Endogenous modulation of pain – no apparent differences in the magnitude of inhibitory response between exercise-induced hypoalgesia and conditioned pain modulation

Background and aims: Recent findings suggest that there may be a link between the tolerance to stretch and endogenous inhibitory pain mechanisms which can be induced with paradigms of exercise-induced hypoalgesia (EIH) and conditioned pain modulation (CPM). Although both methods are known to increase pain tolerance in healthy adults activating a CPM response seems to induce a greater inhibitory response compared with exercise. The aim of the study was to compare the magnitude of inhibitory response induced by EIH and CPM. Methods: Nineteen healthy male participants were included in this randomized, repeated-measures cross-over study conducted in two sessions (EIH & CPM) on separate days. Passive knee extension range of motion (PROM) and pressure pain thresholds (PPT) were assessed before and after the induction of an EIH and CPM response. The EIH was assessed following a 3-minute isometric muscle contraction of the hand flexors and CPM was assessed following a cold pressor test. The difference in PROM and PPTs between measurements was extracted for data analysis. Results: A statistically significant increase in PROM (p < 0.034) and PPT (p < 0.027) was found after both EIH and CPM. However, no differences in PROM or PPTs were seen when comparing EIH and CPM (p = 1.00). Also, moderate and significant correlations in pain sensitivity responses were found between EIH and CPM (Rho = 0.562, p = 0.01). Conclusion: Engaging endogenous pain modulatory systems results in a significant increase in PROM and PPTs however, no differences were seen when comparing EIH and CPM.