3A MEDICINE-BALL SITUP Hold a medicine ball against your chest, and lie on your back on the fl oor. Bend your knees 90 degrees, planting your feet flat. Tuck your chin slightly toward your chest . Keeping your feet and butt on the fl oor, explosively raise your torso off the fl oor and toward your thighs, coming up as far as you can (it's OK to let your lower back come off the floor) . Reverse the motion to return to the starting position. That's one rep.