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The Journal of Neurophysiology Vol. 82 No. 5 November 1999, pp. 2271-2283
Copyright ©1999 by the American Physiological Society
Prince of Wales Medical Research Institute, University of New South Wales, Randwick, New South Wales 2031; and School of Physiotherapy, University of Sydney, Lidcombe, New South Wales 2141, Australia
Herbert, R. D. and
S. C. Gandevia.
Twitch Interpolation in Human Muscles: Mechanisms and
Implications for Measurement of Voluntary Activation. J. Neurophysiol. 82: 2271-2283, 1999. An
electrical stimulus delivered to a muscle nerve during a maximal
voluntary contraction usually produces a twitchlike increment in force.
The amplitude of this "interpolated twitch" is widely used to
measure voluntary "activation" of muscles. In the present study, a
computer model of the human adductor pollicis motoneuron pool was used
to investigate factors that affect the interpolated twitch. Antidromic
occlusion of naturally occurring orthodromic potentials was modeled,
but reflex effects of the stimulus were not. In simulations, antidromic
collisions occurred with probabilities of between ~16% (in early
recruited motoneurons) and nearly 100% (in late recruited
motoneurons). The model closely predicted experimental data on the
amplitude and time course of the rising phase of interpolated twitches
over the full range of voluntary forces, except that the amplitude of
interpolated twitches was slightly overestimated at intermediate
contraction intensities. Small interpolated twitches (4.7% of the
resting twitch) were evident in simulated maximal voluntary
contractions, but were nearly completely occluded when mean peak firing
rate was increased to ~60 Hz. Simulated interpolated twitches did not
show the marked force drop that follows the peak of the twitch, and
when antidromic collisions were excluded from the model interpolated
twitch amplitude was slightly increased and time-to-peak force was
prolonged. These findings suggest that both antidromic and reflex
effects reduce the amplitude of the interpolated twitch and contribute
to the force drop that follows the twitch. The amplitude of the
interpolated twitch was related to "excitation" of the motoneuron
pool in a nonlinear way, so that at near-maximal contraction
intensities (>90% maximal voluntary force) increases in excitation
produced only small changes in interpolated twitch amplitude. Thus
twitch interpolation may not provide a sensitive measure of
motoneuronal excitation at near-maximal forces. Increases in the
amplitude of interpolated twitches such as have been observed in
fatigue and various pathologies may reflect large reductions in
excitation of the motoneuron pool.
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