Abstract: An exercise device for training abdominal muscles comprises a substantially flat lower side and an upper side opposite to the lower side. The upper side has a curved person-facing upper surface including a first sitting portion adjacent to a first back supporting portion having a steeper ascending inclination with respect to the lower side than the first sitting portion, as seen from the first sitting portion to the first back supporting portion, and at least a second sitting portion adjacent to a second back supporting portion having a steeper ascending inclination with respect to the lower side than the second sitting portion, as seen from the second sitting portion to the second back supporting portion. At least a part of the first sitting portion forms an edge portion of the device without having a further inclination or having an ascending inclination less steep than that of the first back supporting portion, as seen from the first back supporting portion to the first sitting portion.