 180 and 360 degrees for torsional rotations of the upper arm an d the forearm, respectively. Donders' law states that arm posture for pointing to a target does not depend on previous positions of the arm. The results of our experiments demonstrate that the upper arm violate s Donders' law. However, the variations in torsion of the upper arm ar e small, typically a few degrees. These deviations from Donders' law h ave been overlooked in previous studies, presumably because the variat ions are relatively small. These variations may explain the larger sca tter of the rotation vectors for arm movements (3-4 degrees) than repo rted for the eye (1 degrees). Unlike for saccadic eye movements, joint rotations in the shoulder during aiming movements were not all single -axis rotations. On the contrary, the direction of the angular velocit y vector varied during the movement in a consistent and reproducible w ay, depending on amplitude, direction, and starting position of the mo vement. These results reveal several differences between arm movements during pointing and saccadic eye movements. The implications for our understanding of the coordination of eye and arm movements and for the planning of 3-D arm movements are discussed.