Delayed-Onset Muscle Soreness Alters Mechanical Sensitivity, but Not Thermal Sensitivity or Pain Modulatory Function

Abstract

Introduction: Many clinical musculoskeletal pain conditions are characterized by chronic inflammation that sensitizes nociceptors. An unresolved issue is whether inflammation affects all nociceptors in a similar manner. Exercise-induced muscle damage (EIMD) has been proposed as a model for simulating clinical inflammatory pain in healthy samples. We sought to test the effect of EIMD on various painful stimuli (pressure and thermal), central pain processing (via the nociceptive flexion reflex) and endogenous pain modulation via conditioned pain modulation and exercise-induced hypoalgesia.

Methods: Eighteen participants (9F, age: 24.6 ± 3.3) were recruited for repeated measures testing and each completed pain sensitivity testing prior to and 48 hours after an eccentric exercise protocol. The participants performed a minimum of 6 rounds of 10 eccentric knee extension exercises to induce muscle damage and localized inflammation in the right quadriceps. Force decrements, knee range-of-motion, and delayed onset muscle soreness (DOMS) were used to quantify EIMD.

Results: There was a significant main effect of time for pressure pain (%diff; -58.9 ± 23.1; p = 0.02, ηp2 = 0.28) but no significant 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 participants had lower pressure pain sensitivity in the right leg only after the damage protocol (%diff; -105.9 ± 29.2; p = 0.002).

Discussion: Individuals with chronic inflammatory pain usually have an increased sensitivity to pressure, thermal, and electrical stimuli, however, our sample, following muscle damage to induce acute inflammation only had sensitivity to mechanical pain. Exercise induced inflammation may reflect a peripheral sensitivity localized to the damaged muscle rather than a global sensitivity like those with chronic pain display.

Keywords: acute inflammation; exercise induced muscle damage; pain modulation; pain sensitivity.

Figure 1

Schematic timeline of experimental visits 2 and 3. %diff – percent difference between pre and post pressure pain thresholds. CPM – Conditioned pain modulation. EIH – Exercise induced hypoalgesia. PPT – Pressure pain thresholds. HPT – Heat pain thresholds. NFR – Nociceptive Flexion Reflex. MVC – Maximal voluntary contraction.

Figure 2

Differences in pain measures from pre damage to post damage in damaged and control legs for pressure pain thresholds (A), thermal pain thresholds (B), and the nociceptive flexion reflex (C). Data from men and women are collapsed and displayed together since no sex difference was observed. *Denotes significant difference between limbs. Values are mean ± SD.

Figure 3

Magnitude, expressed as a percent change, of EIH (A) and CPM (B) in the control and damaged limbs before (Pre) and two days after EIMD (Post). Data from men and women were collapsed together since no sex difference was observed. ^#Denotes a significant main effect for leg with EIH being larger in the damaged leg. *Denotes significant difference between limbs. Values are mean ± SD.