The ab roller features a wheel with handles that run through its center.
The ab roller, with which you perform the wheel rollout exercise, is known for the intense challenge it places on the abs. However, the exercise does also require contributions from other muscles, including some of those in the upper back. Unlike traditional situps or crunches, working out on the ab roller requires muscles in the upper body get involved to help control the wheel as you roll it in and out.
To use the ab roller, kneel on a soft floor or exercise mat and hold the handles of the wheel on the floor in front of you so your palms are facing your knees. Begin with the wheel just in front of your knees, then roll the wheel out away from you while keeping your arms extended. Continue as far as you can, ideally reaching the point where your torso and thighs are just inches from the floor. Roll the wheel back toward your knees by raising your body, then go into the next rep. If the exercise is too difficult, only go as far as your strength allows; you can also perform the exercise up an incline to lessen the difficulty.
Primary Muscles Worked
Although most believe the abs to be the primary worker, the muscle that actually handles most of the work as you perform the ab rollout is the iliiopsoas -- a collection of two major hip flexor muscles. Together, they're responsible for flexing your hips, or bending your waist against resistance. Your rectus abdominis and obliques are also recruited, but they act primarily as stabilizers, contracting to prevent your torso from collapsing to the floor as you roll the wheel away from you.
Contribution from the Upper Back
Muscles surrounding your shoulder joints must work to keep your arms stable as you perform the ab roller exercise. In the upper back, your latissimus dorsi, rhomboids, posterior deltoids and teres major muscles are also recruited to handle this load. The teres major, posterior deltoid and latissimus dorsi, which is the largest muscle in your back, assist by performing shoulder extension, or pushing your arms down into the floor. Your rhomboids, which are located in your upper back between your scapulae, are performing downward rotation.
Increasing the Load on your Back
If your goal is to increase the demand on your upper back muscles, roll the wheel out until your shoulders are fully extended. Once you've mastered the exercise from the knees, begin performing the rollout from your feet instead. Both techniques will increase the stress placed on your upper body and thus cause your upper back muscles to work harder.