In contrast to their nonelectric ancestors, most contemporary treadmills feature all sorts of "extras" from displaying your heart rate to preset programs that vary the way the belt functions in terms of speed and incline without you having to touch any buttons during your workout. Most machines allow you to incline the belt to up to a 10 percent grade, while some top-end machines go up to 15 percent.
What Is Percent Grade?
Percent grade is not to be confused with the angle of the incline in degrees, although at low grades the two are roughly similar. Percent grade can be described simply as "rise over run" -- that is, the amount of vertical change divided by the amount of horizontal change in a given period. So if you are running on a treadmill set to 5 percent, this means that for every 100 horizontal meters you cover, you gain 5 meters in elevation. Obviously on a treadmill you don't actually go anywhere, but these are the physical equivalents.
1 to 3 Percent
If you switch from moving on the level to climbing a grade up to 3 percent, you'll notice the difference in effort at the higher end, and if you were outdoors you would slow down without a conscious and significant increase in effort. A 1-percent grade is so gentle as to be negligible, whereas most runners and walkers are aware of a 2-percent grade. Three percent might be considered a reasonable cutoff point separating a "rise" from a genuine "hill"; for reference, the notorious Heartbreak Hill just after the 20-mile mark of the Boston Marathon climbs through 91 feet in a 3.3 percent grade.
4 to 6 Percent
Walking or running up grades in this range means a significant increase in oxygen cost, noticeable changes in form such as leaning into the "hill" and swinging your arms with aplomb, and a forced slowing of the treadmill belt unless you had it set on a very manageable speed to start with. On real terrain, hills in this range appear moderate to steep and, if fortuitously placed, are apt to earn unique names. The venerated Hayes Street Hill between the 2- and 3-mile marks of the Bay to Breakers in San Francisco, one of the largest 12K races in the world, boasts an average grade of 5.5 percent; the maximum allowed grade on U.S. Interstate Highways is 6 percent.
7 to 10 Percent and Above
Even walking up grades this steep is difficult, and should you try, your quads will remind you that the treadmill starts with the belt horizontal for a reason. City streets that feature 10-percent grades lasting for more than brief stretches are unusual. These types of grades, however, are systematically sought out by endurance athletes such as cyclists and runners for competitive purposes. One of the most popular and storied mountain races in the United States, the Mount Washington Road Race in New Hampshire, rises 4,650 feet through 7.6 miles for an average grade of nearly 12 percent, and the best runners who have ever tackled it have barely managed to average under 8:00 per mile.
Michael Crystal earned a Bachelor of Science in biology at Case Western Reserve University, where he was a varsity distance runner, and is a USA Track and Field-certified coach. Formerly the editor of his running club's newsletter, he has been published in "Trail Runner Magazine" and "Men's Health." He is pursuing a medical degree.