Treadmills and bikes, indoors versus outdoors

Story highlights

  • You burn more calories running outside than on a treadmill, but burn fewer on real bike than a stationary one
  • You are more prone to injury on a treadmill than running outside, but a stationary bike is safer than a real one

Dr. Tom Flaherty is a medical doctor, triathlete and fitness fanatic who lives in New York City.

(CNN)It's that time of year when the allure of a warm gym with comforting TV screens and a post-workout sauna might still seem appealing. But as spring blossoms and the outdoors calls, the question looms: Is it better to exercise indoors or out?

There are pros and cons for both. Indoor workouts are cozy and dry and sometimes safer; outdoor workouts are almost always free and often right on your doorstep. But which is better for your overall health and fitness?
CNN tested the most popular indoor and outdoor exercises, measuring calories burned, heart rate (exercise intensity), likelihood of injury and how they rank on the all-important fun scale.

Running outdoors vs indoors (treadmill)

Calories burned per hour: Outdoors was 970. Indoors was 661.
Winner: Outdoors.
Average heart rate: Outdoors was 163. Indoors was 129.
Winner: Outdoors.
Risk of injury: You are more likely to pick up an injury running on a treadmill than in the great outdoors. On the treadmill you repeat virtually the same running motion every stride, which increases your risk for an overuse injury, like tendinitis. Outside, the more corrections you have to make for running over curbs, up and down hills, and over rough ground, the more you are forced to use different muscles and tendons, so you're less likely to damage them. The treadmill is the single most dangerous piece of exercise equipment, accounting for 24,400 emergency room visits in 2014, according to the U.S. Consumer Product Safety Commission. Although the risk of falling on a treadmill is pretty low, if you do fall, the consequences can be quite severe.
Winner: Outdoors.
Fun scale: Studies show that runners who run outside report increased feelings of well-being and revitalization, and decreased feelings of depression, stress and anxiety. Those feelings might be the result of a connection with nature -- the sun, wind, mountains, trees and so on. Or they could be due to the fact that you're more likely to run outdoors with a running partner or group, and develop a human connection, than if you run on a treadmill inside by yourself.
The gym can be a great place to meet up and work out with friends too, but most people run solo indoors, which can be quite monotonous, and slightly sad when the realization sinks in that you're running but not actually going anywhere on a treadmill.
Winner: Outdoors.
Overall winner: Outdoors.

Cycling outdoors vs indoors (stationary bike)

Calories burned per hour: Outdoors was 570. Indoors was 761.
Winner: Indoors.
Average heart rate: Outdoors was 119. Indoors was 140.
Winner: Indoors.
Risk of injury: Risk of the most common cycling injuries, such as Achilles tendinitis, patellar (knee) tendinitis and saddle sores (a sore butt from being on the seat for too long) is roughly the same. But the risk of falling off a stationary bike and injuring yourself is virtually zero. If you fall off your bike outdoors, the bones you're most likely to break are the clavicle (collar bone) and the scaphoid bone (one of the bones in the wrist).
Winner: Indoors.
Fun scale: Biking outside is a great activity to do with friends, and being in a group makes the ride safer, if you were to fall. A recent study published in Psychology of Sport and Exercise found that our friends' exercise habits have a positive impact on our own exercise habits. So you're more likely to push yourself and have a better workout if you buddy up. In a similar way, a spinning class lets your feed off other people's energy in the studio and work harder as a result. New research suggests that when we work out in groups, we release more endorphins -- the "happy chemicals" responsible for a runner's high, and feelings of fondness toward others -- because you bond as a group.
Winner: Tie
Overall winner: Indoors
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So, for running, if you want to get the best bang for your health buck before the summer, it pays to get the right gear for the weather, brave the morning chill, and head outside. For cycling, don't feel guilty if you stay inside on the stationary bike until the weather perks up. But when summer finally arrives, I still recommend cycling outdoors. It's good for the soul, and your vitamin D levels. Just ride safe and always wear a helmet.