Treadmills are operated by a motor than turns the belt. The motor is sometimes referred to as the "heart" of the treadmill. Without it, the machine is useless. This motor is typically between a 1.5 to 3 horsepower motor that allows for the belt to rotate as you walk, jog or run. What one has to be caution of is that it is not always the size of the motor that makes the best machine. The design of the belt, rollers and the deck have a lot to do with the performance of the treadmill.
The deck is an important component in not only the design of the treadmill but also its performance. The first thing that you have to look at is the frame. You want to make sure that it is solid. Steel is the best option for the frames. The deck is an extension to the frame and what will support all of your weight. The constant pounding from your feet is what this portion of the treadmill will need to be able to support. The shock absorption is the biggest element to the treadmill.
The rollers are what support the belt that in turn allows for you to walk, jog or run. The bigger and heavier they are, the better they are. Their job is to reduce the stress of the belt and motor. This will in turn make the treadmill last long.
Most treadmills have 2 ply belts that are of the same thickness. Anything less would not be recommended. The stronger the belt, the better it will perform and last. The size of the belts range from 14 to 24 inches wide. As for the length, they are anywhere from 45 to 63 inches long. It is recommended that the bigger you are, the bigger your treadmill should be.
This might be the least important of the features. Nonetheless, there are some importance when looking into the electronics and the console. Typically the treadmills come with some built-in workouts already installed. There is usually a heart/pulse monitor as well. You should have speed and incline controls that will allow to adjust for pace and difficulty levels.