There are many conditions that can cause pain in the popliteal fossa when the patient flexes the knee joint. For example, if the patient has acute trauma to the knee or popliteal fossa or long-term chronic strain or injury to the muscles such as the semitendinosus and semimembranosus muscles, the patient will have pain when moving the knee joint that will stimulate the injured muscle tissue. In addition, if the patient has a cyst in the popliteal fossa, the cyst will be irritated when the patient flexes the knee joint, compressing the local tibial nerve, tibial artery or surrounding subcutaneous nerve, which will also cause pain, and will be accompanied by decreased skin sensation and muscle strength in the lower extremity.