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A woman on a treadmill.

A treadmill is a piece of indoors sporting equipment used to allow for the motions of running or walking while staying in one place. The word treadmill originally designated a type of mill which was operated by a person treading steps of a wheel to grind grain (see treadwheel.)

The principle is a belt system (an electric motor or flywheel connected to a circular run of material) where the top of the belt moves to the rear so as to allow a runner to run an equal, and necessarily opposite speed. Thus the speed of the run can easily be set or measured (the rate at which the belt moves is the rate at which the runner moves). The more expensive, heavy-duty versions are motor-driven. The simpler, lighter, cheaper versions are passive, moving only when the walker pushes the belt with their feet, and operates just to resist the motion.

• Known speeds at all times of the run, and being able to change speeds as needed.
• Allows for exact adjustment of gradient and speed.
• As most of the factors of the run are known, the energy expended may be calculated.
• Some treadmills have special features such as step count, heart rate monitors, amount of calories expended.
• Running or walking on a treadmill can be an effective way to workout compared to other complex cardiovascular exercises (similar to non-treadmill walking/running).
• Treadmills can be used in the home to avoid going to a gym. Users can do other things while exercising, such as watching television or reading.
• Users who would not run/walk outdoors (e.g. due to unfavourable weather conditions) may use an indoor treadmill.
• Treadmills offer the benefit of reduced impact since all treadmills offer some sort of shock absorption. Exercising on a treadmill can reduce the strain to the ankles, knees and lower back of users.
• Lack of wind resistance makes running on a treadmill slightly easier than it would otherwise be on an equal elevation grade outdoors.

• Many users find treadmills boring and lose interest after a period.
• Cost of purchase and electricity to run the treadmill is significantly greater than running outside.
• Takes up space in homes (disadvantage reduced by "folding treadmill" option).
• May cause personal injury if not used properly.
• Can make a loud grinding noise if the belt keeps slipping, this can happen on cheap models.
• Lack of wind resistance makes running on a treadmill easier than it would otherwise be on an equal elevation grade outdoors. Training for outdoor races is complicated due to the subtle differences.
• Ordinary treadmills, even the expensive models at gyms and clubs, are not usually able to tilt at the steep slopes used on medical treadmills for standardized stress tests.

As it is basically a conveyor belt, the treadmill can be used for activities other than running. If horses are being tested (especially in jockey racing) they will be put on a specially constructed treadmill.

Treadmills are considered as the most complex type of fitness equipment to repair by repair professionals^{[citation needed]}. They combine mechanical skills with electrical skills, and sometimes require computer skills in situations where the treadmill is used for stress testing in medical facilities.