The hip joint is composed of the acetabulum and the femoral head and is a multiaxial ball and socket joint. An acetabular labrum composed of fibrocartilage is attached to the circumference of the acetabulum to increase the depth of the acetabulum. The acetabular notch is closed by the transverse acetabular ligament, which enlarges the semilunar acetabular surface into a ring shape to embrace the femoral head, and the socket is filled with adipose tissue. The joint capsule of the hip joint is tough and dense, attaching to the acetabular rim and transverse ligament upward, attaching to the femoral neck downward, reaching the intertrochanteric line in the front, and wrapping around the medial two-thirds of the femoral neck in the back.The hip joint can do three-axis flexion, extension, extension, spreading and closing, rotating inwardly, rotating outwardly, and circular rotation, because the head of the femoral head is deep in the acetabular fossa, and the joint capsule is relatively tense, and is restricted by multiple ligaments, the amplitude of its movement is much less than that of the shoulder joint, and it has a greater stability to adapt to its weight-bearing capacity. Because of the relative tension of the joint capsule and the limitation of many ligaments, the range of motion is far less than that of the shoulder joint, and it has a greater stability to adapt to its function of load bearing and walking.