Palmar arch dynamics during reach-to-grasp tasks.

The relationship between arm transport and grip aperture scaling has traditionally been used to study the control of reaching and grasping. Since the palmar concavity forms a postural base of the hand, intuitively it should play an essential role in hand shape modulation during grasping. This role however has not yet been investigated. This study investigated whether hand shape modulation was initiated at the palmar level or at the level of the finger joints during two types of power grasps (spherical, cylindrical). Hand shape modulation was studied in eight healthy adults and was characterized by the change in palmar arch movement during three phases of the reach-to-grasp tasks, traditionally defined as arm transport, hand preshaping and object contact. Palmar arch kinematics changed significantly during arm transport for both types of grasp (spherical 47.4%, cylindrical 63.7%). Approximately 16% more modulation was observed when grasping the cylindrical as compared to the spherical object. This was counterbalanced by relatively less modulation during the preshaping and object contact phases compared to the spherical grasp. These results showed that the three grasping phases occurred concurrently in a common time window. Furthermore, an increase in the duration of either one of the components impacted the execution of the others. A secondary finding was that hand shape modulation began at the same time or within 125 ms of the initiation of arm transport for both grasp types. In most cases (60%), movement in the palmar arch occurred prior to movement at the metacarpophalangeal joint (MCP) of the index and middle fingers. The same was observed when palmar arch modulation was compared with the time of movement initiation in the proximal interphalangeal (PIP) joints of the index and middle fingers (60-75%). Thus palmar and finger joint movement were initiated almost simultaneously and hand shape modulation began early in the reach-to-grasp task. The findings show that hand shape and arm transport were modulated together and suggest that prehensile movement, including both palmar arch and finger configuration, is planned as early as the intent to grasp.