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First published online on June 28, 2002.
 Copyright © 2002 by The Physiological Society
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Received May 5, 2002
Accepted after revision May 30, 2002

Distribution of the forces produced by motor unit activity in the human flexor digitorum profundus

S.L. Kilbreath1*, R.B. Gorman2, J. Raymond3, and S.C. Gandevia2

1 School of Physiotherapy, University of Sydney, PO Box 170, Lidcombe NSW, Australia 1825
2 Prince of Wales Medical Research Institute, University of New South Wales, Sydney, Australia
3 Rehabilitation Research Centre, University of Sydney, Australia

* To whom correspondence should be addressed. E-mail: s.kilbreath{at}cchs.usyd.edu.au.

In humans, the multi-tendoned muscle, flexor digitorum profundus (FDP), produces forces that flex the four distal interphalangeal joints of the fingers. We determined whether the force associated with activity in a single motor unit in the FDP was confined to a single finger or distributed to more than one finger during a natural grasp. The discharge of single low-threshold motor units (n = 69) was recorded at sites across the muscle during weak voluntary grasping involving all fingers and spike-triggered averaging of the forces under each of the finger pads was used to assess the distribution pattern. Spike-triggered averaging revealed that time-locked changes in force occurred under the 'test' finger (that finger on which the unit principally acted) as well as under the 'non-test' fingers. However, for the index-, middle- and ring-finger units, the changes in force under non-test fingers were typically small (< 20 % of those under the test finger). For little-finger units, the mean changes in force under the adjacent ring finger were large (>50 % of those under the test finger). The distribution of forces by little-finger units differed significantly from that for each of the other three fingers. Apart from increases in force under non-test fingers, there was occasionally unloading of adjacent fingers (22/267 combinations), usually affecting the index finger. The increases in force under the test finger correlated significantly with the background force for units acting on the middle, ring and little fingers. During a functional grasp, the activity of single units in the FDP allows for a relatively selective control of forces at the tips of the index, middle and ring fingers, but this is limited for little-finger units.




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