Sudden leg weakness when walking may be related to the following reasons: 1. There may be patellar tenderness. This is often seen in patients who exercise too intensely, resulting in damage to the articular cartilage inside the patella and femur. This can lead to pain around the patella and a tenderness when walking up and down stairs. 2. There may be damage to the meniscus of the knee. Some patients may experience pain in the medial and lateral spaces of the knee joint and may experience weakness due to intense activity or prolonged heavy weight-bearing. 3. This group of patients has severe osteoarthritis due to long-term wear and tear of the knee joint. One of the symptoms is sudden weakness of the legs. 4, there may be an occupancy in the spinal canal leading to spinal stenosis, for example, the patient has a slipped lumbar vertebrae, which may lead to intermittent claudication, followed by a sudden weakness of the legs when walking.