1. Take off your shoes and socks and lie flat on the bed. Put your hands on your sides, lie flat on the bed and adjust your breathing until you breathe evenly.

2. Straighten your legs and lift them slightly, so that your calves can leave the bed without any pressure.

3. Keep this posture and adjust your breathing. If you can't hold on, just put it down and have a rest. Repeat it 20 times a day to extend the time of each support as much as possible.

This is the method that a friend who often exercises told me. You don't need to go out to do anything, and compared with sit-ups, it doesn't hurt your waist and your exercise position is accurate. Persistence is very effective.