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J Neurophysiol (July 7, 2004). doi:10.1152/jn.00516.2004
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Submitted on May 17, 2004
Accepted on July 2, 2004

Common input to motor units of digit flexors during multi-digit grasping

Sara A. Winges1 and Marco Santello1*

1 Kinesiology, Arizona State University, Tempe, AZ, USA

* To whom correspondence should be addressed. E-mail: marco.santello{at}asu.edu.

The control of whole hand grasping relies on complex coordination of multiple forces. While many studies have characterized the coordination of finger forces and torques, the control of hand muscle activity underlying multi-digit grasping has not been studied to the same extent. Motor unit synchrony across finger muscles or muscle compartments might be one of the factors underlying the limited individuation of finger forces. Such 'unwanted' coupling among finger forces, however, might be desirable when a high level of force coupling is required to prevent object slip during grasping. The goal of this study was to quantify the strength of synchrony between single motor units from extrinsic hand muscles as subjects held a device with a five-digit grasp. During the hold phase we recorded the normal force exerted by each digit and the electrical activity of single motor units from each of the four divisions of the m. flexor digitorum profundus (FDP) and one thumb flexor muscle, m. flexor pollicis longus (FPL). The strength of motor unit synchrony was quantified by the common input strength index (CIS; Nordstrom et al. 1992). We found moderate to strong motor unit synchrony between FPL and the index FDP compartment (mean CIS±S.E.: 0.49±0.03) and across most FDP-compartments (0.34 ±0.02). Weak synchrony, however, was found between FPL and the middle, ring and little finger FDP compartments (0.25±0.01). This difference might reflect the larger force contribution of the thumb-index finger pair relative to other thumb-finger combinations in five-digit grasping.




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