Digit Position and Force Synergies During Unconstrained Grasping

Grasping is a complex motor task which requires a fine control of the multiple degrees of freedom of the hand, in both the position and the force domain. In this chapter, we investigated the coordinated control of digit position and force in the human hand while grasping and holding a moving object. We observed a substantial variability between participants in the hand posture. Instead, digit placement was rather stereotyped for repeated grasps of the same participant. The normal forces applied by the digits co-varied with their placement across trials. Specifically, we observed an exponential relationship between finger placement and normal force applied for the thumb and lateral fingers. For the middle and ring fingers, the force responses co-varied in an approximately linear fashion with digit position. Principal component analysis revealed that more than 97 % of the finger force variance was accounted by the first two components (corresponding to the first and the second force synergy). This is consisted with the framework of motor synergy, since two components successfully explained most of the variability in the data.