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This Article Full Text Full Text (PDF) All Versions of this Article: 583/3/1145    most recent jphysiol.2007.135640v1 Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Yu, W. S. Articles by Gandevia, S. C. Search for Related Content PubMed PubMed Citation Articles by Yu, W. S. Articles by Gandevia, S. C. Related Collections Integrative

¹ Prince of Wales Medical Research Institute, and University of New South Wales, Randwick 2031, Australia
² School of Physiotherapy, University of Sydney, Australia

The uncommonly good proprioceptive performance of the long flexor of the thumb, flexor pollicis longus (FPL), may add significantly to human manual dexterity. We investigated the forces produced by FPL single motor units during a weak static grip involving all digits by spike-triggered averaging from single motor units, and by averaging from twitches produced by intramuscular stimulation. Nine adult subjects were studied. The forces produced at each digit were used to assess how forces produced in FPL are distributed to the fingers. Most FPL motor units produced very low forces on the thumb and were positively correlated with the muscle force at recruitment. Activity in FPL motor units commonly loaded the index finger (42/55 units), but less commonly the other fingers (P < 0.001). On average, these motor units produced small but significant loading forces on the index finger (5.3% of their force on the thumb) with the same time-to-peak force as the thumb (50 ms), but had no significant effect on other fingers. However, intramuscular stimulation within FPL did not produce significant forces in any finger. Coherence at 2–10 Hz between the thumb and index finger force was twice that for the other finger forces and the coherence to the non-index fingers was not altered when the index finger did not participate in the grasp. These results indicate that, within the long-term coordinated forces of all digits during grasping, FPL motor units generate forces highly focused on the thumb with minimal peripheral transfer to the fingers and that there is a small but inflexible neural coupling to the flexors of the index finger.

(Received 1 May 2007; accepted after revision 16 July 2007; first published online 26 July 2007)
Corresponding author S. Gandevia: Prince of Wales Medical Research Institute, Easy Street, Randwick, New South Wales 2031, Australia. Email: s.gandevia{at}unsw.edu.au

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