Coordination among the body segments during reach-to-grasp action involving the trunk.

To understand the internal representations used by the nervous system to coordinate multijoint movements, we examined the coordination among the body segments during reach-to-grasp movements which involve grasping by the hand and reaching by the arm and trunk. Subjects were asked to reach and grasp an object using the arm only, the trunk only, and some combinations of both arm and trunk. Results showed that kinematic parameters related to the transport component of the arm and the trunk, such as peak velocity and time to peak velocity, varied across conditions and that the coordination pattern between the arm and trunk was different across conditions. However, parameters related to the grasp component, such as peak aperture, time to peak aperture, and closing distance, were invariant, regardless of whether the hand was delivered to the target by the arm only, the trunk only, or both. We hypothesize that a hierarchy of motor control processes exists, in which the reach and grasp components are governed by independent neuromotor synergies, which in turn are coordinated temporally and spatially by a higher-level synergy.