First of all, it is very important to adjust your breathing during exercise. It is necessary to breathe through the nose. The rhythm of many songs does not necessarily match the frequency of your exercise, so it will affect your breathing rhythm. It is recommended not to sing.

Secondly, it is not recommended to do sit-ups if you want to exercise your abdomen. It is recommended to turn to belly roll, because belly roll is to make abdominal muscles move in a straight line. If you do sit-ups, your strength is mainly on the front thigh, buttocks and neck, and you rarely exercise your abdomen. I hope it helps you.