Force-independent distribution of correlated neural inputs to hand muscles during three-digit grasping

• Published in: Journal of neurophysiology 2010-08, Vol.104 (2), p.1141-1154
• Main Authors: Poston, Brach, Danna-Dos Santos, Alessander, Jesunathadas, Mark, Hamm, Thomas M, Santello, Marco
• Format: Article
• Language: English
• Subjects: Adult; Analysis of Variance; Electromyography - methods; Evoked Potentials, Motor - physiology; Female; Fingers - physiology; Hand - innervation; Hand Strength - physiology; Humans; Male; Muscle, Skeletal - physiology; Young Adult
• ISSN: 0022-3077; 1522-1598
• DOI: 10.1152/jn.00185.2010

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.