title: Force-independent distribution of correlated neural inputs to hand muscles during three-digit grasping.
abstract: The ability to modulate digit forces during grasping relies on the coordination of multiple hand muscles. Because many muscles innervate each digit, the CNS can potentially choose from a large number of muscle coordination patterns to generate a given digit force. Studies of single-digit force production tasks have revealed that the electromyographic (EMG) activity scales uniformly across all muscles as a function of digit force. However, the extent to which this finding applies to the coordination of forces across multiple digits is unknown. We addressed this question by asking subjects (n = 8) to exert isometric forces using a three-digit grip (thumb, index, and middle fingers) that allowed for the quantification of hand muscle coordination within and across digits as a function of grasp force (5, 20, 40, 60, and 80% maximal voluntary force). We recorded EMG from 12 muscles (6 extrinsic and 6 intrinsic) of the three digits. Hand muscle coordination patterns were quantified in the amplitude and frequency domains (EMG-EMG coherence). EMG amplitude scaled uniformly across all hand muscles as a function of grasp force (muscle x force interaction: P = 0.997; cosines of angle between muscle activation pattern vector pairs: 0.897-0.997). Similarly, EMG-EMG coherence was not significantly affected by force (P = 0.324). However, coherence was stronger across extrinsic than that across intrinsic muscle pairs (P = 0.0039). These findings indicate that the distribution of neural drive to multiple hand muscles is force independent and may reflect the anatomical properties or functional roles of hand muscle groups.
authors.affiliation: Department of Kinesiology, Arizona State University, Tempe, Arizona 85287-0404, USA.
authors.forename: Brach
authors.initials: B
authors.lastname: Poston
authors.forename: Alessander
authors.initials: A
authors.lastname: Danna-Dos Santos
authors.forename: Mark
authors.initials: M
authors.lastname: Jesunathadas
authors.forename: Thomas M
authors.initials: TM
authors.lastname: Hamm
authors.forename: Marco
authors.initials: M
authors.lastname: Santello
mesh_terms: D000328:Adult; D000704:Analysis of Variance; D004576:Electromyography / Q000379:methods; D019054:Evoked Potentials, Motor / Q000502:physiology; D005260:Female; D005385:Fingers / Q000502:physiology*; D006225:Hand / Q000294:innervation*; D018737:Hand Strength / Q000502:physiology*; D006801:Humans; D008297:Male; D018482:Muscle, Skeletal / Q000502:physiology*; D055815:Young Adult