Exploring the Mechanisms of Exercise-Induced Hypoalgesia Using Somatosensory and Laser Evoked Potentials

Abstract

Exercise-induced hypoalgesia is well described, but the underlying mechanisms are unclear. The aim of this study was to examine the effect of exercise on somatosensory evoked potentials, laser evoked potentials, pressure pain thresholds and heat pain thresholds. These were recorded before and after 3-min of isometric elbow flexion exercise at 40% of the participant's maximal voluntary force, or an equivalent period of rest. Exercise-induced hypoalgesia was confirmed in two experiments (Experiment 1-SEPs; Experiment 2-LEPs) by increased pressure pain thresholds at biceps brachii (24.3 and 20.6% increase in Experiment 1 and 2, respectively; both d > 0.84 and p < 0.001) and first dorsal interosseous (18.8 and 21.5% increase in Experiment 1 and 2, respectively; both d > 0.57 and p < 0.001). In contrast, heat pain thresholds were not significantly different after exercise (forearm: 10.8% increase, d = 0.35, p = 0.10; hand: 3.6% increase, d = 0.06, p = 0.74). Contrasting effects of exercise on the amplitude of laser evoked potentials (14.6% decrease, d = -0.42, p = 0.004) and somatosensory evoked potentials (10.9% increase, d = -0.02, p = 1) were also observed, while an equivalent period of rest showed similar habituation (laser evoked potential: 7.3% decrease, d = -0.25, p = 0.14; somatosensory evoked potential: 20.7% decrease, d = -0.32, p = 0.006). The differential response of pressure pain thresholds and heat pain thresholds to exercise is consistent with relative insensitivity of thermal nociception to the acute hypoalgesic effects of exercise. Conflicting effects of exercise on somatosensory evoked potentials and laser evoked potentials were observed. This may reflect non-nociceptive contributions to the somatosensory evoked potential, but could also indicate that peripheral nociceptors contribute to exercise-induced hypoalgesia.

Keywords: evoked potential; exercise; healthy subjects; pain rating; pain threshold.

Figure 1

Experimental procedures. (A,B) Show the order of procedures in Experiment 1 (SEPs) and Experiment 2 (LEPs), respectively, when exercise was performed first. In both experiments, evoked potentials were recorded on five occasions during electrical stimulation (Experiment 1) or laser heat stimulation (Experiment 2). Pressure pain thresholds were assessed before and after isometric exercise and before quiet rest in each experiment. Heat pain thresholds were assessed before and after isometric exercise and before quiet rest in Experiment 2 only. A 30 min wash out period was included to ensure possible exercise-induced alterations in pain were gone prior to commencing the next block of evoked potential recordings. This was confirmed by the re-assessment of PPTs (Experiment 1 and 2) and HPTs (Experiment 2 only) prior to the next block of evoked potential recordings. The order of exercise or quiet rest was counterbalanced across participants in each study.

Figure 2

Changes in pain threshold. Individual data for pressure pain thresholds (PPTs; left side of vertical dotted line) and the differences in PPTs for individual participants and the group (mean and 95% confidence interval; right side of vertical line) at m. biceps brachii in Experiment 1 (A) and Experiment 2 (C) and m. first dorsal interosseous in Experiment 1 (B) and Experiment 2 (D). Individual data for heat pain thresholds (HPTs; left side of vertical dotted line) and the differences in HPTs for individual participants and the group (mean and 95% confidence interval; right side of vertical line) at the forearm (E) and hand (F) in Experiment 2 are also shown. Δ baseline is the difference between the pre rest and pre exercise measures and Δ ex (exercise) is the difference between the pre exercise and post exercise measures. Data to the left of the vertical dotted line are plotted against the left-hand y-axis and data to the right of the vertical dotted line are plotted against the right-hand y-axis.

Figure 3

SEP and LEP grand averages for Cz. Somatosensensory evoked potentials recorded at Cz from 16 participants in Experiment 1 (SEPs, A,C,E on the left) and laser evoked potentials recorded at Cz from 16 participants in Experiment 2 (B,D,F on the right). These traces are the grand averages across participants of individual waveform averages from approximately 500 stimuli for the SEPs and from approximately 30 stimuli for the LEPs. Data are shown for SEPs and LEPs recorded during the modulation test in response to different intensities of stimulation (A,B) or immediately before and after exercise (C,D) or rest (E,F). For the modulation test, two stimulus intensities corresponding to either mild or moderate pain were randomly presented within the same sequence of 5 test blocks. For the SEPs, data are shown for 50 ms before and 450 ms following the stimulus onset; the stimulus artifact is visible on each plot and has been truncated for the illustration. For the LEPs, data are shown for 50 ms before and 950 ms following the stimulus onset; the vertical dashed lines represent stimulus onset.

Figure 4

Changes in evoked potential N2P2 amplitude. Each panel presents individual and group data (mean and 95% confidence interval) for the N2P2 evoked potential amplitude to the left side of vertical dashed line and individual and group differences (Δ; mean and 95% confidence interval) in evoked potential amplitude to the right side of vertical dashed line. SEP data from Experiment 1 are in the left panels (SEPs, A,C,E) and LEP data from Experiment 2 are in the right panels (LEPs, B,D,F). (A,B) Responses to mild and moderate (mod) pain stimuli recorded in the modulation blocks. (C,D) Responses recorded before (pre) and after (post) exercise. (E,F) Responses recorded before and after a period of rest. In each of these plots the zero-difference level on the right-hand y-axis is aligned to the group mean for the reference condition of moderate stimulation intensity (A,B), pre-exercise (C,D) or pre-rest (E,F). Data to the left of the vertical dashed line are plotted against the left-hand y-axis and data to the right of the vertical dashed line are plotted against the right-hand y-axis.

Figure 5

Pain ratings for electrical stimuli Experiment 1. Individual and group data (mean and 95% confidence interval) for ratings of pain intensity, pain unpleasantness and anxiety (left side of vertical dotted lines in each graph) before (pre) and after (post) exercise (left panels) or rest (right panels) during Experiment 1. Five ratings were averaged to give a single value for ratings of pain intensity and pain unpleasantness for the sets of electrical stimuli and 3 ratings were averaged to give a single value for anxiety. Individual and group differences (Δ; mean and 95% confidence interval) in ratings from pre to post exercise or rest are shown to the right side of the vertical dotted line in each graph. In each of these plots the zero-difference level on the right-hand y-axis is aligned to the group mean for the pre-exercise reference condition. Data to the left of the vertical dotted line are plotted against the left-hand y-axis and data to the right of the vertical dotted line are plotted against the right-hand y-axis.