Sit-up Exercise Device

FIG. 1 is a top perspective view of a sit-up exercise device, showing my new design; FIG. 2 is a front view thereof; FIG. 3 is a rear view thereof; FIG. 4 is a left side view thereof; FIG. 5 is a right side view thereof; FIG. 6 is a top view thereof; FIG. 7 is a rear perspective view thereof; and, FIG. 8 is an enlarged detail view of area 8 in FIG. 1 . The broken lines in the drawings illustrate the portions of the sit-up exercise device, which form no part of the claimed design. The dash-dot-dash broken lines encircling the enlarged views are for annotative purposes only and form no part of the claim thereof.