Motor-unit activity during isometric and concentric- eccentric contractions of the human first dorsal interosseus muscle Howell, J. N., A. J. Fuglevand, M. L. Walsh, and B. Bigland-Ritchie. John B. Pierce Laboratory, 290 Congress Avenue, New Haven, CT 06519, Department of Biological Sciences, Ohio University, Athens, OH 45701.
APStracts 2:0163N, 1995.
SUMMARY AND CONCLUSIONS
1. Motor-unit activity was recorded with intramuscular, fine-wire electrodes during isometric, concentric, and eccentric activity in the human first dorsal interosseus (FDI) muscle. Twenty-one units from 11 subjects were sampled. 2. During isotonic cycles of shortening and lengthening, 18 of 21 units were recruited during the concentric phase, increased their discharge rates as the concentric movement progressed, then decreased their discharge rate during the eccentric phase, and were derecruited. 3. A different pattern of recruitment was observed in recordings from three units. These units were recruited during the eccentric phase, at a time when other units were decreasing their discharge rate or being derecruited. In two of the units selectively recruited during the eccentric phase, it was possible to determine their isometric thresholds, which were higher than units exhibiting the more common pattern of recruitment. 4. For two of the three units exhibiting selective recruitment during eccentric contraction, the unit was recorded simultaneously with different pairs of recording wires separated by 5 - 10 mm. Each discharge of these units was detected by both electrodes, making it unlikely that movement artifact was responsible for the initiation or cessation of discharge. 5. The recruitment patterns observed suggest that changes in the type or distribution of synaptic inputs to motoneurons during movement can, in some instances, override pre- and postsynaptic factors that shape recruitment order in isometric conditions. 

Received 5 December 1994; accepted in final form 22 May 1995.
APS Manuscript Number J757-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 30 May 1995.