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

Abstract

Objectives 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. Methods 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 3 min 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). Results 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: 0 kPa; 95% CI: −51 to 52; Contralateral thigh: −6 kPa; 95% CI: −42 to 30; neck/shoulder: 19 kPa; 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). Conclusions Pain in the area of an exercising muscle did not reduce local or systemic EIH responses. Trial registration number NCT04354948.

Acute exercise of painful muscles does not reduce the hypoalgesic response in young healthy women – a randomized crossover study

“…Experimental thigh muscle pain was induced by an intramuscular injection of 1 mL sterile hypertonic saline (5.8 %). The injection was administered in the participant's dominant medial vastus muscle, 20 cm proximally on a line from the centre of the basis of patella towards the umbilicus [19] using a 1 mL syringe with a disposable needle (27G × 1 1/2″, 0.4 mm × 40 mm). Isotonic saline (1 mL, 0.9 %) was used as a non-painful control injection [19].…”

“…The injection was administered in the participant's dominant medial vastus muscle, 20 cm proximally on a line from the centre of the basis of patella towards the umbilicus [19] using a 1 mL syringe with a disposable needle (27G × 1 1/2″, 0.4 mm × 40 mm). Isotonic saline (1 mL, 0.9 %) was used as a non-painful control injection [19]. Injections (painful or non-painful depending on the randomization order) were administered by the same researcher (AM), who was not blinded to group allocation.…”

“…PPT was defined as the first time the pressure went from being a pressure to first becoming painful [29], at which point the participants pushed the wired button and PPT was recorded. PPT was assessed over three pre-defined locations based [19]: (1) The dominant medial vastus muscle, 15 cm proximal to the basis of patella in a line aiming at the umbilicus (5 cm distally from the injection site); (2) The non-dominant medial vastus muscle as described for the dominant leg;…”

“…(3) The non-dominant upper trapezius muscle, 10 cm from the acromion on a horizontal line towards the spinous process of the 7th cervical vertebra. Three rounds of PPT recordings were conducted over approximately 3-5 min depending on the individual threshold, separating assessment of the same site by at least 20 s, and the average of each location was used for analysis [19,30,31]. All PPT assessments were conducted by the same assessor (HL) who was blinded to the group allocation.…”

“…Several studies across different painful conditions have observed reduced EIH responses immediately after exercises performed with the painful body region [16][17][18]. Recently, Hansen and colleagues [19] investigated the effect of experimental muscle pain in the exercising muscle on the EIH response compared to exercise in a non-painful muscle in pain free individuals. The local and remote EIH responses were comparable between exercises at painful and non-painful muscles; however, the interpretation of the results was limited by the fact the participants exercised both the painful and non-painful thigh simultaneously using a wall-squat.…”