This excerpt comes from Training Essentials for Ultrarunning by Jason Koop. In his book, Koop reveals the highly effective ultramarathon training methods he uses with elite ultrarunners and shows how you can use them, too.
Chances are, if you are asking that question, you should power-hike.
At slower speeds, walking is a more economical form of locomotion than running. It is only when you speed up to near the 12-min/mi range on level ground that walking is less economical (Falls and Humphrey 1976; Margaria 1976; Glass and Dwyer 2007). The practical decision to run or power-hike has to do with both the situation you are in and the difference in energy costs between the two forms of locomotion. While most elite runners, particularly at the 50K and 50-mile distance, will choose to run anything they can in order to finish faster, they knowingly do so at the expense of economy and have to spike their efforts up the steepest climbs in order to continue running instead of slowing down to a power-hike. At shorter ultra distances, this strategy can work because winning a race is a good tradeoff as long as the increase in effort is reasonable. However, most ultrarunners are not in that position. Most want to finish as fast as they can, but prioritizing economy and effort level over short-term speed, specifically when choosing whether to walk or run, will almost always end up saving the average ultrarunner time. For the average ultrarunner—and definitely anyone flirting with cutoff times—running when you should be power-hiking burns a lot of energy and takes a toll on your system. Any time you gain in the effort will likely be lost (plus additional time) when you are forced to slow down.
Now, what speed you should choose for power-hiking is a more complicated question due to individual variability, course specificity, terrain technicality, and fatigue. The preferred walk-to-run transition speed is around 2.1 meters per second or 12:46 min/mi on flat ground (Beuter and Lalonde 1988; Hreljac 1993; Diedrich and Warren 1995). This means that if you begin walking and gradually increase your speed, you will naturally transition from a walk to a run at about this pace. The scientific explanations vary, but one thing is certain: At speeds slower than the preferred walk-to-run transition point, on level ground, it is energetically optimal to walk (Falls and Humphrey 1976; Margaria 1976; Dwyer 2007). This means running at a 12- to 13-min/mi pace requires more cardiovascular effort and more energy than walking at a 12- to 13-min/mi pace. This balance changes with increases in grade and differences in surface. Generally speaking, the speed at which you should transition slows down as the surface gets more technical and the grades get steeper. To put it in practical terms, if you are running on any normal climb (4 to 15 percent grade) around 18- to 19-min/mi or slower, it’s in your best interest to drop to a power-hike, even at the expense of a few extra seconds at the top. You will be far more economical, and the required effort is substantially lower. As a bonus, you can take the opportunity to take in a few calories.
To illustrate this concept, I put one of my athletes on a treadmill at a pace that was in between a walk and a run for her: an 18-min/mi pace at a 13 percent grade. I had her alternate running and walking for 3 minutes at a time to demonstrate to her the difference in cardiovascular effort required between the two forms of locomotion. The results were easy to see: Running requires a higher heart rate and thus a greater cardiovascular effort than walking at the same speed.
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Adapted from Training Essentials for Ultrarunning: How to Train Smarter, Race Faster, and Maximize Your Ultramarathon Performance by Jason Koop, coach to elite ultramarathoners. In his book, Koop reveals his highly effective ultramarathon training methods for ultrarunners of all abilities. Jason Koop is the Director of Coaching for CTS, coach to elite ultrarunners, and an accomplished ultramarathoner.