Yes You can build abdominal muscles. This action is called sit-ups, which is a kind of abdominal muscle exercise method. The intensity is not as good as sit-ups, but the intensity is low and it is easier to persist. The correct posture is: lie flat, put your feet together, straighten your legs, lift your legs at an angle of 45 to 60 with the bed, don't touch your heels when you put your legs down, and make your legs at an angle of10 with the bed, so that your abdomen is always in a tight state. The action frequency is about 1- 1.5 seconds.