Pre-Exercise Factors Associated with the Magnitude of Exercise-Induced Hypoalgesia in Individuals with Knee Osteoarthritis: A Cross-Sectional, Observational Study

<b>Background</b>: The magnitude of exercise-induced hypoalgesia (EIH) varies across individuals with knee osteoarthritis (OA). Impaired EIH may limit the pain-relieving effects of exercise and reduce exercise adherence. This study aimed to identify key factors associated with EIH in knee OA. <b>Methods</b>: This cross-sectional study included 119 participants (mean age 68 ± 10) with knee OA. Pre-exercise assessments, including validated questionnaires and quantitative sensory testing were undertaken. EIH was evaluated by measuring pressure pain thresholds (PPT) at the knee and forearm before and after quadriceps isometric resistance exercise. Linear regression and mixed models were used to identify factors associated with the magnitude of EIH and sources of variance in EIH. <b>Results</b>: EIH was greater at the knee compared to the forearm (<i>p</i> < 0.01), with considerable inter-individual variability. Older age, less anxiety, and expecting less exercise-induced pain were associated with increased EIH (all <i>p</i> < 0.05). However, all measured variables explained <20% of the variance in EIH, with unobserved between-participant factors estimated to account for ≥45% additional variance. <b>Conclusions</b>: Age, pre-exercise anxiety, and pain expectations are associated with the magnitude of EIH after resistance exercise in people with knee OA. However, the contribution of these factors was modest, with much of the inter-individual variance in EIH remaining unexplained.