Stability Ball Curl
Firm your butt, thighs, and core in one move.
Lie with you back on a mat, legs extend, with lower legs (calves) resting on a stability ball and your arms by your sides, palms facing up. Lift your hips in the air until they form a line from your shoulders to your knees, with only you heels on the ball. Maintaining this bridge position, exhale and use your heels to roll the ball toward your hips and butt (as shown). You'll have to contract your core muscles to keep your body steady and prevent from rolling side to side. Hold for one count, then inhale and push the ball away to the start position. Do 10 repetitions, building up to 20.