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Treadmill vs outdoor running

A constant source of debate for runners involves treadmill vs outdoor running.  Some people who run outdoors swear they will never run indoors on a treadmill; some who run on a treadmill couldn’t imagine running outside.
I’m no long-distance runner, but I was able to run for an hour and a half without any trouble at all on a treadmill recently (despite almost falling off twice when my feet landed on the frame rather than the belt).
It sure felt like treadmill running was easier than outdoor running. And a few people have since agreed with my suspicion. But what is the truth? Treadmill vs outdoor running: which is easier? And why? Let’s find out shall we.
It’s all about the energy required to move. What is the difference in energy expended for outdoor running, when you have to propel yourself forward on stationary ground, compared to treadmill running, where you have a moving belt underneath your feet?
Here are the influencers on required energy:
There is no wind-resistance when treadmill running, but obviously when outdoor running you are having to push your body through stationary air. The difference is significant: when you are outdoor running at 4.0 m/s, about 5% of your total energy goes into just overcoming wind resistance.
Wind resistance is dependant on body size
What about the differences in the actual biomechanics of treadmill running vs outdoor running? Now we can get into some details and actually look at the science behind why treadmills are easier.
A lot of research has been done to compare treadmill running vs outdoor running over the last 30 years. Below are summaries of the key results:
Stride Length and Rate
A study by Elliot, B.C., Blanksby, B.A. Medicine and Science in Sports looked at these two factors.
No significant differences were recorded in stride length, stride rate, support time or non-support time on a treadmill compare to outdoors for men or women when jogging at velocities of between 3.3 and 4.8 m/s. 
As speeds of running increased over 4.8 m/s on a treadmill compared to outdoors: stride length decreased, stride rate increased, and the length of time the support leg is on the ground increased.
So at these higher speeds you are taking shorter strides and getting longer ‘rest’ time each time one of your legs is on the ground. This is countered however by an increased number of strides per minute.
What does this mean in terms of energy use? At higher speeds the greater ‘rest’ time and shorter stride suggests less energy requirement. But an increased rate of stride suggests greater energy requirement. And at low speeds it’s all about the same.
Let’s conclude this by saying that stride length and rate is not a significant factor in our energy equation.
Propelling Your Body
This is the kicker here. The main difference between treadmill running vs outdoor running lies in how your legs have to carry your upper body.
Outdoor running, your leg muscles mostly work on propelling you forward. Treadmill running, because the belt is moving under you, your leg muscles mostly work at re-positioning your legs to keep you stable.
This affects how much work your individual leg muscles need to do.
Treadmill running, the rearward moving belt decreases the need to pull your upper body forward and so requires less work from your hamstrings than outdoor running. However, your hip flexors (right at the top-front of each leg)  have to work harder to provide stability as your planted foot is dragged back (literally) under your body.
(Source: Dave Schmitz PT, LAT, CSCS, PES Health Services at Columbia)
So in treadmill running we have a situation where the hamstrings are less-used and the hip flexors are more used than when outdoor running. This will have an influence on energy output. Hamstrings are b-i-g muscles in the overall scheme of your body – in fact, they are your biggest of all muscles. They use up a lot of energy to expand and contract during each stride. Hip flexors are much smaller and so would intuitively require less energy to squeeze in and out.
Studies estimate that the energy expenditure required by your leg muscles is 3% greater for outdoor running vs treadmill running (running at 4.0 m/s).
We use our legs in different ways on a treadmill
Total Energy Difference
Combine this with our knowledge that overcoming wind resistance outdoor running requires a 5% increased effort (at 4.0 m/s)
And the end result is running at 4.0 m/s you can expect to be working somewhere around 8% harder outdoor running than treadmill running.
So how can you make treadmill running require the same level of energy use as outdoor running?
A 1996 study in the Journal of Sports Science – taking into account the above factors – concluded that a 1% treadmill grade most accurately reflects the energetic cost of outdoor running.
So if you want to match the energy required outdoor running, put your treadmill on a 1% incline.
Other Factors
Aside from energy expended, there is one important factor that makes treadmill running easier than outdoor running:
The treadmill surface is always softer than a hard sidewalk or road surface. That makes it a whole lot easier on all your joints, and without joint pain you can run for longer and be much happier about doing it.
Let’s see. If you set your treadmill to a 1% incline you’re going to get the benefits of the same energy output (and so calories burnt) as outdoor running, plus your joints will be much better protected.

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