Sit-ups: It is a hip joint movement, so it depends on the contraction of iliopsoas and rectus femoris, while rectus abdominis only tightens, not contracts.

In fitness, the action of sit-ups itself is unscientific, which is a misunderstanding of many people's fitness. Everyone thinks that sit-ups are for abdominal muscles, but in fact they are for leg strength. If they often do sit-ups instead of stretching exercises, they will strengthen the iliopsoas muscle, leading to pelvic forward tilt and lumbar lordosis, which will cause back pain.

If you want to practice rectus abdominis, don't change the angle of hip joint, just roll up the trunk to make the body feel rolled up, that is, don't lift the lumbar spine, which is the contraction force of rectus abdominis.