Delayed Hypoalgesia to a Heat Stimulus is Induced by Eccentric Exercise and is Greater for Red- than Dark-Haired Women

Abstract Objectives: This study sought to examine the potential delayed effect of eccentric exercise on pain perceptions of heat stimuli presented to red-haired and dark-haired women. Methods: Healthy college-aged women capable of physical exercise were sought for participation in the study. They were to assess pain intensity and unpleasantness of thermal stimuli [at 45, 47 and 49 °C] applied to their forearm and posterior lower leg before and 24 and 48 h after 18 eccentric actions of the non-dominant elbow flexors at an intensity of maximal voluntary concentric contraction strength. Women of similar age and ability served as controls and the exercise for the controls (n=6) was concentric exercise, resulting in little or no muscle injury. Results: Eighteen healthy adult women participated. Twelve [five with bright red hair and five with dark hair] subjects performed eccentric exercise. At 24- and 48-h after the eccentric exercise, the volume of the exercised arm increased by 2.2% [±1.5%] and by 5.0% [±2.2%], arm stiffness increased as evidenced by decreases in elbow range of motion of 8.3% [±6.8%] and 8.3% [±6.8%] and isometric arm strength decreased by 29.6% [±14.6%] and 17.3% [±14.0%], respectively. Analysis of variance revealed that compared to controls, pain intensity and unpleasantness were reduced by about five percent 24 and 48 h after eccentric exercise, except for the absence of an effect at 45 °C for unpleasantness. From baseline to 24 h post-exercise, significantly larger reductions in pain intensity [−17.0 versus −4.5%] and pain unpleasantness [−19.0 versus 2.3%] were found for red-haired compared to dark-haired women. Conclusion: Research summarized in this study documents eccentric exercise-induced hypoalgesia 24–48 h post-exercise that was greater in women with red hair, a prevalent phenotype among those with malfunctioning melanocortin-1 receptors.