Get access to everything we publish when you sign up for Outside+.
Those old enough to have learned to drive with a manual transmission were probably told they had to shift gears when the tachometer reached a certain RPM. Once you became proficient at it, you simply knew when to shift, by feel or sound, and didn’t need to look at the gauge, much less think about it. The same is true of changing between running and walking uphill, but, surprisingly, it isn’t as simple, involving many variables. In fact, it is so complex that the choice of when to run or walk up a hill was the focus of a mountain runner student’s recent honors thesis.
Jackson Brill, a Salomon-sponsored runner and soon-to-be-graduate of the University of Colorado Boulder, wrote his thesis on “To Run or Walk Uphill: A Matter of Inclination” toward earning his degree in Integrative Physiology. In researching it, he worked with his advisor, prominent CU faculty member, Dr. Roger Kram, Ph.D., Integrative Physiology, who runs the locomotion lab that did the original Nike 4% testing.
Brill’s thesis centers on the point at which uphill trail and mountain runners transition to walking. He measured three different speeds at which this can occur. First is the “Preferred Transition Speed” (PTS), where people prefer to switch—any slower, humans prefer to walk, any faster, they prefer to run. The second is the “Energy Optimal Transition Speed,” (EOTS), where exercise economy—the cost required to maintain a certain speed—indicates transitioning to walking is mechanically better. (Note: Brill shies away from saying “more efficient” as there is no way to truly measure mechanical power in runners.) Finally, there is the heart rate optimal transition speed (HROTS); a third measure that is basically the same idea as EOTS but using heart rate as the efficiency indicator. At HROTS, heart rate is the same between walking and running. Slower than this speed, walking heart rate is lower than running heart rate. Faster than HROTS, vice versa.
Brill’s study set out to examine the effect that incline had on PTS and EOTS, and to determine “how heart rate is influenced by gait selection.” Brill’s hypothesis was that at certain speeds it would be more efficient to walk on steeper inclines and that both speed and incline play into PTS and EOTS. In other words, that both measures would get slower at steeper inclines.
“I thought this would occur because both walking and running are more metabolically demanding at steeper inclines and, thus, there would be greater drive to minimize energetic cost,” he says. “Finally, I hypothesized that HROTS and EOTS would be equal at each incline. I thought this would occur because heart rate generally correlates with energetic cost during steady state endurance exercise.”
Brill based his study on testing 10 “healthy, high-caliber, male trail and mountain runners.” He tested the runners in two sessions, one where the treadmill was set at 0 degrees and 15 degrees and a second at 5 degrees and 10 degrees. PTS and EOTS were determined from metabolic cost data for walking and running at three or four speeds per incline near the expected EOTS.
Expected and Unexpected Research Findings
Brill’s study and analysis produced expected and unexpected results. Consistent with prior research, the study showed that at all inclines walking generally required less metabolic power at slow speeds and running required less at faster speeds, and that the transition would arrive at a slower speed on steeper inclines. Also consistent with prior research was that PTS would be less than EOTS at shallow inclines. The reasons we transition sooner than what would be most metabolically efficient is unclear, but theories point to biomechanical factors, such as sparing fatigue on specific muscles.
This changed at a higher incline, however. At 15 degrees, PTS and EOTS were the same. Since this was only on average (not all of the subjects showed this change), Brill is cautious with drawing conclusions, “especially because no prior research looked at PTS and EOTS on these steep inclines and, thus, nobody else has validated such a finding,” he says. However, he observes: “There’s physiological plausibility for PTS and EOTS to converge at steeper inclines since the greater intensity of the steeper inclines means that subjects are closer to their VO2 max and energetic cost or oxygen consumption begins to become a limiting factor at higher intensities, unlike lower intensities on the more gradual inclines.”
Unexpectedly, the study determined that HROT did not equal EOTS at all inclines and, accordingly, that heart rate is not a reliable predictor of when a runner will shift to walking. Therefore, athletes and coaches shouldn’t rely on heart rate monitors to govern gait.
As part of Brill’s written conclusion, he states: “Energetic, biomechanical, and neuromuscular factors may influence gait transition, and these should be studied in further detail, especially on inclines commonly experienced by trail and mountain runners, where the question of gait transition has large performance implications.” He says he’d love to delve into the effects of fuel utilization and carb sparing, local fatigue, and the relative strength and weakness of specific lower leg muscles.
Brill points out that the study was limited by the fact that the subjects weren’t able to place their hands on their quadriceps or knees to facilitate knee extension during late stance due to the constraints of the mouthpiece and breathing tube that collected expired air. This may have influenced metabolic cost and discomfort, especially at 10 and 15 degrees, and thus artificially distorted the results. Brill’s thesis also recognizes that the limitations of lab-based research eliminated a variety of relevant factors such as the steepness of the incline, length of the climb, ground surface, and the overall duration of the effort, which all weigh on an individual’s gait selection. Those factors are crucial, along with fueling choices, a runner’s unique leg strengths and weaknesses, use of poles or no poles, at what point in the run the incline comes, and, perhaps most importantly, whether there are other runners to pass or be passed by, or observers to cheer or jeer.
Impact of the Study
Brill says he thought this study was “important because many trail and mountain running coaches and athletes believe that deciding whether to walk or run uphill is solely determined by speed or solely determined by incline.” He wants runners and coaches to understand the “nuance and complexity of gait selection.” Additionally, many trail and mountain running coaches and athletes rely on cardiovascular or energetic models in their training—in the sense of VO2 max and anaerobic threshold workouts—and he wanted to determine whether that reliance was well founded. “Furthermore,” he says, “since coaches and athletes often utilize heart rate monitors to approximate cardiovascular stress or energetic cost, I also wanted to learn if this was a useful tool for approximating EOTS.”
Beyond heart rate, Brill says, “The practical importance of this finding is that if someone says ‘I always switch to walking if I’m going slower than 12 minutes per mile’ or, alternatively, ‘I always switch to walking if I’m going steeper than 10 degrees,’ they’re dumb, because ultimately the speed of transition—whether we’re talking PTS, EOTS, or the unknown transition speed that optimizes performance—is a function of both incline and speed, not just one or the other.”