Running on a treadmill offers distinct advantages to running outside. Though self-described "hardcore" runners may scoff at those who eschew the scenic outdoors, running on a treadmill provides a softer surface, freedom from traffic and a guarantee of favorable weather. Most motorized treadmills can be inclined to at least a 10 percent grade, and some runners naturally wonder how their paces under such conditions translate to speed on flat ground with equivalent effort.
Considerations
Running on a treadmill is a qualitatively different experience from running on a nonmoving surface. Because you set the speed and this speed is constant, your pace cannot vary slightly as it tends to do outdoors even when you feel like you're holding a constant speed, because if you slow down even a little, you'll fall off the back of the deck. Also, on a treadmill, the "what goes up must come down" edict that applies to ordinary hills is not at issue, as you can run endlessly up a grade without the need to pound your legs on a corresponding decline.
Adjusting for Air Resistance
When you run on a treadmill, because your body remains in one place, you do not have to work to overcome the air resistance that you confront when running outdoors even in perfectly still air. As a result, to compensate for this nominal advantage, you need to incline the treadmill to a 1 percent grade to create the equivalent -- in terms of energy cost -- of running outdoor on level ground. According to exercise physiologist Greg McMillan, the difference in oxygen utilization when not having to deal with air resistance is significant -- about 7 percent.
Adjustments at Lower Inclines
When comparatively slow runners move from a flat running surface to an incline, their equivalent paces drop by a greater percentage than speedier runners. For example, if you run on the flat at 12:00 pace per mile, or 5 mph, and incline the belt to a modest 5 percent grade, your equivalent pace is 9:38 per mile, an effective improvement of 20 percent. In contrast, someone running 6:00 pace per mile who moves the belt to 5 percent gains only 36 seconds per mile, an 8 percent improvement.
Adjustments at Higher Inclines
The higher the grade at which you choose to set the treadmill, the less difference each additional increment makes in terms of equivalent level-ground effort. Our putative 12:00-per-mile runner chugging along up a 8 percent grade is doing the same work she would do to hold 8:38 pace on the flat, but knocking the grade up to 10 percent only gives her 15 seconds per mile of "improvement." A 6:00-per-mile flat-surface runner is in effect hammering out 5:02 miles at 8 percent and 4:49 miles at 10 percent. Above 10 percent, running form deteriorates for most people and compromises any attempted workouts.
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