Discharge rate modulation of trapezius motor units differs for voluntary contractions and instructed muscle rest

Abstract

This study examined discharge rate modulation at respiratory (0-0.5 Hz) and beta (16-32 Hz) frequencies in trapezius motor units active during voluntary contractions and during periods of instructed rest under conditions of low and high psychosocial stress. In separate sessions, single motor unit activity was recorded from the trapezius muscle of healthy women during low-intensity voluntary contractions and during periods of instructed muscle rest that followed voluntary contractions. The level of psychosocial stress during periods of instructed muscle rest was manipulated using a verbal math task combined with social evaluative threat which increased perceived anxiety, heart rate, and blood pressure (P ≤ 0.002). Discharge rate modulation was quantified by the mean power of motor unit discharge rate profiles within frequency bands of interest. Under low stress conditions, motor units active during instructed rest had greater power at 0-0.5 Hz (P = 0.002) and less power at 16-32 Hz (P = 0.009) compared to those active during voluntary contraction. Exposure to the stressor increased the amount of motor unit activity during instructed rest (P = 0.021) but did not alter the power of discharge rate modulation at 0-0.5 Hz (P = 0.391) or 16-32 Hz (P = 0.089). These results indicate that sustained motor unit activity during periods of instructed muscle rest has a lesser contribution from inputs at beta frequencies and a greater contribution from inputs at respiratory frequencies than present during low-intensity voluntary contractions. Furthermore, increases in motor unit activity when exposed to stressors during periods of instructed rest are not caused by changes in inputs at respiratory or beta frequencies.

Figures

Fig. 1

Representative example of the procedure used to identify minimum motor unit discharge rates during voluntary contractions. The target force was initially set to a level that resulted in regular discharge of an isolated motor unit and was then reduced every 30 s until motor unit discharge was no longer observed. The lowest contraction level at which regular motor unit discharge was observed (indicated by vertical dashedlines) was selected for analysis. In this example, the contraction level corresponding to the minimum discharge rate was 1.9% maximum, and the subject could not reduce the contraction intensity further without fully relaxing, whereby the motor unit ceased to discharge

Fig. 2

Representative example of the procedure used to quantify the amount of motor unit activity present during instructed muscle rest. The top two panels show the intramuscular EMG recorded in the 50 s following a voluntary contraction under low stress (left) and high stress (right) conditions in the same subject. The bottom panel shows an enlarged view with greater time resolution (1 s duration) for the same signal, enabling individual motor unit action potentials (MUAP) to be identified. Each time the EMG signal crossed a threshold that was manually positioned to exceed baseline noise (horizontal dashed lines) a MUAP was identified, as indicated by the event raster above each plot. The amount of motor unit activity was quantified as the total number of MUAP events identified in the rest period, divided by the duration of the rest period (expressed as MUAP/s)

Fig. 3

The mean (a) and coefficient of variation (b) of discharge rate for motor units active during periods of instructed muscle rest and during low-amplitude voluntary contractions performed under low stress conditions. Error bars represent 95% confidence intervals

Fig. 4

a Representative examples of power spectra derived from the instantaneous discharge rate profiles of motor units active during instructed muscle rest (light gray line) and voluntary contraction (dark gray line) in the same subject. Dashed vertical lines indicate the frequency bands over which the mean power was quantified. Note that peaks in the power spectra at 8–12 Hz which correspond to mean dis charge rates are shifted toward higher values for the voluntary contraction. b, c Group averages for the mean power at respiratory (0–0.5 Hz) and beta (16–32 Hz) frequencies. Error bars represent 95% confidence intervals. d A representative example of the power spectra derived from respiratory chest wall movement for the same subject as in panel A

Fig. 5

a Group average for the amount of motor unit activity (MUAP/s) present during instructed muscle rest under low and high stress conditions. Error bars represent 95% confidence intervals. b–d Relations between the change in the amount of motor unit activity and the change in blood pressure (b), heart rate (c) and state anxiety (d) from low to high stress conditions

Fig. 6

The mean (a) and coefficient of variation (b) of discharge rate, and the mean power at respiratory (0–0.5 Hz) (c) and beta (16–32 Hz) (d) frequencies for motor units active during periods of instructed muscle rest during low and high stress conditions. Error bars represent 95% confidence intervals