There are two types of people in this world: those who run on treadmills, and those who run outside. I’m 32 and I have never run on a treadmill until a month ago when I did a fundraising run with a group of others at my gym.

Now I’m no long-distance runner, but I was able to run for an hour and a half without any trouble at all on the treadmill that day (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 outside running. And a few people have since agreed with my suspicion. But what is the truth? Is it easier to run on a treadmill or run outside? 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:

1. WIND RESISTANCE

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
 
2.  BIOMECHANICS OF RUNNING

What about the differences in the actual biomechanics of treadmill running compared to 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 to 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 and 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 over 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.
 
The Mental Challenge

There is one aspect, not related to energy requirements, that effects how easy your run will be.

As I see it, being distracted when running makes things a whole lot easier. Distractions include listening to music or watching tv while you workout. When you focus on something else rather than your body, you don’t notice your body so much. This helps to alleviate any running discomfort. This is why running with someone or in a group is much easier than running by yourself. There is no better way to distract you from what you are doing than my conversing with someone.

Listening to music distracts from effort
 
But focusing on external distractions can also take away from the actual intensity that you can go to and the overall exertion that you feel. And sometimes (most of the time) actually feeling that effort and intensity is a large part of the fun. Like for my daily 1 km time-trials (which I’ve been experimenting with since the motorway run) I  run without my ipod so I can focus exactly on the level of exertion on my body.

To wrap this all up, we can say that treadmill running with distraction (people, tv, music) is going to feel easiest and require slightly less energy. Outdoor running with no music is going to feel hardest, but you will benefit from expending more energy (and likely burning more calories).

I always say make it easy for yourself to get your exercise done. Obviously easy for you is different to easy for a professional triathlete. Just be clear on what level of effort you want to reach at each run and do whatever it takes to reach it.

Source: Body Transform