The gluteus maximus, quadriceps, calf triceps, and erector spinae can keep the body upright. 1. Gluteus maximus: It is the largest muscle in the whole body in terms of volume and strength. It plays an important role in maintaining the body upright, walking, running and other sports. When the gluteus maximus muscle is fixed near, it can make the thigh extend back and rotate internally; when it is fixed far, it can make the pelvis tilt back and the torso straighten to maintain the human body in an upright position. 2. Quadriceps: located in the anterior thigh, it plays an important role in maintaining the stability of knee joint in daily life. When the knee joint is near-fixed, it can promote thigh flexion and calf straightening; when it is far-fixed, it will pull the femur forward with the knee joint stretching movement. 3. Calf triceps: a muscle group located at the back of the calf, it has the function of fixing the ankle and knee joints and preventing the body from tilting forward. When the calf triceps is fixed near, the calf and ankle can be flexed and extended, but when it is fixed far, the calf will be pulled backward to keep the knee straight. 4. Vertebral spine muscle: It is an important muscle in the back from the head to the sacrococcygeal bone, and plays an important role in maintaining the uprightness of the human body. When the erector spinae muscles on both sides of the spine contract at the same time, it will promote the backward extension of the spine.