The following reasons are considered for tendon pain when the thigh is bent: First, if there is tendon pain at the back of the knee when the thigh is bent, it is generally considered to be a meniscus injury, especially the posterior horn of the meniscus. When the knee is bent, the posterior horn of the meniscus will squeeze between the posterior condyles of the femur as well as the posterior tibial plateau, which will cause pain at the posterior side, and sometimes there is a case of being unable to straighten the leg after bending it. This is because the torn part of the posterior horn of the meniscus is just mechanically jammed in the joint space, which affects the movement of the joint. Secondly, pain on the side or front of the knee joint when the thigh is bent can be considered to be caused by synovitis. When the knee bends, the tension of the synovial membrane increases, and if synovial inflammation occurs, it may be manifested as pain in the lateral or anterior aspect of the knee joint.