In 1818, British prisons adopted William Cubitt's treadmill as a tool of hard labor. 208 years later, the machine is a fitness staple, but its modern medical and exercise history evolved separately from the original punishment device

Today, the treadmill is one of the most recognizable pieces of exercise equipment in the world. It appears in gyms, homes, rehabilitation centers and medical laboratories, where people use it to walk, run, improve cardiovascular fitness or undergo controlled exercise testing. Its basic promise seems almost absurdly simple: move your feet while remaining in the same place.

That apparent simplicity hides a much stranger history.