Postural Hand Synergies for Tool Use

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.

Evidence of muscle synergies during human grasping

“…This clearly proves that kinematic synergies are present; our figures are in agreement with previous work, given the simplicity of the tasks at hand (e.g., [28]). The grasps considered can be captured (at 75% variance) by using three linear combinations of X glove , meaning that most of the grasps share the same three characteristic "eigengrasps".…”

“…A description of the grasps according to the synergies confirms that each grasp employs each synergy to very different degrees. This analysis reminds of that found for kinematic patterns in, e.g., [28,29,4].…”

“…Indeed, when taking a large number of every-day grasps into account, most movements of the fingers of the hand can be described with a very limited number of principal components (i.e., 3-5 [28], considerably less than the number of degrees of freedom of all fingers combined, i.e., 25 [31]). Complementary to that, this work tries to answer the following, so-far unanswered question: are analogous synergies also present at the level of the exerted grasp forces and, consequently, of the activation of the forearm muscles?…”

“…As shown by Santello et al [28,29], the simultaneous motion of the fingers is characterised by coordination and covariation patterns that reduce the number of independent degrees of freedom to be controlled by the neural system. Still, although some constraints on the musculotendon system, as well as on the peripheral and central neural system, can be identified, a clear relationship between the finger kinematic constraints and the underlying muscular activity remains to be analysed.…”

“…Muscular activity measured via sEMG relates in the simplest case nearly linearly to the force exerted by a muscle [16]; but given the complexity of the arm/forearm/hand musculoskeletal system, there is no a priori indication as to whether such a principle should exist, although finger position synergies have been found in [28]. Some results mapping sEMG activity to finger, wrist and arm position have actually appeared, e.g.…”