Lie faceup with your hips and knees bent 90 degrees; hold the ball with both hands, arms straight. Bend your elbows and lower the ball toward your chest, pressing your hands firmly against the ball. Brace your abs, extend your arms in front of you, curl your shoulders off the floor, and straighten your legs. Hold for one or two seconds, then reverse to return to start. Make about 20 reps.