In order to reduce the joint and back pain that can result from running, almost all motorized treadmills are made with some sort of cushioning to help absorb the shock of the user's feet landing on the tread belt. The most basic way this is accomplished is through the use of rubber grommets that hold the deck of the treadmill onto the frame. However, most treadmills have additional cushioning provided by proprietary systems that are typically some combination of springs and automobile-type shock absorbers. The resistance level is often adjustable. Serious runners tend to like more resistance than others, in order to more closely simulate running on a hard surface.

For more information on choosing a treadmill, see

How to Choose a Treadmill

Treadmill Buying Tips

Big Guy – Big Treadmill

What is iFit?