A big advantage of using a treadmill over running outside is that virtually all motorized treadmills offer some sort of shock absorption capabilities. The risk of common running injuries to the feet, knees, and back can be reduced through this reduction of impact force on these body parts. Treadmills provide this absorption by placing cushioning devices between the running belt and deck.
Cushioning devices typically consist of springs, rubber, or some combination of the two. As with a car, a simple spring will cause bouncing up and down. This counter force is exerted on the user’s joints. What is needed is a real absorption of the shock, so that a down movement will produce a counter movement only to the starting point, not beyond. Thus, the rubber, or rubber and springs combination, is preferable to springs alone. Perhaps the most basic method of shock absorptions is 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 of cushioning devices 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
More Treadmill Information