Muscle activation during unilateral and bilateral contractions
studied with motor nerve and cortical stimulation.
R., Herbert, D. and Gandevia S. C.
Prince of Wales Medical Research Institute, University of New South
Wales and Faculty of Health Sciences, University of Sydney,
Australia
APStracts 2:0539A, 1995.
Voluntary muscle activation was measured with twitch interpolation in
11 subjects during attempted maximal voluntary contractions (MVCs) of
the right thumb adductor muscles either in isolation ("thumb
alone"), or as they simultaneously performed MVCs of left thumb
adductors or left elbow flexors ("both thumbs" or "thumb
and elbow", respectively). During "thumb alone"
contractions, median voluntary activation of the right thumb
adductors was 90.3%, and subjects fully activated the thumb adductors
on 22% of all contractions. Transcranial magnetic stimulation of the
cortex during maximal voluntary efforts produced small, twitch-like
force increases, suggesting that at least part of the voluntary
activation failure was attributable to sub-optimal corticospinal
drive. Maximal voluntary force produced by the right thumb adductors
in the 3 conditions differed by less than 2% (p = 0.21), and the
ability to activate the thumb adductors in the "both thumbs"
condition was only marginally less than during "thumb alone"
contractions (median 88.6%, p = 0.004). Thus subjects are usually
unable to fully activate their thumb adductors with maximal voluntary
efforts, and simultaneous maximal contractions of contralateral
muscle groups have little effect on this ability.
Received 29 June 1995; accepted in final form 28 November 1995.
APS Manuscript Number A703-5.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 12 December 95