Differential Effects Of Exercise Induced Muscle Damage On Multiple Noxious Stimuli

Abstract

Many clinical musculoskeletal pain conditions are characterized by chronic inflammation. Inflammation leads to the release of inflammatory cytokines that activate or sensitizes nociceptors leading to pain sensitivity. An unresolved issue is whether inflammation affects all nociceptors in a similar manner. Exercise induced muscle damage in the days following a bout of eccentric exercise has been proposed as a valid model of simulating clinical inflammatory pain to be used in healthy samples. PURPOSE: We sought to induce localized inflammation using eccentric exercise to test various painful stimuli (pressure, thermal, and electrical) regarding an individual’s pain sensitivity in a limb that was eccentrically exercised versus a limb that was not exercised. METHODS: 18 healthy participants (9F, age: 24.6 ± 3.3) completed baseline pressure pain thresholds (PPT), thermal pain threshold (TPT), and nociceptive flexion reflex (NFR) in both legs and again 48 hours after a muscle damaging protocol. The participants performed a minimum of 6 rounds of 10 eccentric leg extension exercises to induce muscle damage and localized inflammation in the right quadriceps only. RESULTS: Significant main effect of time for PPT (%diff; -58.9 ± 23.1; p = 0.02, ηp2 = 0.28) but no main effect was observed for limb (%diff; -15.5 ± 23.9; p = 0.53, ηp2 = 0.02). In contrast there was a significant interaction between time and limb (p < 0.001, ηp2 = 0.47) whereby post hoc correction revealed participants had lower PPT’s in the right leg only after the damage protocol (%diff; -105.9 ± 29.2; p = 0.002). No main effects or interactions were observed for TPT and NFR. CONCLUSION: Those with chronic inflammatory pain usually have an increased sensitivity to pressure, thermal, and electrical stimuli whereas our sample with acute inflammation only had sensitivity to mechanical pain. Exercise induced inflammation may reflect a peripheral sensitivity localized to the damage muscle rather than a global sensitivity that those with chronic pain display.