It definitely makes you mentally tougher. This happens because of an absence of air resistance on the treadmill. Studies have indicated that heart rate and oxygen consumption levels come closest to those recorded during outdoor running when this slight incline is used. Be aware, however, that constant incline running can cause injuries. If that s on an incline, you re really lengthening your achilles tendon.
Treadmill running is easier on some muscles and harder on others.
Although it mimics land running more closely than any other activity, treadmill running is a marginally different biomechanical experience. So, if someone runs just on the treadmill for several months it could take a few weeks to feel good running on land. One difference is that the relatively soft, steadily moving belt is kinder to the lower-body muscles and joints than are stable, hard outdoor surfaces. This can result in significant muscle soreness, especially in the quadriceps, for runners who do all or most of their training on a treadmill then go back to the roads. Treadmill runners also grow unaccustomed to running turns, and may overstress their joints when they race on a track or twisting race course.