Only women report increase in pain threshold following fatiguing contractions of the upper extremity

The perception of pain in response to a noxious stimulus can be markedly reduced following an acute bout of exercise [exercise-induced hypoalgesia (EIH)]. Sex differences in EIH frequently occur after exercise but may be confounded by the sex differences in muscle fatigue. The purpose was to determine if sex differences in pain relief occur after an exercise protocol when muscle fatigue is similar for both young and older men and women. Pain perception of 33 men (15 young) and 31 women (19 young) was measured using a pressure pain stimulus on the left index finger before and after maximal velocity concentric contractions of knee extensors or elbow flexors (separate days). During the 2-min pressure pain test, participants verbally indicated the onset of pain (pain threshold) and reported pain intensity (0-10) every 20 s. Only women experienced an increase in pain threshold (30 +/- 27 to 41 +/- 32 s) following elbow flexor exercise (trial x sex: p = 0.03). Neither men nor women experienced an increase in pain threshold following knee extensor exercise, and pain ratings were unchanged after exercise with either limb (p > 0.05). The pain response to exercise was similar in young and older adults (trial x age: p > 0.05), despite older adults demonstrating greater fatigability than young adults for the elbow flexor and knee extensor exercise tasks. Under controlled conditions where muscle fatigue is similar, sex differences in EIH occur in young and older adults that is site specific (upper extremity). Only women experience EIH following acute single limb high-velocity contractions.