Yoga balls, also called fitness or stability balls, are large, inflatable balls that help you improve your balance while working your abdominal, back and pelvic — or core — muscles. A variety of standard exercises can be made more challenging by performing them on a yoga ball. Make sure you get a ball that is the right size for your height. Your feet should be flat on the floor and your knees bent at a 90-degree angle when you sit on the ball, according to MayoClinic.com.

Sit and Lift

Hold the yoga ball in your hands. Lie on your back on a mat or carpeted floor. Raise your legs into the air and hold them so that your calves are parallel to the floor. Sit up slightly, so that your neck and shoulders are off the floor. Place the ball between your calves. Lower your legs with the ball and pull your arms behind your body. Keep your shoulders and neck up. Reach forward and bring your legs back up as you grab the ball. Repeat five times.

Push-Ups

Get into the standard push-up position, with your stomach facing down and your palms flat on the floor. Slide the yoga ball under your abdomen. Slowly roll forward so that the ball slides under your thighs or under your calves. Keep your hips and shoulders in a straight line. Bend your elbows as you bring your abdominal muscles in, slowly lowering your chin and chest to the floor. Hold for a few seconds if you are able, and then bring yourself back up. Repeat 10 times.

Bridge

Lie on your back and place the ball under your ankles. As you inhale, lift your hips off the floor, so that your legs form a bridge. Your back should stay flat on the floor. Hold the position for two breaths, then slowly lower your hips to the floor, exhaling as you do so. Repeat 10 times. As you get more advanced, try lifting one leg into the air while you go into bridge.

Squats

Hold the ball in both hands and lift it over your head. As you bend your knees into a squat, bring the ball forward, keeping your arms stretched out in front of you. Your back should stay straight. Use your abdominal muscles to turn to the left, keeping your arms extended and the ball forward. Slowly turn back to center, then turn to the right. Repeat 10 times.

Froggy

Place the yoga ball at your feet. Stand with your feet pointed outward, a bit wider than hip-width apart. Squat all the way down, bending your knees. Grab the ball with both hands. Spring upward, lifting the ball over your head as you straighten up. Try to jump as high in the air as possible. Return to the squat and place the ball on the floor. Repeat five times.