What's the noun for treadmillers? Here's the word you're looking for.
treadmill
(sports) A piece of indoor sporting equipment used to allow for the motions of running or walking while staying in one place.
(historical) A mill worked by persons treading upon steps on the periphery of a wide wheel having a horizontal axis. It is used principally as a means of prison discipline.
A mill worked by horses, dogs, etc., treading an endless belt.
“The gaol held 30 prisoners when full, and is still home to the treadwheel walked by the inmates at 10-minute intervals for eight hours a day.”
“The gaol retains the only surviving treadwheel at a prison in England and Wales, while replica thumbscrews are available to take home from the gift shop.”
“They used masses of wooden scaffolding, and pulleys operated by treadwheels and cranked round by labourers or animals.”
“The local authorities kept careful statistics of the treadwheels, which provided extensive information on human work, of a nature far more accurate than earlier studies.”
“Along with cranks, treadmills, and treadwheels, they allowed human and animal power of machinery in the absence of electric machinery.”