The main part of flat push-ups is abdominal muscles, including rectus abdominis, oblique muscle outside abdomen and oblique muscle inside abdomen. In addition, this movement also involves some muscles of the back, waist and buttocks, such as latissimus dorsi, iliopsoas and gluteus maximus. Through the practice of this movement, you can effectively enhance the muscle strength and endurance of these parts and improve the stability and balance of your body.