INFLUENCE OF OBJECT POSITION AND SIZE ON HUMAN PREHENSION MOVEMENTS

Prehension movements of the right hand were recorded in normal subject s using a computerized motion analyzer. The kinematics and the spatial paths of markers placed at the wrist and at the tips of the index fin ger and thumb were measured. Cylindrical objects of different diameter s (3, 6, 9 cm) were used as targets. They were placed at six different positions in the workspace along a circle centered on subject's I-rea d axis. The positions were spaced by 10 degrees starting from 10 degre es on the left of the sagittal axis, up to 40 degrees on the right. Bo th the transport and the grasp components of prehension were influence d by the distance between the resting hand position and the object pos ition. Movement time, time to peak velocity of the wrist and time to m aximum grip aperture varied as a function of distance from the object, irrespective of its size. The variability of the spatial paths of wri st and fingers sharply decreased during the phase of the movement prio r to contact with the object. This indicates that the final position o f the thumb and the index finger is a controlled parameter of visuomot or transformation during prehension. The orientation of the opposition axis (defined as the line connecting the tips of the thumb and the in dex finger at the end of the movement) was measured. Several different frames of reference were used. When an object-centered frame was used , the orientation of the opposition axis was found to change by about 10 degrees from one object position to the next. By contrast, when a b ody-centered frame was used (with the head or the forearm as a referen ce), this orientation was found to remain relatively invariant for dif ferent object positions and sizes. The degree of wrist flexion was lit tle affected by the position of the object. This result, together with the invariant orientation of the opposition axis, shows that prehensi on movements aimed at cylindrical objects are organized so as to minim ize changes in posture of the lower arm.