Contributions of gait and trunk movements to prehension: perspectives from world- and body-centered coordinates.

The present paper reviews a series of prehension experiments recently conducted at Simon Fraser University's Human Motor Systems Laboratory, and attempts to place them into the larger context of multi-segmental control theory. Two related lines of experiments are reported: (a) experiments involving prehension during walking, and (b) experiments involving trunk-assisted reaching. Three-dimensional analyses of movements were performed via both world- and body-centered coordinates. Our results are supportive of the idea that both types of tasks are carried out using task-specific synergies. Furthermore, we assert that the actions of these synergies are comprised of variable contributions of different movement systems and result in smooth, world-centered end-point trajectories. We show evidence that this "motor equivalence" is the result of increasing the complexity of a given task. Finally, the implications of the present findings on prevailing motor control theory are discussed in terms of the theoretical mechanisms underlying the coordination of the transport and grasp components of prehension.