Individual Fluctuations Of Exercise-induced Hypoalgesia

PURPOSE: A single session of physical exercise can reduce pain perception for up to 45 minutes. In a recent systematic review, we reported that this exercise-induced hypoalgesia (EIH) is not reliable in healthy adults and recommended that future studies ensure assessor blinding and strict control of source of variations. This study aimed to assess the reliability of EIH, taking those methodological adaptations into account. METHODS: We recruited 40 healthy males (18-30 years). Their pressure pain threshold (PPT) was measured at the rectus femoris muscle, forearm muscles, and tibia periosteum by a blind assessor before and after two identical cycling exercise sessions (25 minutes; 75% heart rate reserve; 1 week apart). EIH was defined as the Postexercise - Preexercise PPT. We used a linear mixed model to estimate EIH across sessions and computed intraclass correlation coefficients [ICC(2,k)] to assess reliability. RESULTS: We analyzed data from 39 participants (22.4 ± 2.8 years) and observed a significant increase of post vs pre exercise PPT only at the quadriceps (p < .001). EIH was not different between sessions. Despite reliable pre-exercise PPT at all sites (all ICCs >0.7), the reliability of EIH was poor, with ICC estimates of -0.05, -0.44, and -0.13 for the quadriceps, forearm, and tibia, respectively. CONCLUSIONS: Our results suggest that while EIH is consistent at a group level in an exercising body part, its reliability is poor. This may be due to an unknown fluctuation of the mechanism(s) driving EIH or its small effect size compared to the measurement error of PPT. Supported by: This work is supported by a FRS-FNRS FRIA grant, Belgium