Coordination of intrinsic and extrinsic hand muscle activity as a function of wrist joint angle during two-digit grasping

Abstract

Fingertip forces result from activation of muscles that cross the wrist and muscles whose origins and insertions reside within the hand (extrinsic and intrinsic hand muscles, respectively). Thus, tasks that involve changes in wrist angle affect the moment arm and length, hence the force-producing capabilities, of extrinsic muscles only. If a grasping task requires the exertion of constant fingertip forces, the Central Nervous System (CNS) may respond to changes in wrist angle by modulating the neural drive to extrinsic or intrinsic muscles only or by co-activating both sets of muscles. To distinguish between these scenarios, we recorded electromyographic (EMG) activity of intrinsic and extrinsic muscles of the thumb and index finger as a function of wrist angle during a two-digit object hold task. We hypothesized that changes in wrist angle would elicit EMG amplitude modulation of the extrinsic and intrinsic hand muscles. In one experimental condition we asked subjects to exert the same digit forces at each wrist angle, whereas in a second condition subjects could choose digit forces for holding the object. EMG activity was significantly modulated in both extrinsic and intrinsic muscles as a function of wrist angle (both p < 0.05) but only for the constant force condition. Furthermore, EMG modulation resulted from uniform scaling of EMG amplitude across all muscles. We conclude that the CNS controlled both extrinsic and intrinsic muscles as a muscle synergy. These findings are discussed within the theoretical frameworks of synergies and common neural input across motor nuclei of hand muscles.

Skeletal evidence of craft production from the Ch'iji Jawira site in Tiwanaku, Bolivia

“…4), as well as medial and lateral finger phalanges (Fig. 5), which may show precision gripping or precision handling of objects (Johnston et al, 2010;Landsmeer, 1962;Long et al, 1970).…”

Changes in hand muscle synergies in subjects with spinal cord injury: Characterization and functional implications

“…Weiss Sign Language alphabet to examine whether muscle synergies could serve as a predictive framework for new hand postures; the synergies identified in this study were generally subject specific. Johnston et al 12 found evidence of synergistic control of extrinsic and intrinsic hand muscles during a two-digit task. These studies generally did not investigate the presence of synergies during the different types of hand movements used in ADLs (with the exception of Ref.…”

The superposition principle applied to grasping an object producing moments outside anatomically-defined axes

“…These redundant degrees of freedom allow for an infinite number of solutions when performing whole-hand object manipulation (Zatsiorsky and Latash, 2008). Despite this, the central nervous system (CNS) tends to select certain families of solutions, or synergies, when grasping objects (Santello and Soechting, 2000;Rearick and Santello, 2002;Shim et al, 2003Shim et al, , 2005aJohnston et al, 2005Johnston et al, , 2010Shim and Park, 2007). A better understanding of such control mechanisms can lead to improved rehabilitation of neuromuscular dysfunction (Nowak, 2008;Alberts and Wolf, 2009;Schieber et al, 2009), and the development of more dextrous robotic limbs (Bae et al, 2005;Choi et al, 2008;Lee et al, 2009;Yang et al, 2009).…”