Thumb invariance during prehension movement: effects of object orientation.

The aim of the present study was to examine the contribution of the thumb and index finger during the task of reaching to grasp a cylinder positioned at different orientations. To this end an axis was defined between a marker positioned on the subjects' wrist and the target. For each frame the perpendicular distances of the thumb and index finger from this axis were determined. The perpendicular distance was greater for the index finger than the thumb, confirming a relative stability of the thumb during natural prehension and supporting the notion of the thumb as a guide for the transport component of reaching. Further, index finger perpendicular distance was varied according to object orientation. When the object was positioned at an angle that requires hand pronation, the perpendicular distance for the index finger was the greatest. It is concluded that changes in the index finger distance are necessary to allow the thumb to maintain stability in order to provide appropriate movement guidance.