Alterations in transport path differentially affect temporal and spatial movement parameters.

Many studies have examined the coordination of reach-to-grasp movements. However, there is debate regarding the mechanism of coordination between the transport and grasp components. The current study investigated the stability of temporal and spatial measures for reaches in which transport path was altered early or late in the reaching action. Transport alteration was accomplished by placing an obstacle either 10 cm (near) or 20 cm (far) from the hand starting position. Obstacle location affected the formation of transport path such that maximum wrist elevation coincided with the location of the obstacle. Kinematic analyses revealed that reaches over the near obstacle significantly prolonged transport time and time to maximum velocity compared with reaches over the far obstacle. A similar pattern of results was observed for the grasp component; reaches over the near obstacle resulted in a prolongation of grip duration, time to maximum aperture, and time to maximum opening and closing velocity. Grip closing velocity was decreased in the obstacle conditions. These results confirm findings from earlier studies that have shown that changes in the transport component affect grasp formation. A spatial and temporal analysis of grasp opening and closing was also performed. Grasp closing time varied significantly between conditions, while closing distance or the distance traveled by the wrist after maximum aperture remained essentially constant across conditions. These results suggest that the central nervous system may be using a spatial controller to coordinate prehensile components.