The treadmill was commonly used to punish prisoners with hard labor

Well, it's not the Iron Maiden or the Brazen Bull, but the treadmill of today's fitness centers does have a "tortuous history" as Dan Kopell writes at Wirecutter:

Inventor William Cubitt subscribed to the "no pain, no gain" philosophy. His "Tread-Wheel," which was described in the 1822 edition of Rules for the Government of Gaols, Houses of Correction, and Penitentiaries (published by the British Society for the Improvement of Prison Discipline and for the Reformation of Juvenile Offenders), was presented as a way for prisoners to put in an honest day's labor. Prisoners used treadmills in groups, with up to two dozen convicts working a single machine, usually grinding grain or pumping water, sometimes for as long as eight hours at a stretch. They'd do so "by means of steps " the gang of prisoners ascend[ing] at one end " their combined weight acting upon every successive stepping board, precisely as a stream upon the float-boards of a water wheel."...

This was considered to be more humane, at least compared with earlier methods of punishment, which centered on hanging or exile to British colonies. Hard labor on a treadmill for a fixed term, the theory went, could rehabilitate an offender, who could then return to society and family. Never mind that the prisoner was often left shattered by the experience. Oscar Wilde spent two years on the treadmill as punishment for "gross indecency with certain male persons." In a poem about his incarceration, he wrote: "We banged the tins, and bawled the hymns, /And sweated on the mill: /But in the heart of every man /Terror was lying still."

"The Torturous History of the Treadmill" (Wirecutter)