Lie flat on your back with your arms fully extended behind you, holding the medicine ball in your hands.
Raise your legs straight up in the air so that the flats of your feet are facing the ceiling.
Keeping your arms straight (locked at the elbows), bring the medicine ball up over your chest and lift your shoulders off the mat, pushing the ball toward your feet.
Return to starting position by lowering your upper body all the way back to the floor and keeping your arms locked in the straight position. Your feet should remain lifted in the air. Let the medicine ball touch the floor. That counts as one rep.