POSTURAL HAND SYNERGIES FOR TOOL USE

Subjects were asked to shape the right hand as if to grasp and use a l arge 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 ad opted distinct hand shapes for the various objects, the joint angles o f the digits did not vary independently. Principal components analysis showed that the first two components could account for >80% of the va riance, implying a substantial reduction from the 15 degrees of freedo m that were recorded. However, even though they were small, higher-ord er (more than three) principal components did not represent random var iability 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 adjustment s. Because the postural synergies did not coincide with grip taxonomie s, the results suggest that hand posture may be regulated independentl y from the control of the contact forces that are used to grasp an obj ect.