Since we often encounter people with pain on the posterior side of the knee, we did a structural consideration of the cause of posterior knee pain and figured out the mechanism of the pain. It turns out that for vertebrates that move on all fours, only humans have this problem because no other animal has a knee joint that needs to be straightened like humans do. The posterior knee joint is also called the N-fossa. It is a depressed area. It is a nearly diamond-shaped fossa. The upper medial side is the N cord muscle and the outer side is the biceps femoris. The lower medial side is the medial head of the gastrocnemius muscle and the lateral side is the lateral head of the gastrocnemius muscle. In this fossa, from posterior to anterior, there is the N fascia, the neurovascular and its lining fat from the thigh towards the calf, the N muscle, and the posterior wall of the joint capsule. the N muscle is barred to travel obliquely from the external superior to the internal inferior and does not completely cover the posterior wall of the joint capsule. When walking, the foot is held in place by ground resistance and the body leans forward, causing the knee joint to straighten, prompting the gastrocnemius muscle behind the calf to contract and the ankle joint to extend and stomp. There is a straightening action of the knee joint during this process. This action produces a pull on the posterior wall of the joint capsule, the biceps femoris and N cord muscles that flex the knee, the gastrocnemius and N muscles that extend the ankle, and the fascia. The knee is fully extended with maximum stretch, the knee straightens for a long time when upright or walking slowly and maintains the maximum stretch for a long time, and the stretch is short when walking quickly or running. Therefore, the magnitude of the knee extension action is greatest when walking upright or slowly and least when running. From middle age to old age, the walking speed gradually becomes slower, the single leg support time becomes longer, and the time spent in the N-fossa being tensed (stretched) increases. In addition, from middle age to old age, the brittleness of myofascial tissue increases and cannot withstand the damage caused by repeated flexion and extension fatigue. Therefore, slow walking speed and tissue intolerance to fatigue are the causes of posterior knee pain in middle-aged and elderly people, and the repeated and prolonged tension of the myofascia in the N fossa not only results in fatigue injury to the tissues in the N fossa, but also in myofascial adhesions due to the injury, and synovial cysts can occur when the synovial fluid between the myofascia is wrapped. The posterior wall of the joint capsule can herniate out along the weakness of the myofascial gap. Whether it is a fatigue injury or a synovial cyst, it will naturally cause discomfort or pain behind the knee. Understanding this, maintenance prevention and treatment is simple. Posterior knee pain can be avoided by avoiding excessive knee extension, prolonged standing upright and prolonged walking at a slow pace (long walks). For people who already have posterior knee pain, the best treatment is to avoid walking long distances or standing upright for long periods of time. The terms “long periods” and “long distances” vary from person to person. The time and distance that can cause posterior knee fatigue and discomfort is what is referred to as “long” and “long distance”.