I have been in the army, and there are too many ways to say it. Sleep at night, lie flat on the bed, sit-ups with both hands, feet together, 30 degrees too high, and don't bend your knees for ten minutes. This method is to exercise my abdominal muscles. Push-ups put my chest on the ground but don't touch the ground, and keep your eyes on the front 10 minutes. The next method is to exercise my waist muscles and big arms.