Running is good for health, but running outside is better than running on a treadmill. The reasons are the following.
- It can get boring running on a treadmill. Staring at a wall while running on treadmill can get monotonous. Running outside is refreshing as you get stimulus from the scenery around you. You get to breathe fresh air and you can change your route if you get bored.
- Technically the treadmill does the running for you. It generates the momentum for you to run. When you are running outside you generate the momentum. Your brain pushes your body forward not the treadmill.
- Treadmill has a soft, predictable and flat surface unlike the uneven rugged surface of outside terrain. Because of this the muscles aren't taxed on a treadmill as much as they are when running outside. When you are running outside more muscles are recruited to stop you from falling on the bumps on the road. You have to constantly balance on the uneven surface of the road unlike the even surface of a treadmill and therefore use more muscles.
- You will also have to encounter wind resistance when running outside, this means you will burn more calories running 1 km outside than on a treadmill. You can increase the incline on a treadmill by 1% to give the effect of wind resistance.
Treadmills are useful when the weather outside is not good and conducive for running or if you are living in a city and there is no place to run or it is too dangerous to run due to heavy traffic.