Abstract: A portable, easy to use exercise device suited for various types of sit-up type exercises to increase muscular flexibility and generally strengthen the abdominal and back muscles while substantially minimizing stress to the lower back region. The exercise device includes a platform formed of a rigid material structured for supporting an exerciser's body weight. The platform includes a top region, a bottom region, a first end zone, and a second end zone. The top region of the platform defines an upper support surface for contacting and supporting the exerciser. The upper support surface has a generally sinusoidal configuration between the first and second end zones. The upper support surface includes a convex support portion for supporting the exerciser's back so as to cause it to partially arch when the exerciser reclines back. The upper support surface further includes a concave cradle portion for supporting and cradling the exerciser's buttocks during the performance of exercises.