This study investigates the association between weight-bearing recreational physical activities and the risk of incident knee osteoarthritis, while examining whether this association varies based on lower-limb muscle mass (LMI). The Rotterdam Study, a large prospective cohort, included 5003 participants aged 45 years and older. Data on recreational physical activities, knee pain, and knee radiographs were collected at baseline and follow-up. The primary outcome was incident radiographic knee osteoarthritis, and the secondary outcome was symptomatic knee osteoarthritis defined by x-ray and knee pain questionnaire. Key findings: - Higher weight-bearing activity was associated with an increased odds of incident knee osteoarthritis (odds ratio [OR], 1.22; 95% CI, 1.10-1.35; P < .001), while non-weight-bearing activity was not (OR, 1.04; 95% CI, 0.95-1.15; P = .37). - In stratified analyses based on LMI tertiles, the association of weight-bearing activity with incident osteoarthritis was observed only among participants in the lowest LMI tertile (OR, 1.53; 95% CI, 1.15-2.04; P = .003), but not among those in the middle or high LMI tertile. The study suggests that weight-bearing activity may increase the risk of knee osteoarthritis, particularly in individuals with low LMI. This finding has important clinical implications for tailored physical activity recommendations to optimize benefits while minimizing the risk of developing osteoarthritis.This study investigates the association between weight-bearing recreational physical activities and the risk of incident knee osteoarthritis, while examining whether this association varies based on lower-limb muscle mass (LMI). The Rotterdam Study, a large prospective cohort, included 5003 participants aged 45 years and older. Data on recreational physical activities, knee pain, and knee radiographs were collected at baseline and follow-up. The primary outcome was incident radiographic knee osteoarthritis, and the secondary outcome was symptomatic knee osteoarthritis defined by x-ray and knee pain questionnaire. Key findings: - Higher weight-bearing activity was associated with an increased odds of incident knee osteoarthritis (odds ratio [OR], 1.22; 95% CI, 1.10-1.35; P < .001), while non-weight-bearing activity was not (OR, 1.04; 95% CI, 0.95-1.15; P = .37). - In stratified analyses based on LMI tertiles, the association of weight-bearing activity with incident osteoarthritis was observed only among participants in the lowest LMI tertile (OR, 1.53; 95% CI, 1.15-2.04; P = .003), but not among those in the middle or high LMI tertile. The study suggests that weight-bearing activity may increase the risk of knee osteoarthritis, particularly in individuals with low LMI. This finding has important clinical implications for tailored physical activity recommendations to optimize benefits while minimizing the risk of developing osteoarthritis.