Sit-ups are not good for the waist and abdomen, and can only be practiced to the upper part of the abdominal muscles. Why don't you use belly rolls instead and make four groups?

Abdominal muscles: 4 groups at each end.

Supine leg lifting group 4

Belly rolling? quadruplet

Abdominal muscle exhaustion in each group is still 15 to 25. The rest time of each group is 20 seconds to 30 seconds.

Practice more than four times a week.

Abdominal rolling

Liangtouqi