Supervised exercise therapy (SET) is the recommended first treatment for patients with leg artery disease (peripheral arterial disease, PAD) causing pain when walking. However, approximately 40% of patients do not benefit meaningfully and go on to require a procedure to open the blocked arteries within three months. This study investigates whether body composition measurements - specifically the quality of muscle and the amount of belly fat - taken from a CT scan already performed as part of routine care, can identify before treatment begins which patients are unlikely to respond to exercise therapy. If confirmed, this approach would allow doctors to use information from a scan patients are already having, with no additional tests, to better match patients to the right treatment from the start.