Is Running Outside Better Than the Treadmill? Here’s What the Research Says

Whether you’re shooting for a PR or simply training to finish a race, one thing is certain for runners everywhere: If you’re putting in the work, you want to make sure you’re going to get the results to show for it.So it’s only natural to question whether your regular running routine is helping to maximize your performance—or if it’s actually holding you back. And one common cause for concern has to do with where you choose to run: Do treadmill runs train your body just as well as outdoor sessions, or might you be leaving some benefits on the table if you default to indoors?The topic of treadmill runs versus outdoor runs has been highly debated for years among runners—and researchers join in the conversation, too. Most recently, a new study suggests outdoor running simply pushes your body harder than treadmill running does, even if you use a slight incline to try to narrow the gap.But does that mean running outdoors is always the better option compared to the treadmill? Below, running experts dig into what you need to know about treadmill versus outdoor running—and how to make the most of your training, wherever you’re clocking miles.The Energy Cost of Running Outside vs. TreadmillIn the recent study, which was published in the journal PLOS One, researchers recruited 12 young, highly trained male runners who regularly ran at least five days a week, averaging about 40 to 50 miles. They had them complete two running tests, one on a treadmill with a 1 percent incline and the other on an outdoor track.Each used a progressive protocol, starting easy and getting faster with each three-minute stage. The tests began at a 12:00 mile pace and ended at just under a 6:30 pace. Throughout the trials, researchers measured the runners’ oxygen cost and other metabolic responses.The researchers found energy cost and oxygen cost were higher and running economy was worse when participants ran outdoors versus indoors. The differences in running economy were most apparent when the runners approached their second ventilatory threshold (which they reached, on average, when running a pace of about 7:10 to 7:22)—meaning, as they picked up the intensity.“That second ventilatory threshold separates the hard work from the very, very hard work,” says Grace Horan, MS, an exercise physiologist on the Sports Rehabilitation and Performance Team at Hospital for Special Surgery who was not affiliated with the study. “So as they approached that threshold between hard and very hard, the difference did become more apparent based on their data.”This roughly falls in line with prior research suggesting that differences in energy cost between treadmill and outdoor running can depend on how hard you’re running—which is when tweaking the incline can come into play.In particular, a 2019 meta-analysis in Sports Medicine that looked at 34 studies found that the two had similar oxygen demands at easy or moderate paces. But at near-maximal intensities—above 80 percent of VO₂ max, or roughly a 6:00 to 7:30 pace for the runners included in the review—oxygen consumption was lower on a treadmill with a 0 percent incline than during outdoor running. When the incline was bumped up to 1 percent, though, the oxygen cost was similar.Oxygen demand can play a role in performance, as it contributes to fatigue. “Lower oxygen consumption at a given submaximal pace is beneficial,” says Horan. “It basically means that you’re able to save some energy—you don’t have to spend as much energy or utilize as much oxygen to maintain the same submaximal pace.” So that means you may be able to hit faster paces on a treadmill without feeling like you’re working quite so hard.While the more recent study’s results do lean in favor of outdoor running being “harder,” there are a few important limitations to keep in mind before drawing sweeping conclusions. For one, the recent study was super small, and enrolled only young, male runners, so it’s not possible to say whether the findings would be generalizable to different demographics. And the device they used to measure metabolic metrics tends to overestimate readings, particularly at higher intensities, which could overinflate the differences, says Horan.Finally, the directive to maintain exact pacing outdoors—plus the audio reminders piped into the participants’ ears every 100 meters—could have contributed to the higher strain of outdoor running, too. “It’s incredibly hard for humans to be able to achieve [precise pace] without any kind of deviation,” she says, which can lead to a more stressful environment.All in all, the findings shouldn’t lead you to change anything about your training, Horan says. Both outdoor and indoor runs have a place on your plan.How Is Running Outdoors Different Than Running on a Treadmill?In addition to how hard you’re physically working, a few key points create slightly different experiences between running outdoors and hitting the belt.The big one is wind resistance: “There’s a lot more air resistance and aerodynamic drag that a runner has to fight against to propel themselves forward outside on ground,” says Horan. “With a treadmill, that’s kind of keeping that forward propulsion for you or keeping that pace for you.”And the differences in outdoor running versus treadmill running are likely to become greater the harder you go and the more pronounced the effects of air resistance become, per a review in Sports Medicine. “The faster you run, the stronger the wind feels, and the more work that’s required to move against that wind,” RRCA-certified running coach Alysha Flynn, founder of What Runs You and official training advisor for Every Woman’s Marathon, tells Runner’s World.Outdoor running also brings constant surface and elevation changes. The most obvious would be hills, which demand a lot more from your glutes and hamstrings to power you up, says Flynn. Of course, you can add incline to your tread, but outdoor hills are more consistently variable, which can ask more out of your body—and the eccentric nature of actual downhills means your quads work overtime, too.But even if your route seems relatively flat, you’re still dealing with subtle variations that you don’t have to worry about on a treadmill belt, she says. This includes surface changes as you hop onto a sidewalk from asphalt, the slight pitch or camber on streets, or the turns you make to round a track or move onto another street. “Changes in your footing and all of that adds more demand to your body,” says Flynn. “Your body has to adjust for it in real time to control your balance and the direction you’re going.”These slight changes also require more out of your small, stabilizing muscles, says Horan, which don’t get challenged as much on a treadmill. For instance, a PLOS One study found reduced activation of ankle-stabilizing muscles when running on a tread versus outdoors.These variations—along with the fact that humans are not robots—can make it tougher to hold exact paces outdoors, whereas a treadmill takes care of that for you.But on the flip side, holding an exact pace for an entire run can make treadmill running feel harder for some folks. “Outdoors, runners naturally speed up and slow down slightly,” says Flynn. On the treadmill, you don’t have that slight variation, which can be tiring physically and mentally.The boredom of staring at the same four, unchanging walls around you can also make treadmill running feel harder. So too can simply doing something your body is not used to. “I think a lot of it is psychological,” says Horan. “A lot of it is like, ‘I’m running on a belt that is moving for me and I don’t typically do that. This feels wild.’ And that certainly can drive up the rates of perceived exertion.” And that psychological aspect can make it seem more taxing, even if physiological measures still give outdoor running the “harder” edge.How to Make the Most Out of Your Treadmill Runs“The treadmill and outdoor running are two different environments for training,” says Flynn. “You just have to understand what each offers, then choose the one that better supports the purpose of your workout and what you need that day.”In fact, there are some situations in which a treadmill may be better able to help you hit that workout goal. For instance:Hard, Precisely Timed RunsWorkouts that require you to hold specific, hard paces can be a great choice on a treadmill, whether we’re talking about short intervals or longer tempos. “Doing it on a treadmill will allow for that pace-precision and take some thought out of it for yourself, rather than trying to do it on the road,” says Horan.You don’t have to keep checking your watch to make sure you’re on target—as long as you haven’t flown off the belt, you know you’re on pace.Truly Easy RunsOn the other hand, treadmills can also be a solid tool to ensure you actually take an easy run easy. While you might be tempted to speed it up on the road, you’re locked into the pace you chose on a treadmill unless you physically push the buttons to change it. “I might lean into that for an athlete who is really struggling to manage their pace to learn better pacing,” Flynn says. “It’s a nice tool to reinforce effort-based running.”Progression Runs to Find a FlowIf you live in a city or run on a route where there are lots of people or traffic, you know all too well how it feels to have to slam on the brakes right when you’re getting into a rhythm. Usually, a quick pause isn’t a problem, but if you’re doing a progression run, it can really pull you out of your flow.“For some runs, we really want them getting to a pace and being able to hold that or progress their pace without any interruptions,” Flynn says. “Getting on a treadmill would help them be able to do that, otherwise that run would be broken up because of their environment.”Hill Work in Flat AreasOutdoor routes often come with changes in elevation, but depending on where you live, you might not have that option—which can be a problem if you’ve signed up for a race that’s rich in hills.A treadmill allows you to play around with the incline and train with the hill stimulus, says Flynn. That way, you can learn how it feels to approach an incline when you’re already gassed, or to call on more from your glutes and hamstrings when they’re already feeling it. “For at least part of the run, you can focus on pulling back the pace and intensity during the upward incline and coming back to race pace for the descent,” Horan adds.Safety“The best training tool is sometimes the one that makes consistency possible,” says Flynn. A treadmill allows you to continue your scheduled workout when environmental factors may have led you to skip outdoors. These include: super cold, snowy, or icy weather; heavy downpours; or wildfire smoke.Plus, if you need to run in early morning or evening hours, loss of daylight could make you feel less safe—so taking it inside might be the option that lets you get that run done.For nearly 10 years, Christa has created health, fitness, nutrition, and wellness content that’s steeped in science but engaging enough that people actually want to read it. She’s tuned in to all the latest research that people with an athletic lifestyle need to know, and prides herself on helping her readers apply it to their everyday lives.