Like a new pair of shoes: How treadmills solve impact reduction
If you look at a good pair of running shoes, they have a thick sole and thin front. The design is to reduce as much impact when your foot lands and to be able to produce as much force as possible to take off when running. Think about running on sand. It’s hard to take off because it is soft. If the front of the shoe had suspension, it would be harder to take off.
Between the deck and frame of the treadmill, you have what is known commonly as the durometer or elastometer. Essentially it is a rubber block, which makes it softer to run on. With most treadmills the intensity of the rubber block is the same. With the Horizon treadmills as shown above, they vary the intensity of the block to better replicate the running motion and like a pair of running shoes – soft on impact, firm on take off.