Discharge behaviors of trapezius motor units during exposure to low and high levels of acute psychosocial stress

Abstract

This study investigated the effects of acute psychosocial stress on trapezius single motor unit discharge behaviors. Twenty-one healthy women performed feedback-controlled isometric contractions under conditions of low and high psychosocial stress in the same experimental session. Psychosocial stress was manipulated using a verbal math task combined with social evaluative threat that significantly increased perceived anxiety, heart rate, and blood pressure (P < 0.001). Motor unit discharge behaviors including the threshold and discharge rate at recruitment [7.7% (5.7%) maximal voluntary isometric contraction and 7.3 pulses per second (pps) (6.8 pps), P > 0.121, N = 103] and derecruitment [6.0% (4.4%) maximal voluntary isometric contraction and 6.5 pps (4.1 pps), P > 0.223, N = 99], the mean [11.3 pps (2.3 pps), P = 0.309, N = 106] and variability [2.5 pps (0.91 pps), P = 0.958, N = 106] of discharge rate, and the proportion of motor units exhibiting double discharges (21%, P = 0.446) did not change across stress conditions. Discharge rate modulation with changes in contraction intensity was highly variable and similar across stress conditions (P > 0.308, N = 89). Rate-rate modulation of concurrently active motor units was also highly variable (r = -0.84 to 1.00, N = 75). Estimates of DeltaF for motor unit pairs with rate-rate modulation >or=0.7 were positive and similar across stress conditions [4.7 pps (2.0 pps), P = 0.405, N = 16]. The results indicate that acute psychosocial stress does not alter trapezius motor unit discharge behaviors during a precisely controlled motor task in healthy women.

Figure 1

Example data from one subject in the low pre condition showing, from bottom to top, shoulder elevation force (solid line) and target force (dashed line), RMS surface EMG, raw intramuscular EMG, and the instantaneous discharge rate profiles of two concurrently active motor units expressed in pulses per second (pps). Recruitment and derecruitment thresholds were assessed as the magnitude of the RMS surface EMG at the point at which regular discharge of motor unit action potentials began and ceased, as indicated by the vertical dotted lines for the lower threshold reference motor unit. The discharge rates at recruitment and derecruitment were calculated as the mean instantaneous discharge rate over the first five action potentials following recruitment and the last five motor unit action potentials preceding derecruitment respectively, as indicated by the black data points in the discharge rate profiles. Motor unit discharge behavior throughout the contraction was characterized by the mean and range of instantaneous discharge rates, with variability assessed as the RMS error from a 5^th order polynomial fit to the discharge rate profile, as indicated for the reference motor unit. The PIC magnitude was estimated by ΔF, calculated as the difference between the discharge rate of the reference motor unit at the time of recruitment and derecruitment of the test motor unit. In this example the test motor unit was recruited when the reference motor unit was discharging at 12.3 pps, and derecruited when the reference motor unit was discharging at 7.3 pps, giving a ΔF of 5.0 pps (12.3 – 7.3 pps). The positive ΔF value indicates that the discharge rate of the reference motor unit was lower at test motor unit derecruitment than it was at test motor unit recruitment.

Figure 2

Mean arterial blood pressure, heart rate and perceived anxiety at the beginning of the experimental session (baseline) and after each experimental condition. Data points represent mean ± 95% CI. Asterisks indicate significant differences across conditions (P<0.001).

Figure 3

Panel A shows an example of consecutive double discharge from a single subject during the high stress condition. The top panel shows the intramuscular EMG, with the section outlined by vertical dotted lines depicted below on an expanded timescale. The bottom panel shows the instantaneous discharge rate of the motor unit discharge, expressed in pulses per second (pps). Panel B shows an example from a different subject during the high stress condition of a discrete jump in the discharge rate of one motor unit (upper panel) while a concurrently active motor unit (lower panel) shows no change in discharge rate.

Figure 4

An example from a single subject of the relation between the smoothed discharge rate profile and RMS surface EMG for two concurrently active motor units during the low pre condition. Discharge rate modulation was calculated as the slope of this relation for the ascending (filled circles) and descending (open circles) limbs of the contraction.

Figure 5

Panel A shows the discharge rate modulation on the ascending (filled boxes) and descending (open boxes) limbs of the contraction for the low pre, high and low post conditions. The boundaries of the boxes represent the 25^th and 75^th percentiles, while the line within the box represents the median value. Error bars above and below the box indicate the 90th and 10th percentiles respectively, and individual data points outside these values are represented by filled circles. Panel B shows the rate-rate modulation (r) of concurrently active motor unit pairs from low pre (triangles), high (circles) and low post (squares) test conditions. The dashed horizontal line indicates rate-rate modulation of 0.7, the minimum value required for inclusion in ΔF calculations (see text for more details).