There are several main reasons for the lack of knee strength: 1, due to damage to the nerves innervating the knee, such as the femoral nerve damage to the muscles can not be innervated, which will lead to the lack of knee strength. 2, damage to the quadriceps muscle, or patella, patellar tendon damage, will be in the knee joint activities when there is a severe pain, which leads to the lack of knee strength. 3, due to meniscus injury, meniscus injury, the femur and tibia will be damaged to the location of the injury to produce pressure, irritation, pain, resulting in knee weakness. After meniscus injury, when performing knee flexion and extension activities, the femur and tibia will extrude and irritate the damaged position, which will also cause pain and lead to weak knees.4. Synovial folds syndrome, that is, some of the proliferated synovial folds in the knee joint are extruded by the femur and tibia, which will also lead to weak legs. The last one is patellofemoral joint instability, which mainly manifests itself as more serious friction on one side of the patella, obvious cartilage degeneration as well as pain, which can also lead to the lack of strength in the knee joint and weakness in the leg.