By Hunter Bennett, Lecturer in Exercise Science, Adelaide University. Originally published in The Conversation (opens in a new tab) in July 2026.
You hop on the treadmill, set your pace, and easily knock out a couple of kilometres.
Then a few days later you head outside to run the same distance. You try and to keep the same speed you did at the gym, but it destroys you.
What's going on? Is it the weather? Your mood? Or is running outside genuinely harder?
The conditions are different outside
There are a couple of key reasons why running outside might feel harder than on a treadmill.
First, when you run outside, you push into the ground to propel yourself forward. But when you are running on a treadmill, the belt moves under your feet, helping with this propulsion (albeit very slightly). Basically, the machine does a small amount of the work for you.
Second, there's air resistance outside. Research has suggested (opens in a new tab) running on a treadmill is slightly easier because there is no air resistance caused by wind or even a slight breeze.
Studies show setting the treadmill to a 1% incline (opens in a new tab) makes it more similar to running outside, by mimicking the difficulty added by air resistance.
Finally, the undulating terrain you have to deal with outside can make running more challenging. Even a small hill can make you work harder than when running on flat ground.
What about when the conditions are the same?
A large body of research has compared the demands of running outside with running on a treadmill under matched conditions (for example, at the same speed).
A recent systematic review (opens in a new tab) combined 34 studies on this topic, and found that overall running on a treadmill was physically easier compared to running on the ground, resulting in:
- lower oxygen uptakes – how much energy your body needs to create to maintain your running speed
- less lactate accumulation – how hard your body is working to produce energy during running
- lower heart rates – how hard your heart is working to pump blood and oxygen to your muscles while you run.
But while these measurements showed running on a treadmill was physiologically easier, the same study found people felt it was harder (although only slightly).
This could partially be due to airflow. Without the natural breeze you get when moving outside, your body can overheat faster (opens in a new tab), which could make it feel slightly harder.
Of course, it could also be that some people simply find staring at a wall on a treadmill to be boring, which makes it feel harder.
So, which will make you fitter?
Based on the research above, it seems possible that running outside might increase your fitness more than training on a treadmill. However, there is very little research answering this question directly.
One study (opens in a new tab) in active young males found six weeks of outdoor running led to slightly greater improvements in fitness than six weeks of treadmill running, despite all training sessions being the same duration and intensity.
Similarly, another study (opens in a new tab) in athletes found six weeks of sprint training performed outside led to much greater improvements in sprint speed, compared to six weeks of sprint training on a treadmill.
With this in mind, if you went for a 2 kilometre run on the treadmill, it might be slightly less effective at increasing your fitness than a 2km run outside. But you will still get benefits, and the treadmill is still a great option if running outside is not possible, or without treadmill running, you wouldn't run at all.
Other considerations
Running both outside and on a treadmill can have some unique advantages depending on your goals and needs.
There is research suggesting exercising outside in nature might have psychological benefits (opens in a new tab), including reduced feelings of depression and improved mood, compared to exercising inside.
So, if you are someone who exercises for mental health, running outside might be might be better.
Conversely, treadmills provide a consistent running environment that allows you to precisely control your pace and distance. This might make them better if you are returning from an injury and don't want to do too much too soon.
Final thoughts
When you run outside your body has to work slightly harder to propel you forward. This means that it can provide a slightly more challenging workout.
But the treadmill isn't a shortcut.
If your goal is to improve health and fitness, both work. Rather than picking one over the other, think of them as different tools in your training kit that you can use depending on your goals.
This article is republished from The Conversation (opens in a new tab) under a Creative Commons licence. Read the original article (opens in a new tab).
