1. Fat pad strain The fat pad fills the gap at the front of the knee joint and has the function of strengthening the joint stability and reducing friction. The cause of fat pad strain may be due to trauma or long-term friction that causes fat pad congestion, hypertrophy and inflammation, and adhesions with the patellar ligament, thus limiting knee joint movement. This injury occurs in people over 30 years of age who walk, hike or squat frequently. Patients may feel pain in the knee joint, which is worse when fully extended, but there is no restriction of joint movement. The symptoms are obvious after exertion. 2. Traumatic synovitis of the knee joint The synovial membrane of the knee joint is one of the main structures that make up the knee joint. Synovial cells secrete synovial fluid, which can keep the cartilage surface of the joint slippery and increase the range of motion of the joint. Injury to the synovial membrane due to trauma or overexertion can produce a large amount of fluid, which increases the pressure in the joint and, if not eliminated in time, can easily cause joint adhesions and affect normal activities. Patients will feel pain, swelling and pressure in the knee joint, and the synovial membrane has a frictional and astringent sound. The most obvious feature of the pain is that when the knee joint is actively and extremely extended, especially when there is a certain resistance to do the knee extension movement, the pain in the lower part of the patella will be increased, and the pain is also significantly increased when it is passively and extremely flexed. 3, osteoarthritis of the knee joint This condition is mostly seen in middle-aged and elderly people, mostly women. Overload is the main cause of the disease. The knee joint will be swollen and painful, and sometimes there will be friction sounds when moving the joint. The knee may have an inversion deformity and be accompanied by medial pain. 4. Meniscus injury Meniscus injury is a common injury in athletes. When the lower extremity is weighted, the foot is fixed, and the knee is slightly flexed, a sudden excessive internal rotation of the knee extension or external rotation of the knee extension (for example, in volleyball, the player suddenly turns to fishtail the ball while defending) may cause a meniscus tear. A meniscus injury is characterized by a distinct tearing sensation in the knee, followed by joint pain, limited motion, and a walking limp. The joint exhibits swelling and slipping sensations and pops when the joint is moved. 5. Knee ligament injury The stability of the knee joint is relatively poor when it is slightly flexed, and if it is suddenly subjected to external forces that lead to valgus or valgus, it may cause medial or lateral collateral ligament injury. Medial collateral ligament injuries account for the majority of clinical cases. In this injury, for example, the patient will have a clear history of trauma, pain and pressure on the medial side of the knee, increased pain with passive abduction of the lower leg, and swelling on the medial side of the knee, with petechiae appearing after a few days. Knee motion will be limited.