The treadmill's main function today is for the user to gain cardiovascular exercise. Treadmills generally burn the same amount of calories and give users the same cardiovascular exercise as if they were walking or running outside or on a track.
Working out on a treadmill regularly can cause the user's leg muscles to expand and grow stronger. Because running on a treadmill is very similar to running on a road or track, the force that is expended by the feet and legs is identical.
The first treadmills were not workout machines, but rather machines that used animals to run threshing machines and other various farm equipment. The first use of a treadmill as a workout device occurred in the 1950s.