What was the treadmill originally designed for?

Using a treadmill, the constant walking, maybe even up at an angle, can seem like torture. It was designed for use by prisoners, providing useful labor. Originally called a treadwheel, up to 40 prisoners at a time were forced to walk on planks, rotating a giant wheel for up to eight hours a day. The power generated by it was used to grind corn or pump water. It was also used for punishment (torture). Happy Labor Day!