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The Journal of Neurophysiology Vol. 82 No. 1 July 1999, pp. 501-504
Copyright ©1999 by the American Physiological Society
RAPID COMMUNICATION
1NeuroMuscular Research Center and Department of Biomedical Engineering, Boston University, Boston, Massachusetts 02215; and 2Division of Organization and Work Science, Norwegian University of Science and Technology, N-7491 Trondheim, Norway
Westgaard, R. H. and
C. J. de
Luca.
Motor Unit Substitution in Long-Duration Contractions of the
Human Trapezius Muscle. J. Neurophysiol. 82: 501-504, 1999.
We examined the activity pattern of low-threshold motor
units in the human trapezius muscle during contractions of 10 min duration. Three procedures were applied in sequence: 1)
static contraction controlled by maintaining a constant low level of the surface electromyogram (EMG)-detected root-mean-square
signal, 2) a manipulation task with mental
concentration, and 3) copying a text on a word
processor. A quadrifilar fine-wire electrode was used to record single
motor unit activity. Simultaneously, surface electrodes recorded the
surface EMG signal. During these contractions, low-threshold motor
units showed periods of inactivity and were substituted by motor units
of higher recruitment threshold. This phenomenon was not observed
during the first few minutes of the contraction. In several cases the
substitution process coincided with a short period of inactivity in the
surface EMG pattern. Substitution was observed in five of eight
experiments. These observations may be explained by a time-variant
recruitment threshold of motor units, sensitive to their activation
history and temporal variation in the activity patterns. We speculate that the substitution phenomenon protects motor units in postural muscles from excessive fatigue when there is a demand for sustained low-level muscle activity.
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