Designed to the natural curvature of the spine.  Its stable design keeps your ab mat firmly in place allowing you to focus solely on your ab workout. Flat non slip base made from high density foam making situps possible on any surface. The stable design keeps your mat in place so you can focus on your workout. No more balancing. Unlike fitness balls and ab wheels the Reebok core mat will not roll or wobble on you so you can concentrate on your core exercises.