An improved exercise device including a sole with an upper surface on which a wearer's foot rests and a lower partially ground contacting surface. The sole has a heel, a midfoot, a forefoot and a toe region such that when the heel region lower surface and the forefoot region lower surface are in ground contact, the upper surface does not incline upwards from the forefoot to the heel region. The forefoot lower surface of the sole is generally radiused in a cylindrical curve about the line connecting the mid points of the first and second metatarsal-phalangeal joints of the wearer. When the wearer contracts their calf muscles, the heel section is lifted away from the ground, requiring work against gravity.