A Trail Runner's Guide to Power Meters

Just about every runner knows the benefits of ditching the pavement and heading cross country. Softer surfaces reduce impact. You improve your balance and build supportive-muscle and core strength. You tackle steeper hills, taxing your cardio system with less stress on your frame. In sum, trail running is a better workout—not to mention the quiet and the views.

On the trail, you may be working harder, but you’re typically covering less ground than on roads. A trail run is hard to quantify for those following a regimented training plan, and it looks less impressive on Strava.

A new device—the running power meter—may help solve the problem of quantifying effort on difficult terrain, as well as help you train more effectively on trails. The power meter, commonly used by cyclists, delivers one number that represents the effort you are expending at any given second.

The Promise of Power

The most user-friendly power meter available to runners today, the Stryd, collects data from a pod mounted on a chest strap or attached to your shoe, and delivers a reading in watts (force times velocity) on your cell phone or smart watch. At least two other companies offer power as one of many stride measures: RPM² gathers information from smart insoles and SHFT combines data from a chest and shoe pod.

Power can tell you precisely how hard you are working for how long, which, according to power advocates, makes it perfect for the structured, data-driven trail runner.

“Being able to quantify the overall effort is really valuable,” says ultrarunner Andy Reed of Canmore, Alberta. “I don’t think at all in pace, because I can be running at 12-minute mile, for example, and it can be a pretty hard effort, depending on the terrain.”

Runners traditionally have used pace, heart rate or “feel” to judge effort. The power meter’s big advantage over pace is that it takes into account factors such as slope and footing. Head up a switchback and your power ramps up, corresponding to your increased effort—even as your pace slows. Similarly, if you move from a smooth path to a rocky or rooty trail you’ll see a higher power reading at the same pace, reflecting the lower efficiency of dancing from side to side and climbing over and around obstacles.

In each of these scenarios, heart rate would also go up as effort increases. But heart rate is notoriously variable.

“I find power more helpful than heart rate,” says Reed. “There are so many other variables that play into heart rate—if you had a few beers the night before, or didn’t sleep particularly well, or maybe you’re a little sick—those things can have an impact on your heart rate, which you don’t see in the power meter. The wattage doesn’t lie.”

More problematically, heart rate reacts slowly, creating a lag. Climb a hill, for example, and you won’t see your heart rate increase for the first minute, so you don’t know if you’re going hard enough. Then, as you crest the hill, your heart rate continues to climb and takes its time slowly coming down, even when the effort has long since peaked.

“In training, power worked great because if I was pushing too hard for my intended target, I would know almost immediately,” says John Schneider of Pittsburgh, a marathoner converting to trail running. “As opposed to running with heart rate, where, by the time your heart rate climbs past your desired zone, you have already been pushing too hard for a while, or running with pace, where the ups and downs on a trail make it a poor correlation to effort.”

The power meter reacts almost instantly to changes and is consistent day after day regardless of your level of recovery or other external factors. Thus, once you’ve learned your power-level equivalents for different paces or thresholds, you can use power to judge your work at a specific point in a run or over the course of a day or week of training. Instead of going out for eight miles at 7:30 pace, for example, you convert your thinking to an hour at 250 watts.

To figure out your power equivalents, Stryd CEO Robert Dick recommends an initial “critical-power” test, a rather brutal, max-effort 1,200m, followed, after a full recovery, by an all-out 2,400m. If you survive, the test delivers a list of “power zones” in watts and pace equivalents corresponding to familiar thresholds and race paces.  Those who have used the meter report that they quickly learn to gauge effort in terms of their power number.

“My training is totally based on power zones now, and it works well,” says Reed. “It’s quite incredible how quickly you switch over to it. I can go out for a run now and pretty much know roughly what wattage I’m at.”

Limits of Power

While users report that power works well on ascents and over iffy terrain, it falls short on descents, particularly when they get technical. The problem is that gravity is supplying the power and your role is to keep upright and uninjured. So power readings coming down the steeps tend to be significantly lower than perceived effort, and it is near impossible, and dangerous, to speed up to match a goal power level such as aerobic threshold.

“On trail runs where I’m looking to keep a certain power range, I tend to use the lap average,” says ultrarunner Bud Talbot of Denver. “I don’t try to match it every second.”

On severely technical trails, Talbot and others agree that it is best to use the power meter only on ascents, where it tracks with perceived effort even when you’re reduced to a walk, and revert to feel when going down.

For most trails and most paces, runners can keep the power within a desired range, and the run will provide the desired workout. “If you’re keeping it at that number the whole time, it should be darn close in terms of your energy usage,” says Dick. And that should set you free to choose a trail more often, knowing, and documenting, that you’re doing the work, even if you’re not racking up miles as quickly.

Besides using power to gauge overall effort and keep in appropriate ranges on climbs during a run, here are suggestions for trail workouts where power can help.

How to Use It

Uphill Intervals

Best performed on a moderate hill. “I find the power meter very helpful for interval sessions, especially shorter intervals,” says Reed. “If I’m doing three-minute hill repeats, by the end of that three minutes, my heart rate may still be rising, whereas when I set out with the power meter, within a couple of seconds it jumps up to the value.”

Instead of doing 12 x 400m on the track, for example, do 12 x 90 seconds climb at the power level that you’d sustain while racing a 5K. The hill needn’t be a consistent slope as the power will adjust, but shouldn’t be significantly up and down or too technical.

Power Fartleks

On a moderately rolling trail, for example, run seven sets alternating Threshold and Easy power levels of 1’T 1’E, 2’T 2’E, 3’T 3’E. The Stryd app will identify these levels after you do the critical power test.

“Without Stryd, it would have been really tough to gauge that my effort and progress were kept appropriately challenging,” says Schneider. “I could be pushing a 7-minute mile on the level that turns into a 10-minute mile on a hill—but as long as I’m hitting my target power zone, I’m confident that I’m targeting the physiological adaptations that I’m working on that day.”

Short Max Bursts

On a steep ascent, run 10 to 12 seconds at max effort, walk down for full recovery and repeat up to 10 times. These short bursts work the neuro-muscular system, and the goal is to open all the stops each time.

While runners can do this using effort, the power meter can help keep each repeat consistent and let you know when it is time to shut it down: when an all-out effort doesn’t reach the same power level.

This article originally appeared in our October 2016 issue.