Just sitting on an exercise ball challenges your core because you have to stabilize your torso muscles in order to not roll off. These exercises, even when they aren't focusing on your core muscles, are challenging because you have to keep your balance as you do them. The exercises are listed first, and the details of each exercise follow:

• Hip rolls with leg extension

• Abdominal curl with leg extension

• Single leg stretch

• Criss cross

Hip Rolls with Leg Extension

Lie on the floor with the ball just in front of your butt, your right leg across the ball, your left leg extended up in the air, and your arms out from your sides at a slight angle on the floor. Be sure your back is gently against the floor.

On an exhale, tilt your hips toward the left without letting your lower back lose contact with the floor. Let the ball roll with you and keep your left leg extended. On an inhale, return to the start position and roll to the other side. Repeat 16 times, eight to each side.

Abdominal Curl with Leg Extension

Lie on the floor, with your back pressed gently to the floor, and your hands behind your head, elbows out. The ball should be between your feet and lower legs, and your knees should be bent, with toes pointed. Your abs should be contracted, with your hips even on the floor.

On an exhale, bring your head, neck, and shoulders off the floor as you straighten your legs. Keep your back gently pressed to the floor. You should feel this in your abs. On an inhale, come down slowly. Repeat sixteen times. You can do two sets if you want.

Single Leg Stretch

Lie on your back with the ball in your hands, above your torso. Press your lower back gently into the floor as you raise your legs to a 45-degree angle from your hips. Keeping your shoulders away from your ears, raise your head, neck, and shoulders off the floor. Bend your right knee and bring it in toward your chest. The ball should be just above your knee. Hold this position for two counts, then switch legs without moving your upper body and hips. Do this eight times on each leg.

Criss Cross

Lie on your back with the ball in your hands, above your torso. Press your lower back gently into the floor as you raise your legs to a 45-degree angle from your hips. Keeping your shoulders away from your ears, raise your head, neck, and shoulders off the floor. Bend your right knee and bring it in toward your chest.

As you do this, twist toward your left slightly, bringing your hand and the ball down toward your left knee. Hold this position for two counts, come back to the start position, and then go to the other side. Do this eight times on each leg.