Real-world exercise and disease activity in axial spondyloarthritis: does the association vary by age, BMI, or socioeconomic status?

Exercise is recommended in axial spondyloarthritis (axSpA), yet evidence on habitual exercise in routine care and whether its association with disease activity varies by age, body mass index (BMI), or socioeconomic position remains limited. We assessed the association between self-reported regular exercise during the previous year and disease activity (ASDAS) at the study visit. We performed a cross-sectional analysis from two multicentre registries (REGISPONSER and RESPONDIA), including 1233 patients with axSpA. The exposure was self-reported regular exercise in the preceding year. Missing data were handled using multiple imputation. We estimated the association between exercise and ASDAS using stabilised inverse probability of treatment weighting (IPTW) based on a propensity score model including age, sex, BMI, HLA-B27 status, and country (Spain vs. Latin America). Doubly robust augmented IPTW (AIPW) and sensitivity analyses using a clinically expanded confounder specification and sex-stratified models were also performed. We then explored whether the exercise–ASDAS association varied across age, BMI, and socioeconomic position (Graffar scale) by fitting weighted interaction models, using spline terms for age and BMI, and deriving marginal effects across the observed ranges and by Graffar levels. Regular exercise was associated with lower disease activity. In IPTW analyses, exercise was associated with a reduction in ASDAS of − 0.153 points (95% CI − 0.276 to − 0.031; p = 0.014). Results were consistent in doubly robust AIPW analyses (− 0.155, 95% CI − 0.277 to − 0.034; p = 0.012) and in a clinically expanded sensitivity analysis (IPTW: −0.157, 95% CI − 0.279 to − 0.035; AIPW: −0.158, 95% CI − 0.280 to − 0.036). Exploratory analyses suggested possible variation in the association across age, BMI, socioeconomic position, and sex, but global interaction tests were not statistically significant, and sex-stratified estimates in women were imprecise. Self-reported regular exercise during the previous year was associated with modestly lower ASDAS at the clinical visit, consistently across IPTW and doubly robust estimators. These findings are consistent with current recommendations supporting regular physical activity as a core component of axial spondyloarthritis management; however, the cross-sectional design, retrospective self-report of exercise, and the potential for residual confounding and reverse causation preclude causal inference. Not applicable