Curls and sit-ups can exercise the stomach, but relatively speaking, curls are better than sit-ups, regardless of whether it is to train the abdominal muscles or lean stomach. Because when doing sit-ups, the thigh muscles and hips will be involved, resulting in relatively little abdominal force, there will be compensation. Curl-ups can mainly exercise the rectus abdominis muscle, after the hip is fixed, relying mainly on the abdominal muscles to generate force, so that compensation is reduced. Sports injuries, when doing sit-ups will increase the burden on the waist, the pressure on the lumbar spine is very large, long practice will inevitably occur lumbar spine injuries, disc herniation, etc.. The lower back is pressed against the bed during the abdominal roll exercise, which will not produce too much damage to the lumbar spine. If the purpose of the exercise is to train the abdominal muscles, thin stomach or worry about the lumbar spine injury, it is recommended to choose the curly abdominal exercise better. But if the abdominal exercise really can not do up or want to exercise the iliopsoas muscle, you can choose sit-ups, the specific choice varies from person to person.