Corrugator Muscle Activity Associated with Pressure Pain in Adults with Neck/Shoulder Pain

Abstract

Background and Objectives: No studies have reported corrugator muscle activity associated with pain in people with pain. This study aimed to develop an objective pain assessment method using corrugator muscle activity with pressure pain stimulation to the skeletal muscle. Methods: Participants were 20 adults (a mean ± SD age of 22.0 ± 3.1 years) with chronic neck/shoulder pain. Surface electromyography (sEMG) of corrugator muscle activity at rest (baseline) and without and with pressure pain stimulation applied to the most painful tender point in the shoulder was recorded. Participants evaluated the intensity of the neck/shoulder pain and the sensory and affective components of pain with pressure stimulation using a visual analogue scale (VAS). The percentages of integrated sEMG (% corrugator activity) without and with pressure pain stimulation to the baseline integrated sEMG were compared, and the relationships between the % corrugator activity and the sensory and affective components of pain VAS scores were evaluated. Results: Without pressure stimulation, an increase in corrugator muscle activity due to chronic neck/shoulder pain was not observed. The % corrugator activity with pressure pain stimulation was significantly higher than that without stimulation (p < 0.01). A significant positive correlation between corrugator muscle activity and the affective components of pain VAS scores with pressure stimulation was found (ρ = 0.465, p = 0.039) and a tendency of positive correlation was found for the sensory component of pain VAS scores (ρ = 0.423, p = 0.063). Conclusions: The increase in corrugator muscle activity with pressure pain stimulation to the tender point in adults with chronic neck/shoulder pain was observed, although increased corrugator muscle activity resulting from the chronic neck/shoulder pain was not. These findings suggest that corrugator muscle activity with pressure pain stimulation can be a useful objective indication for tender point sensitivity assessment in the skeletal muscle with pain.

Keywords: algometer; corrugator muscle; facial expression; neck/shoulder pain; pain; pain assessment; pressure pain; visual analogue scale.

Figure 1

Experimental setting. Participants on a massage chair with electrodes attached to the forehead to record corrugator muscle activity.

Figure 2

The ensemble average of rectified surface electromyography (sEMG) of corrugator muscle activity (a) without (blue) and (b) with pressure pain stimulation (orange) to the tender point for 5 s in 20 chronic neck/shoulder pain participants. The means of rectified sEMGs of both sides of the corrugator muscles in each participant, which were generated via rectified sEMGs converted from bilateral sEMGs, were used to determine the ensemble average in order to depict the average activity curve of the corrugator muscle. The vertical and horizontal axes represent muscle activity and time, respectively.

Figure 3

Corrugator muscle activity in 20 chronic neck/shoulder pain participants without (blue) and with pressure pain stimulation (orange). The “% corrugator activity” indicates the percentage of the integrated surface electromyography (sEMG) without and with pressure pain stimulation to the baseline integrated sEMG. The bottom, middle, and top lines in the boxes correspond to the 25th, median, and 75th percentiles, respectively. The whiskers extend from the 10th to 90th percentiles. The circles indicate arithmetic means. There was a significant difference between without and with pressure pain stimulation. ** p < 0.01.

Figure 4

(a) Correlations between the intensity of sensory and affective components of pain with pressure pain stimulation, and correlations between the intensity of (b) sensory and (c) affective components of pain with pressure pain stimulation and chronic neck/shoulder pain on visual analogue scale (VAS). There were significant positive correlations between them. The uppermost dot represents two data points in (a).

Figure 5

Correlation between corrugator muscle activity and the intensity of (a) chronic neck/shoulder pain, (b) sensory component of pain with pressure pain stimulation, and (c) affective component of pain with pressure pain stimulation on visual analogue scale (VAS). “% corrugator activity” indicates the percentage of the integrated surface electromyography (sEMG) without and with pressure pain stimulation to the baseline integrated sEMG. A significant positive correlation was found between corrugator muscle activity and the intensity of the affective component of pain.