Soreness and weakness of leg bends may be synovitis, meniscus injury, popliteal cyst, joint effusion, and so on. 1. Synovitis: the cause of synovitis, mainly due to fatigue, cold, osteoarthritis secondary to synovial hyperplasia and hypertrophy, joint cavity with fluid, so there will be soreness and swelling, serious patients will appear joint activity is limited, can not squat up. 2. Meniscus injury: it is caused by knee trauma or joint degeneration, if the injury is serious, it will cause pain and weakness of the joints, softness when walking, and popping when flexion and extension activities, which can be diagnosed clearly through the MRI film of the knee joint. 3. Popliteal cyst: in the back side of the knee joint, if the cyst is bigger and compresses the blood vessels and nerves, it can cause soreness and weakness. 4. Joint effusion: joint effusion caused by various reasons, such as too much exercise intensity or trauma, joint effusion increases and localized soreness and weakness occurs. Therefore, the causes of leg curvature soreness and weakness are mainly due to the above reasons, if the symptoms are obvious, it is recommended to go to the hospital for examination.