What’s the Perfect Slope? Researchers Study Uphill Mountain Running

Joe Gray, 31, of Colorado Springs won the Power of Four Vertical K race up Ajax Mountain in Aspen, Colo., on July 18. Gray covered the route, which began at 7,952 feet and gained 3,258 feet over 2.72 miles, in a new course record of 37:30. Photo: Aspen Snowmass

Running uphill on steep inclines is never easy, but researchers at the University of Colorado Boulder have discovered a range of slope angles that would allow an athlete to ascend a mountain most quickly.

“For either running or walking, slopes between 20 and 35 degrees require nearly the same amount of energy to climb the hill at the same vertical velocity,” said Rodger Kram, an associate professor in CU-Boulder’s Department of Integrative Physiology and senior author of the study.

The study, which was recently published in the Journal of Applied Physiology, is believed to be the first to examine the metabolic costs of human running and walking on such steep inclines. Kram said the findings also indicate that you can still get a good aerobic workout simply by walking up very steep trails.

Several competitive mountain runners co-authored the study, including 2013 World Junior Mountain Running Champion Amanda Ortiz, a CU cross country runner and undergraduate student in the Department of Integrative Physiology.

The research focused on the challenges posed by vertical kilometer (VK) races, which have grown in popularity in recent years in mountainous regions of the U.S. and Europe. Athletes run or walk up steep slopes ranging between 10 degrees and 30 degrees in order to ascend 1,000 meters over a distance of less than 5 kilometers, or about 3.1 miles.

PHOTOS: Aspen’s Power of Four VK and 50K Races

The world record for a VK course is 29 minutes, 42 seconds and was set in Fully, Switzerland on a course with an average slope of 31 degrees. By comparison, elite runners can finish a 10K race on flat ground in less than 30 minutes.

The extreme slopes of VK courses inspired Kram and his colleagues to design a specialized treadmill that can reach inclines of up to 45 degrees. Standard gymnasium treadmills only reach a maximum incline of around 9 degrees, he said, while a typical black diamond ski run slopes about 25 degrees.

“We wondered if there was an optimal incline for beating the VK world record,” Kram said. “If you were running up just a one degree incline, you’d have to run over 70 miles per hour to beat that record, which is impossible. And if the slope were 90 degrees, you’d need a rope and rock climbing equipment, which isn’t feasible either. We figured there was a perfect angle somewhere in between.”

Instead, the researchers found that there was actually an optimal range of slope angles, a “Goldilocks plateau” of between 20 and 35 degrees where the degree of incline “doesn’t really matter and the same rate of ascent requires the same rate of energy expenditure,” Kram said.

Kram summarized the findings via an analogy: “Imagine that you are standing in Colorado at a trailhead where the base elevation is 9,000 feet. Your friend challenges you to race to the summit of the mountain, which tops out at 12,280 feet, i.e. 1,000 meters of elevation gain. There are several different trails that go to the summit. They are all pretty steep and some are extremely steep. One trail averages 10 degrees incline and the sign says it is 3.6 miles long. A second trail averages 30 degrees, but is only 1.25 miles long. A third trail averages 40 degrees, but only 1 mile long. To get to the summit the fastest, which trail should you choose and should you walk or run?

“Based on our research, we now know that choosing the second trail (30 degrees) and walking as fast as you can within your aerobic capacity is the fastest way to go,” he said.

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The study focused on a vertical rate of ascent of just over 1 foot per second, a pace that the high-level athletes could sustain aerobically during the testing. At that speed, walking used about nine percent less energy than running. So, sub-elite athletes can ascend on very steep uphills faster by walking rather than running.

The study examined 15 competitive mountain runners as they ran and walked on the treadmill at seven different angles ranging from 9 to 39 degrees. The treadmill speed was set so that the vertical rate of ascent was the same.  Thus, the treadmill speeds were slower on the steeper angles. The athletes were unable to balance at angles above 40 degrees, suggesting a natural limit on the feasible slope for a VK competition.

“Very few people walk or run up such steep slopes, but by going to extremes, we broaden our horizons and investigate the limits to human performance,” Kram said.

As for amateur runners who aren’t planning to try a VK race anytime soon?  These findings indicate that you can still get a good aerobic workout simply by walking up very steep inclines, Kram said.

Competitive mountain runners Nicola Giovanelli, a visiting scholar from the University of Udine, Italy; Keely Henninger, who now works for Nike; and Ortiz co-authored the study. The University of Colorado Boulder Undergraduate Research Science Training (BURST) and Undergraduate Research Opportunity Program (UROP) supported the research.

Boulder/Denver area runners who are interested in being a participant in future uphill running experiments should contact uphillrunstudy@gmail.com

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