With minor adjustments, a treadmill run replicates an outdoor run.
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Running on a level road or trail is not the same as running on a level treadmill. The combination of a moving belt and the lack of air resistance makes a level treadmill run easier, allowing you to run at a faster pace at the same effort level. A study done in the United Kingdom found that you have to set the treadmill at a 1 percent grade in order to replicate the energy cost and speed you would run outdoors.
Many runners would prefer to train outdoors than on a treadmill. Outdoors, you can run with others, enjoy the scenery or just revel in having the sun on your shoulders or the wind in your hair. It's also easier for some runners to train hard outdoors; when asked to run at the same perceived effort level both outside and on a treadmill, some people will run faster outdoors.
Treadmills have distinct advantages for runners when the roads are icy, the trail is muddy or the temperature outside is extremely hot or cold. In these situations, treadmills can allow you to perform a quality workout inside when an outdoor workout could cause discomfort or injury. Treadmills can also help you replicate a specific race course and practice eating and drinking while on the run -- something that will help you in a race outdoors.
Researchers at Kansas State University found that running on a treadmill reduced the risk for a tibial stress fracture. They found that peak mean tibial axial compression, tension strains, peak mean tension and compression strain rates were significantly higher during outdoor running than during treadmill running. They concluded that outdoor running is better when trying to improve your bone density, but treadmill running is better if you're trying to avoid a tibial stress fracture.
Patterns and Pace
In most respects, however, running on a treadmill, when a 1 percent incline adjustment is made, will accurately replicate an outdoor run. Runners use the same biomechanical patterns on a treadmill that they use outdoors. You can also more easily practice running at your goal pace on a treadmill because you can set the speed you want to go and train yourself to run at that pace.