Exercise-induced hypoalgesia in healthy males is best explained by mechanisms selectively affecting deep-tissue pain sensitivity within exercising body parts.

Background: The mechanisms driving the transient reduction in pain sensitivity following a single bout of physical exercise-a phenomenon known as exercise-induced hypoalgesia (EIH)-remain unclear despite proposed contributions from central and peripheral processes. So far it is unknown whether EIH selectively modulates the perception of specific types of nociceptive stimuli and whether this modulation is confined to exercising limbs or generalized to the entire body. Methods: In 38 healthy males (18-30 years) the effects of a single session of moderate/high-intensity cycling exercise (25 min at 75% of the heart rate reserve) were assessed against a control condition (25 min of light cycling exercise) on the sensitivity to stimuli preferentially activating skin mechano- and heat-sensitive nociceptors versus deep-tissue nociceptors, applied to the exercising lower limbs versus the non-exercising upper limbs. Pressure pain thresholds (PPT), heat pain thresholds (HPT), cold detection thresholds (CDT), and mechanical pinprick ratings (PP), as well as auditory detection thresholds (ADT) serving as control, were recorded before and immediately after the exercise and control conditions. Results: Linear mixed models revealed that exercise increased the PPT selectively at the exercising limb. No significant effects of exercise were found for other types of stimuli, except for a reduction of CDT at the exercising limb, which was not significant when controlling for skin temperature. Conclusions: These findings suggest that processes preferentially modulating deep-tissue nociceptor sensitivity within exercising muscles contribute to EIH in young healthy males.