The weakness of the knee when climbing stairs is due to the following reasons: 1. It is often caused by overuse of the knee joint, or cold in the knee joint, which then causes damage to the articular cartilage associated with the patella or femur. In the process of climbing stairs, the high pressure load on the patellofemoral joint may lead to pain in the original damaged area. 2. It may be caused by meniscal damage to the knee joint, such as damage to the meniscus due to long-term wear and tear, or tearing during strenuous activity, and there will be significant pain here. The pain can be even more pronounced when the patient squeezes this area during stair climbing. This pain induces atrophy of the muscles around the joint and consequently a feeling of weakness.3. Similar symptoms can occur due to degeneration of the knee joint, such as when there is osteoarthritis in the knee joint.