Association of Pain During Exercise With Exercise-Induced Hypoalgesia in People With Knee Osteoarthritis

A paradoxical relationship between pain during exercise and the hypoalgesic effect of exercise has not been studied well in the knee osteoarthritis (OA) population. We sought to investigate the relation of pain evoked during exercise to exercise-induced hypoalgesia (EIH) and to determine if the efficiency of conditioned pain modulation (CPM), a proxy of the descending pain inhibitory system, mediates this relationship in people with knee OA.

We used baseline data from two clinical trials for people with symptomatic knee OA (n = 68). The maximum pain rating (0-10) during a series of knee exercises was defined as the outcome. EIH was assessed as an increase (ie, improvement) in the pressure pain threshold (PPT) after a bout of exercises. Efficient CPM was defined as an increase (ie, improvement) in PPT after a painful conditioning stimulus (forearm ischemia). We performed a causal mediation analysis to examine the association between pain during exercise and EIH as well as the mediating role of CPM efficiency on the relation of pain during exercise with EIH.

People with knee OA who had at least a one-unit increase in pain with exercise were 43% more likely (odds ratio [OR] 1.43, 95% confidence interval [CI] 1.05-1.94) to experience subsequent EIH than those without pain increase. The efficiency of CPM did not mediate the relationship between pain during exercise and EIH (OR 1.00, 95% CI 0.96-1.04).

Our finding suggests that some amount of discomfort or pain during exercise may have beneficial analgesic effects; however, this is not likely via activation of the descending pain inhibitory system.