Hypoalgesia after exercises with painful vs. non-painful muscles in healthy subjects – a randomized cross-over study

Abstract

Exercise-induced hypoalgesia (EIH) is a decrease in the pain sensitivity after exercise. Individuals with chronic pain show less EIH after one exercise session compared with pain-free individuals possibly due to pain in exercising muscles. The primary aim of this randomized controlled cross-over study was to compare the EIH response at the exercising thigh muscle following exercises performed with painful vs. non-painful muscles. Secondary aims were to explore if a reduced EIH response was confined to the painful muscle, and whether the muscle pain intensity and the EIH responses were negatively associated. In two sessions, 34 pain-free participants received a painful (hypertonic saline, 5.8%) injection and a control (isotonic saline, 0.9%) injection in the right thigh muscle before performing a 3min isometric wall squat exercise. Pressure pain thresholds (PPTs) were assessed at both thighs and the left neck/shoulder at baseline, after injections and after exercise. Pain intensities in the thighs were rated on numerical rating scales (NRS: 0-10). Hypertonic saline induced moderate thigh pain at rest (NRS: 4.6±2.1) compared to the control injection (NRS: 0.3±0.4; p<0.001). EIH at the thighs and neck/shoulder were not different between sessions (Injected thigh: 0kPa; 95% CI:-51 to 52; Contralateral thigh:-6kPa; 95% CI:-42 to 30; neck/shoulder: 19kPa; 95% CI:-6 to 44). No significant associations between pain intensity ratings immediately after the Painful injection and EIH responses at any assessment sites were found (right thigh: β=0.08, 95% CI:-12.95 to 20.64, p=0.64, left thigh: β=-0.33, 95% CI:-27.86 to 0.44, p=0.06; neck/shoulder: β=-0.18, 95% CI:-15.11 to 4.96, p=0.31). Pain in the area of an exercising muscle did not reduce local or systemic EIH responses. NCT04354948.