Abstract
BACKGROUND: This study aimed to examine the independent and joint associations of chronic heat exposure and physical activity (PA) with the risk of developing knee osteoarthritis (KOA). METHODS: We analyzed data from 352,904 participants in the UK Biobank cohort, free of KOA at baseline. Chronic heat exposure was quantified as the proportion of days with daily maximum temperature > 27 °C during the 5 years before baseline and dichotomised at the 75th percentile. PA was assessed at baseline via the International Physical Activity Questionnaire and classified as low, medium, or high. Incident KOA (ICD-10, M17) was ascertained through linked health records. Cox proportional hazards models evaluated associations. Sensitivity analyses included alternative heat cut-offs, alternative heat windows, 1- and 2-year landmark analyses, and an accelerometer sub-cohort. RESULTS: Over a mean follow-up of 11.3 years, 23,146 KOA cases were identified. In fully adjusted models, high heat exposure was associated with a modestly lower KOA risk compared with low heat exposure (HR 0.90, 95% CI 0.87–0.93). Relative to high PA, both medium and low PA were associated with lower KOA risk (HR 0.93 for both). In joint analyses, high heat exposure was associated with lower KOA risk within each PA category; within the high-heat group, both medium and low PA showed similarly reduced risk relative to high PA. Findings were directionally consistent across sensitivity analyses. CONCLUSIONS: Chronic heat exposure was associated with a lower risk of incident KOA, and higher total weekly physical activity volume was associated with a higher KOA risk. In joint analyses, high heat exposure was associated with lower KOA risk within each physical activity category. CLINICAL TRIAL NUMBER: Not applicable. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12889-026-26608-w.