The functions of the psoas major are to flex the hip joint, externally rotate the lower extremities, and assist in the upright position of the body, among others. The specific functions are as follows: 1. Flexion of the hip joint: the psoas major muscle connects to the side of the lumbar vertebrae, stops at the femur small rotor downward, and crosses the hip joint in the front. Therefore, when the psoas major muscle contracts, its most important function is to make the hip joint flexion. 2. External rotation of the lower limbs: because the psoas major muscle stops at the small rotor below the hip joint, therefore, in coordination with the contraction of the other muscles around the hip joint, if the psoas major muscle is strongly contracted, it can make the femur rotate outward along its long axis. At this time, the patient’s entire lower limb can be simultaneously rotated to the outside, which means that the patient’s toes are turned to the outside. 3. Assisting the body to remain upright: the psoas major muscle spans a wide range. When it contracts, it can pull forward on the lumbar vertebrae, pelvis, hip joints and other parts of the body. At this time, if the muscles behind the above mentioned structures exert force at the same time, it can keep the above mentioned parts straight, and then help the human body to stand upright. In fact, the psoas major muscle can also help the body to perform a series of complex activities, such as walking, jumping, sit-ups and so on.