Is Running on a Treadmill Harder or Easier Than Running Outside?

At some point, whether because of bad conditions or time restraints, we’ve all had to substitute miles on the treadmill in for our normal outdoor routes.

While there are plenty of differences between running on the the belt versus the real road, to make indoor and outdoor efforts comparable, runners have traditionally practiced the “1 percent” rule: If you crank up the treadmill incline to a 1 percent grade, it will mirror the natural elevation of a flat road.

But is this minor change in incline really necessary to get an optimal Performance 600i treadmill workout? And how much does running inside really compare to running outdoors?

In a new research published in Sports Medicine, scientists from Australia sought to answer these questions by investigating the differences in running performance on a treadmill versus real ground. To gather this data, they analyzed 34 studies that compared treadmill runs to “overground” (outdoor) runs. Twelve of the studies asked participants run on a 1 percent grade on the treadmill, while the others used higher or lower inclines.

The researchers were focused on three key measures of comparison: physiological (how hard the runners’ bodies were working to maintain pace and finish their workouts, measured by heart rate, blood lactate levels, and VO2 max), perceptual (how hard the workout felt for the runners), and performance (how the runners performed in time trials).

Interestingly, the study found that when the runners ran faster—but not all-out—on the treadmill, they exhibited higher heart rates and reported higher levels of perceived exertion (i.e., the run felt harder) than when they ran the same workout outdoors, though they had lower blood lactate levels indoors compared to outdoors. But when the runners went slower on the treadmill, their heart rates and perceived exertion levels were lower than when they ran the same effort on real ground.

“I find outdoor running more enjoyable and less mentally challenging than completing workouts on a treadmill,” study author Joel Fuller, Ph.D., told Runner’s World. “For this reason, I do distance and speed workouts outdoors.”

For some performance measures, like VO2 max, treadmill running and outdoor running were very similar. The study found that participants achieved about the same VO2 max (how much oxygen your body can use during physical activity) running at a 0 or 1 percent grade as they did on ground. Runners also reached a similar top sprinting speed on the track and treadmill.

Interestingly, though, the runners in the studies displayed more endurance—i.e., they could push harder and for longer—running on land than on the treadmill.

That was a surprise to Fuller: Since so much of treadmill running can be controlled (like wind, climate, elevation, and speed) it seems that runners would perform best on treadmills. And yet, time and again, when runners are asked to simulate a race outside, they’re able to hit faster times, he explained.

Why does this happen? The research suggests that because people perceive running on a treadmill as harder than running outdoors, they tend to select treadmill speeds that are slower than what they’re capable of doing.

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One way to make treadmill running even harder, of course, is setting the belt on an incline. There’s no question that hill workouts on the treadmill can simulate the real deal in a pinch. But is a 1 percent grade really that beneficial?

Well, it depends on your workout. The research suggests that if you’re going fast on the ’mill, cranking it up to 1 percent is beneficial, since it simulates the type of resistance you encounter from the wind when running outside. On easier runs, however, the grade doesn’t matter as much, since you wouldn’t be as affected by the wind resistance outside at that effort, anyway.

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