There are many reasons why you might want to run on a treadmill even if you do most of your running outside. Maybe the weather is bad or too hot. You may have various training routines that would only suit a treadmill. These would be speed work, intervals, tempo or incline running. This is essential when you want to run faster or are training for a race.
Treadmill running does not mimic running outside. There is no wind resistance so running is easier even if you're running at the same speed. The belt on motorized treadmills also pull away from you which is not something that happens outside. There maybe variations in gradient that a treadmill can't mimic because they change so frequently outdoors.
The recommendation has regularly been that to mimic outside running you should run on a treadmill at a 2% incline. This would give you a similar time as if you were encountering wind resistance. This has long been thought to be the correct answer, even experts in running will advise a 2% incline.
Is that however correct? The best way to measure this is the actual effort involved in running on that incline compared to running on a level road outside. The measurements would need to be done scientifically and multiple times to check if the right answer is indeed 2%.
So what does the scientific research suggest?
There was a study done on exactly this and was accepted and published in the Journal of Sports Sciences. This is a respected scientific paper and the studies published are accepted as true. The research was to accurately test what gradient of treadmill running would reflect the energy cost in running outdoors.
Being science they couldn't just have one person doing some running outside and inside. Then asking how it felt and reading of the time. The study used 9 trained male runners who were used to running outside and on treadmills. They had them do a variety of runs and measured the oxygen consumption of expelled air from the runners lungs.
They had the runners run at 6 different speeds with a rest of 6 minutes between each of those runs. They repeated this 6 times to make sure of the accuracy in their results. They also did this on 5 different treadmill gradients plus they ran on a level road surface so they had results for outdoor running to compare.
The paper says they found the results to be very similar amongst all the runners and that they had the definitive answer. A treadmill at a gradient of 0% and 1% were similar to the efforts of running on the road. Gradients of 2% and 3% were very different. They concluded that to replicate outdoor running a treadmill should be set at a 1% incline.
But that's not all
Running on a permanent incline on a treadmill is not the most natural of experiences for the runners body. You never encounter a 1% incline throughout a whole race or any other run. Your body's natural gait has to slightly adjust to this gradient. This means that treadmill running at a 1% incline should be used sparingly.
The advice for runners is to only do specific running work on treadmills that can't easily be replicated outside or are just more convenient. This includes speed work, tempo's, intervals etc. Many runners do this already and always have a treadmill to hand for specific workouts as needed. As long as you're not doing all your running on a 1% treadmill incline then you'll be ok.
Source: Andrew M. Jones & Jonathan H. Doust tandfonline.com/doi/abs/10.1080/02640419608727717. Journal of Sports Sciences.