Once called "treadwheels," treadmills were used in the past as a way of punishing inmates at prisons and were used in the very early part of the 1800s. Although private health clubs have been in existence from the late 1800s, commercial treadmill machines weren't used for exercise purposes until the late 1960's. Because of the way a treadmill will operate, it is recommended that users of this type of exercise machine raise the incline of the running belt to at least a one percent grade since running on a zero percent grade on a treadmill actually approximates running downhill outside.