I wrote about prison treadmills before. They were invented in England in 1817
by Sir William Cubit, who observed prisoners lying around in idleness
and put himself to the task of "reforming offenders by teaching them
habits of industry".
Forty-four prisons in England adopted it as a form of
hard labour that could also grind grain or pump water.
However, as it turns out, in at least one jail prisoners were only "grinding the wind": they were walking a treadwheel that was connected to a giant fan built on the courtyard. By this apparatus the resistance necessary for rendering the tread-wheel hard labour was obtained.
The system is explained in "The criminal prisons of London and scenes of Prison Life" (1862), written by Henry Mayhew & John Binny. Starting on page 299, they describe the method of "hard labour", and the technologies used for it: the treadwheel, crank labour & the shot drill. Great reading.