One of the more persistent myths in running is the idea that running on a treadmill is “easier” than running overground. Here’s a typical formulation of the myth, which I found on the website of the Houston Chronicle:
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.
While it is true that, at faster speeds, the energy cost of running on a treadmill is lower than the energy cost of running overground, it is not true that this results in a lower perceived effort level on the treadmill. In fact, precisely the opposite is true. Studies have shown that running on a treadmill at any given pace feels harder than running outdoors despite the fact the cardiometabolic demand is lower.
How is this possible? It’s pretty simple, actually. Heart rate is not the only determinant of perceived effort. A variety of other factors, including psychological factors, also affect how hard it feels to run at a given pace. Indeed, a 2011 study by Brazilian, Italian, and American researchers found that overground running feels easier than treadmill running simply because it’s more fun. But I happen to think there’s another factor at play, which is the slightly greater degree of control one has when running outdoors.
When you run outdoors, your pace is never perfectly steady. Even when you’re trying to run at a perfectly steady pace, there are micro-fluctuations in rhythm, whereas on the treadmill you are locked into a rigidly unvarying rhythm. There is evidence that this lack of freedom slightly increases perceived effort. For example, a study involving rowers found that perceived effort was lower when a certain wattage was maintained voluntarily than when the same wattage was automatically enforced.
What’s more, because perceived effort has a much stronger effect on performance than heart rate does, runners are also faster outdoors than they are on the treadmill. Don’t believe me? Too bad! It’s a proven fact. In a 2014 study by researchers at the State University of Maringa in Brazil, 18 recreational runners were asked to perform one-hour time trials on a treadmill and on an outdoor track. On average, they covered 11.8 km on the treadmill and 12.2 km on the track. In other words, they performed 3.3 percent better outdoors. Yet their heart rates were lower on the treadmill.
Somebody reading this post is thinking, “Treadmill running may be harder and slower than outdoor running for most runners, but I’m an exception. I know from experience that I can run faster at a lower effort level on a treadmill than I can outside.”
The problem with this objection is that it’s based on the assumption that the speed/pace data you see on the treadmill’s information display is accurate, and this is seldom the case. Most treadmills are poorly calibrated. If you pick a treadmill at random, step onto the belt, and set the speed at 7.0 mph, you might actually be running at 6.6 mph, 6.9 mph, or 7.3 mph. I own a treadmill of reasonably high quality, and its speed readings only remain accurate for about six months after each calibration. My service plan limits me to one “free” recalibration per year, and by the time the tech comes out to my home, the speed is usually off by about 3 percent—and always in the same direction. Specifically, it’s telling me I’m running 3 percent faster than I really am. So a runner who used my treadmill in this uncalibrated state and didn’t know it needed calibrating might think that he or she is able to run faster more easily on a treadmill than outdoors.
It’s not really time but usage that causes a treadmill to lose calibration. My wife and I use our machine anywhere from three to ten hours per week. Consider how much more usage the typical fitness club treadmill gets. Unless these machines are serviced every other week or so (and most aren’t), they are likely to provide unreliable speed/pace information. You truly never know what you’re getting on a fitness club treadmill. It would be a fun experiment to go to a gym wearing a properly calibrated running accelerometer and run on five different treadmills, each set at 7.0 mph. I wouldn’t be surprised if your device gave you five different pace readings.
Don’t get me wrong: Treadmill running is real running. Heck, Christine Clark won the 2000 U.S. Olympic Trials Marathon after training almost exclusively on her home treadmill. But you can’t trust the speed/pace information a treadmill gives you, and even on an a well-calibrated treadmill, you can’t compare your speed or pace to your performance outdoors.