 Crunches are executed while lying on the back with the feet flat on the ground together or 12 inches apart (whichever is more comfortable), knees bent at a 90 degree angle, and arms on the ribcage or chest.One crunch is completed when the upper body is lifted until both arms touch the thighs and then lowered until the shoulder blades touch the ground.The arms must be in constant contact with the chest or rib cage; the buttocks must be in constant contact with the ground.