Lie flat on the bed, legs straight, toes hooked, upper body motionless, legs slowly raised until close to the head. You can lift your legs faster at first, and practice as slowly as possible later. When you are slow, you can sweat by lifting it back and forth twice, which not only exercises your abdominal muscles, but also pulls your hamstrings. It's not easy to get hurt, and it can also help to tighten the legs.