Lie on your back, knees on your chest, legs straight, lift your left leg, and grab your thighs from behind with your hands. Then slowly pull your knees to your chest until you feel a stretch in your legs, and keep this action for 8 seconds. If the skill of this movement can improve the difficulty a little, on the basis of this movement, look up and put your forehead close to your knees. Hold for 8 seconds and then change to the other leg, and do it three times for each leg.