Objectives: This study describes the longitudinal association between objectively assessed physical activity (PA) and knee structural change measured using MRI. Methods: 405 community-dwelling adults aged 51-81 years were measured at baseline and approximately 2.7 years later. MRI of the right knee at baseline and follow-up was performed to evaluate bone marrow lesions (BMLs), meniscal pathology, cartilage defects, and cartilage volume. PA was assessed at baseline by pedometer (steps/day). Results: Doing =10 000 steps/day was associated with BML increases (RR 1.97, 95 CI 1.19 to 3.27, p=0.009). Participants doing =10 000 steps/day had a 1.52 times (95 CI 1.05 to 2.20, p=0.027) greater risk of increasing meniscal pathology score, which increased to 2.49 (95 CI 1.05 to 3.93, p=0.002) in those with adverse meniscal pathology at baseline. Doing =10 000 steps/day was associated with a greater risk of increasing cartilage defect score in those with prevalent BMLs at baseline (RR 1.36, 95 CI 1.03 to 1.69, p=0.013). Steps/day was protective against volume loss in those with more baseline cartilage volume but led to increased cartilage loss in those with less baseline cartilage volume. (p=0.046 for interaction). Conclusions: PA was deleteriously associated with knee structural change, especially in those with pre-existing knee structural abnormalities. This suggests individuals with knee abnormalities should avoid doing =10 000 steps/day. Alternatives to weight-bearing activity may be needed in order to maintain PA levels required for other aspects of health.