Many treadmills are padded, making them a good option if you're overweight or are injury-prone (especially with knee issues) and want to decrease the impact.

The unshod runners' style causes more flex in the foot's springlike arch ankle and knee and engages more foot and calf muscles blunting the impact on the body and making for a more comfortable "ride."

As your foot strikes the ground, your knee should be slightly flexed so that it can bend naturally on impact.