Swiss-Ball Leg Curl
STEP 1. While lying on your back, place your lower legs on a Swiss ball. Then, put your hands flat on the floor next to your hips.
STEP 2. Push your hips up so that your body forms a straight line from your shoulders to your knees. Without pausing, pull your heels toward you and roll the ball as close as possible to your butt.
STEP 3. Pause, then roll the ball back until your body is in a straight line again.