The obliques are the muscles found on the sides of the stomach. They are activated when you make lateral and rotational trunk movements. Strengthening the obliques can help in the functioning of daily activities like golfing, shoveling snow, throwing hay bales and placing grocery bags in a trunk. When using a yoga ball to work your obliques, your body is off balance which forces you to contract multiple muscle fibers to maintain form.

Side Planks
Do side planks, which are variations of traditional planks, from a face-down position. Pin the ball against a wall, place your right forearm on top and place your feet in a staggered position on the floor. Your forearm should be perpendicular to your body and your left arm against your side. Steadily raise your hips and form a straight line from your shoulders to your feet. Hold this position for 20 to 30 seconds before switching sides.

Seated Rotations
Work your obliques with seated rotations using a medicine ball. Sit on a yoga ball while holding the medicine ball at arm's reach in front of your chest. Place your feet shoulder-width apart. Steadily rotate your torso to your left and right sides in a steady motion for 15 to 20 repetitions. When doing this, keep your lower body still and head in line with the ball.

Roll-ins
Place your lower shins on the ball and hands shoulder-width apart on the floor and perform roll-ins, which work your obliques and lower abs and are done in a face-down position. After lifting your hips and forming a straight line from your shoulders to your heels, roll the ball toward your body as you move your knees toward your left shoulder. After squeezing your obliques for a second, roll the ball back out and repeat going to your right side. Alternate to each side for a set of 15 to 20 repetitions.

Pendulums
Lie on your back with your arms out to your sides, legs lifted above you and the ball pinched in between your ankles and perform "pendulums," which work the obliques, lower back muscles and inner thighs. Your feet should be parallel to the ceiling. In a controlled fashion, alternate lowering the ball to your right and left sides as far as possible for 15 to 20 repetitions.

Lateral Flexion
Use a yoga ball and medicine ball to do lateral flexion, Place your right hip on the yoga ball, feet in a staggered stance against a wall and your arms straight overhead holding the medicine ball. Slowly bend laterally over the ball and lift yourself up by contracting your obliques. Repeat for 15 to 20 repetitions before switching sides. If this is too difficult, drop the medicine ball and place your hands on the sides of your head.