The present invention relates to a low-impact, time-efficient way to exercise both the cardio-respiratory system and the muscular system, particularly the abdominal and back muscles. The exerciser uses his muscles to oscillate a pivoted carriage within a frame. In one embodiment, the exerciser pedals a crank to pump hydraulic fluid through a hydraulic circuit to a hydraulic cylinder connected between the carriage and the frame so as to urge the carriage to oscillate with respect to the frame. A limit sensor detects when the carriage is positioned at one extreme of its oscillation and signals the hydraulic circuit to reverse the flow of hydraulic fluid to the hydraulic cylinder.