@roger,
roger wrote:Why call it any kind of mill, though? Mills usually produce some useful product or result like lifting water, or maybe grinding grain.
Treadmills were introduced before the development of powered machines, to harness the power of animals or humans to do work, often a type of mill that was operated by a person or animal treading steps of a treadwheel to grind grain. In later times treadmills were used as punishment devices for people sentenced to hard labour in prisons. The terms treadmill and treadwheel were used interchangeably for the power and punishment mechanisms.
More recently treadmills are not used to harness power, but as exercise machines.
(WIKIPEDIA)