Firing patterns of low-threshold trapezius motor units in feedback-controlled contractions and vocational motor activities.
The study aimed to examine firing patterns of low-threshold trapezius motor units, with attention given to motor unit recruitment threshold. Different motor tasks were explored: shoulder elevation, arm movement in typing, and the motor response to mental stress. Contraction amplitudes in the range from 1 to 10% of the surface electromyographic (SEMG) signal at maximal voluntary contraction (1-10% EMGmax) were studied, representing the range of trapezius muscle activity commonly observed in daily living. Single motor unit activity was recorded by a quadrifilar fine-wire electrode. A surface electrode simultaneously recorded the SEMG signal. Low-threshold motor units showed a small increase in mean firing rate, from 10.5 to 12.5 pulses per second ( p<0.01), in constant-amplitude contractions when SEMG amplitude increased from <2% to >4% EMGmax. After the first few minutes, firing rates were similar for all motor units in a contraction, despite different recruitment thresholds. Firing rates of motor units with threshold <2% EMGmax were the same in constant-amplitude contractions, contractions with vocational arm movement, and contractions with imposed stress for SEMG amplitude at the same level. High-frequency firing patterns were observed in dynamic contractions, limited to bursts of up to a second duration. We conclude that low-threshold trapezius motor units have similar, stable firing rates in sustained contractions, independent of task and recruitment threshold, but with a small increase for increasing contraction amplitude.
Westad
C
C
Department of Industrial Economics and Technology Management, Norwegian University of Science and Technology, N-7491 Trondheim, Norway.
Mork
P J
PJ
Westgaard
R H
RH
D000200:Action Potentials / Q000502:physiology*; D000328:Adult; D000704:Analysis of Variance; D005246:Feedback / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009043:Motor Activity / Q000502:physiology*; D009119:Muscle Contraction / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*