6 Reasons to Run On the Treadmill Instead of Outside

Not so much. With record-high temperatures and humidity reaching 90 percent, chasing miles on the treadmill makes for a much better, less insanely sweaty run than the open road. Of course, getting outside for sunshine and fresh air provides its mental and physical benefits, but sometimes it really is better to find your stride on a machine.

Here are some of the benefits of running on a treadmill vs. outside, and six scenarios that say, "Hey, let's stay inside and cruise through some distance in the comfort of AC and the predictability of tread terrain."

1. When simply standing on a street corner makes you sweat.

Check the weather before you even leave the house, says David Siik, co-founder and creative director at Precision Run. He suggests only attempting short runs on those 80- and 90-degree days when the humidity hits 40 percent or higher. "Heat indexes of 95 degrees or higher is my recommended breaking point—just don't do it, it's not worth it," he says. "If you do head out and within 10 minutes you feel like you are working harder than usual, feel heavy, or start to feel a little cold and clammy, get back inside." (Related: What Running In the Heat Does to Your Body)

Marni Wasserman, a coach at Mile High Run Club, recommends checking the dew point, which considers both heat and humidity. When the dew point hits 70 or 80 degrees, you probably want to turn to the tread. Wasserman also says to skip the outdoor heat if you haven't been hydrating. (For example, if you had one or two too many glasses of wine last night.) "The longer the run, the more fluids you'll need heading into it because the more you'll be sweating," she says. (More here: Is It Safe to Work Out In a Heat Wave?)

2. When you're making your comeback post-injury.

One of the major benefits of running on a treadmill is that it's a softer surface and therefore less force on your joints. While running on trails or a track offer more give than pavement, many manufacturers design treadmills to specifically absorb the shock of each step. The Woodway tread used in Precision Running classes, for example, has rubberized slats meant to lighten the impact of your run. The Peloton machine has similar features. The reduced shock on the joints equals reduced risk of injury.

Running on a treadmill also means you don't have to worry about moving laterally to dodge sticks, trees, cars, or people, says Siik. "That simple plane of work can really help to reduce stress and inflammation to areas recovering from injury," he adds. 

Another benefit: If the same pain starts to kick back in, you can stop right there on the tread and not have to worry about getting back home, says Wasserman, who adds that the treadmill can also help you ease back into running by allowing you to set a more relaxed pace and stick to it, without over-pushing yourself.

3. When you need more motivation to speed through intervals.

It's easy to slow down on the open road during running interval workouts, even when you're trying to get in some sprint drills and all-out efforts because you don't have someone (or something) forcing you to move faster. The treadmill, on the other hand, requires you to go hard to keep up with the belt. "The treadmill is painfully honest. It cannot lie to you," says Siik. "If you put in 10 mph, the treadmill will hold you accountable to that speed unless you change it or step off. That freedom from the ability to cheat is a wonderful accountability tool."

Another benefit of running on the treadmill is that you can make micro-adjustments to your run that gets you to push harder without making it crazy dramatic—say adding 0.1 percent incline or 0.1 mph. "This creates a type of engagement that makes the run so much more fun and dynamic," says Siik. (Related: What Is a Fartlek Run?)

One catch, though: If you tend to step off the belt between sprints (rather than slowing down) or you're scared to go higher than a certain number for fear of falling off the machine, you might want to consider the outdoors your best bet for interval workouts, says Wasserman. Those signs could mean the numbers are actually holding you back.

4. When you don't have hills in your 'hood.

If you live in a mostly flat area, but you've signed up for a race with steep incline climbs, then you'll want to become BFFs with the treadmill. "If you have access to the course elevation profile, you can try to mimic it on the tread to make long runs more interesting. Or, if you take the inclines faster, you can get a feel for the climbs at race pace," says Wasserman. She recommends doing rolling hills at 3 to 6 percent incline when you don't have the course to copy. "You can also play around with hill sprints to build leg strength, improve form, and boost power—they're really tough and will make you feel like you're flying once you drop the hill," she says. Not to mention, it's a lot easier than finding the perfect hill to do sprint repeats IRL.

5. When you want to protect your skin.

Crushing runs outside multiple days a week means the sun continuously beats down on you, especially if you forget sweat-proof sunscreen before you head out the door. So you might want to go inside occasionally just to keep your skin under cover, says Siik. "Although you should enjoy running outdoors, if you supplement the 'harsh' days (heavy exposed sun and high heat or cold, windy, slippery weather) you lift the burden and fatigue on not only your body but your skin," he says. "Imagine spending 40 percent less of your life being destroyed by the sun or chapped from the wind but keeping up the same level of fitness and cardiovascular health—win, win!"

In addition to the sun wreaking havoc on your skin, air pollution can do the same. And the drawbacks of low air quality go further than that, too. If your runs regularly take place near busy, car-crowded streets, the pollution can negatively affect your fitness level and your overall health. So take it inside from time to time to breathe in some cleaner air.

6. When you haven't run in a while.

Taking a treadmill-focused class not only brings a social element to a typical solo run, but you also get expert advice from the coaches talking you through the run and helping you set goals for pace or incline. "There is nothing more unique and educational than a coach who stands next to you, in front of you, compassionately observing and giving real-time feedback, correction, and praises," says Siik. Sign up for a treadmill session and see how it goes—you might find a return to the tread after a long hiatus brings your passion for the sport right back.