So it is a little difficult for muscles to recover after an exercise. After all, the body is not used to it.

There is a misunderstanding here: sit-ups don't have to be that high.

Because only the first half here is effective for abdominal muscles, and the second half is completely started by hip and waist muscles.

Lying on your back with flat legs is very effective and can completely replace the movements you practice.

The only way to practice abdominal muscles is to be nervous all the time, don't relax, each group is exhausted, and slowly experience the expansion and contraction of related muscles in order to practice well.

Your muscles are too tired. Go to the masseur. Could be ligament damage. Pay more attention to rest.

Practice abdominal muscles up to three times a week, with enough rest in the middle.