The influence of initial hand posture on the expression of prehension parameters.

This paper describes the transport and grasp kinematic parameters associated with four initial hand postures (palm flat and thumb against the hand, palm flat and thumb extended laterally, index and thumb in opposition, and index and thumb in opposition and elbow flexed 90 degrees). A group of healthy adult subjects reached for and picked up a wooden dowel placed midsagittally, at one of three distances (20 cm, 25 cm and 30 cm). The initial posture of the hand and arm altered transport (peak velocity and peak negative acceleration) as well as grasp (peak angle and time to peak angle) parameters, particularly when the elbow was flexed 90 degrees. The pattern of results was reproduced in a pointing paradigm. The findings are discussed in the context of joint space models of reaching.