Postural Hand Synergies for Tool Use

TLDR

Objects are typically grasped using a range of grips, such as precision and power grips. Participants shaped their right hand to grasp a large set of familiar objects while researchers recorded the angular positions of 15 finger and thumb joint angles to capture static hand posture. Although subjects adopted distinct hand shapes, the 15 joint angles were largely coordinated, with two principal components explaining over 80 % of the variance and higher‑order components revealing subtle, non‑random adjustments that indicate hand posture is governed by a few postural synergies plus finer control, independent of grip taxonomy.

Abstract

Subjects were asked to shape the right hand as if to grasp and use a large number of familiar objects. The chosen objects typically are held with a variety of grips, including "precision" and "power" grips. Static hand posture was measured by recording the angular position of 15 joint angles of the fingers and of the thumb. Although subjects adopted distinct hand shapes for the various objects, the joint angles of the digits did not vary independently. Principal components analysis showed that the first two components could account for >80% of the variance, implying a substantial reduction from the 15 degrees of freedom that were recorded. However, even though they were small, higher-order (more than three) principal components did not represent random variability but instead provided additional information about the object. These results suggest that the control of hand posture involves a few postural synergies, regulating the general shape of the hand, coupled with a finer control mechanism providing for small, subtle adjustments. Because the postural synergies did not coincide with grip taxonomies, the results suggest that hand posture may be regulated independently from the control of the contact forces that are used to grasp an object.