Gray matter density in the primary somatosensory cortex explains individual differences in the intensity of delayed onset muscle soreness

Imaging studies have shown that, within the primary somatosensory cortex (SI), increased gray matter density (GM) is associated with lower pain thresholds, (i.e. increased pain sensitivity), in healthy volunteers. However, reports in the literature also suggest GM decreases in SI is associated with persistent pain, while GM increases in SI is associated with pain habituation. To investigate this relationship, we examined the baseline relationship between SI-GM and lumbar pressure pain thresholds in 15 (mean age 22±2.4 years, 67% female) healthy individuals before completing a single session of unfamiliar eccentric exercise to the lumbar extensor muscles, and SI-GM with the intensity of delayed onset muscle soreness (DOMS) felt at 48 hours. Specifically, we correlated pressure pain thresholds to SI-GM and conducted a linear regression with 48-hour pain intensity as the dependent variable. Due to limited sample size, we choose step-wise regression to avoid creating an oversaturated model, while still trying to statistically control for potential co-founding factors. We considered age, sex, fear of pain, baseline pressure pain threshold, total cortical GM, and left SI-GM for inclusion into the step-wise regression. We used a liberal criterion of 0.1 to be entered and 0.2 to remain in the model. At baseline the mean (±SD) pressure pain threshold was 19.23 (±8.4) lbs and was negatively correlated with SI-GM (r=-0.37, p ≤ 0.09). At 48 hours, the mean (±SD) pain intensity was 15.13 (±12.54) mm on a 100mm line. Our final regression model included only SI-GM, (standardized β=-0.67, t=-3.26, p ≤ 0.01), which accounted for 41% of the variance in DOMS pain intensity at 48 hours (F1,13=10.65, p ≤ 0.01). Our results support the position that more SI-GM is associated with lower pressure pain threshold (higher pain sensitivity), but that following a standardized muscular injury, more SI-GM in associated with less pain experienced.