Muscular and Postural Synergies of the Human Hand

Abstract

Because humans have limited ability to independently control the many joints of the hand, a wide variety of hand shapes can be characterized as a weighted combination of just two or three main patterns of covariation in joint rotations, or "postural synergies." The present study sought to align muscle synergies with these main postural synergies and to describe the form of membership of motor units in these postural/muscle synergies. Seventeen joint angles and the electromyographic (EMG) activities of several hand muscles (both intrinsic and extrinsic muscles) were recorded while human subjects held the hand statically in 52 specific shapes (i.e., shaping the hand around 26 commonly grasped objects or forming the 26 letter shapes of a manual alphabet). Principal-components analysis revealed several patterns of muscle synergy, some of which represented either coactivation of all hand muscles, or reciprocal patterns of activity (above and below average levels) in the intrinsic index finger and thumb muscles or (to a lesser extent) in the extrinsic four-tendoned extensor and flexor muscles. Single- and multiunit activity was generally a multimodal function of whole hand shape. This implies that motor-unit activation does not align with a single synergy; instead, motor units participate in multiple muscle synergies. Thus it appears that the organization of the global pattern of hand muscle activation is highly distributed. This organization mirrors the highly fractured somatotopy of cortical hand representations and may provide an ideal substrate for motor learning and recovery from injury.

Discharges in Human Muscle Receptor Afferents during Block Grasping

“…The biomechanical situation in the block grasping task was, of course, much more complicated than what was considered above: the load was most likely changing during the movements, e.g., because of varying degrees of cocontractions (Weiss and Flanders, 2004). Moreover, whenever the parent muscle was deactivated, elastic recoil of its tendon might elongate the muscle fascicle whether the whole muscle was shortening or lengthening (Hoffer et al, 1989;Roberts et al, 1997).…”

Learning effects on muscle modes and multi-mode postural synergies

“…We would like to emphasize a major difference of our approach from those of a number of research groups who have also used matrix factorization techniques to identify muscle groups, within which levels of muscle activation scaled in parallel (d'Avella et al 2003;Ivanenko et al 2004;Weiss and Flanders 2004;Ting and Macpherson 2005;Tresch et al 2006). Both lines of research follow the general idea that a smaller number of variables (M-modes in our study or "synergies" in most other studies) can be used to construct behaviors with different characteristics (Loeb et al 2000;Saltiel et al 2001;d'Avella and Bizzi 2005;Ting and Macpherson 2005).…”

“…M-modes were defined in those studies using principal component analysis with factor extraction. Other matrix factorization techniques have been used to identify such muscle groups (d'Avella et al 2003;Ivanenko et al 2004;Weiss and Flanders 2004;Ting and Macpherson 2005;Tresch et al 2006), but the authors of those studies addressed such groups as "synergies", while we view them as elemental variables forming a space, within which synergies are organized.…”

Neuromechanics of Multifunctionality during Rejection inAplysia californica

“…As limbs move, the net torques exerted by their muscles may change sign, and thus muscle function depends on mechanical context. Rather than ignore these complexities, the nervous system exploits them to generate qualitatively different behaviors (Buneo et al, 1997;Weiss and Flanders, 2004).…”