A manually powered treadmill includes a frame; a plurality of bearings coupled to the frame; a running belt at least partially supported by the plurality of bearings and adapted for rotational movement relative to the frame, the running belt having a running surface, at least a portion of which is curved; a braking system configured to selectively resist the rotational movement of the running belt; and a safety device coupled to the frame, wherein the safety device is structured to substantially prevent rotation of the running belt in a first direction and substantially permit rotation of the running belt in a second direction, opposite the first direction.