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When a 400-yard tunnel in the opening mile of last fall’s Chicago Marathon garbled the pace data on her GPS watch, Nicole Lane didn’t panic. The 25-year-old was aiming to dip below the threshold of two hours 45 minutes to qualify for this year’s Olympic Trials in Atlanta, but she wasn’t too concerned about splits. Instead, her coach, former 2:16 marathoner Steve Palladino, had given her strict instructions the night before: aim for a power output between 218 and 219 watts. “There were definitely times when I felt pretty good and thought about speeding up,” she recalls, “but seeing the power get higher than what Steve gave me, I would slow down.”

The first wearable power meter for running, from a Boulder, Colorado, startup called , was launched in 2015. Since then, engineers and biomechanists have been squabbling over the true definition of running power—and curious runners have been checking it out on the trails. Heavyweight competitors Garmin and Polar have now joined the fray with running power apps that harness the monitoring abilities of their existing devices, and Stryd recently launched a radical revamp of its foot pod that enables it to measure and  in real time. Power, in other words, is making its bid to be the new running metric of choice. And for Lane, who crossed the finish line in 2:42:26 and secured her trip to Atlanta with an average power of exactly 218 watts, the verdict is already in.

The golden promise of power—in physics terms, the force with which your foot smacks the ground multiplied by its velocity at the moment of contact—is that it tells you exactly how hard you’re working at any given moment. “It’s the best proxy we have for quantifying perceived effort,” says Tom Schwartz, coach of the  running group in Boulder. Pace is affected by hills and wind and footing; heart rate can be skewed by factors like dehydration and sudden pace changes. But power doesn’t lie. Collect enough data during your training sessions, the theory goes, and the algorithm will tell you exactly which target power you can sustain over a given distance.

In cycling, where power meters are well established, they’ve more or less lived up to the promise—so effectively, in fact, that in 2018, Tour de France race director Christian Prudhomme  in competition, because they “annihilate the glorious uncertainty of sport.” But measuring power in cycling is relatively straightforward, since it simply depends on how hard you’re mashing the pedals.

The picture is murkier with running. Do you count the power required to swing your leg forward through the air, or only the power against the ground? How do you account, from one side to the next, for the “free” energy stored in the spring-like tendons whose contribution to your forward motion changes when you go up or down hills? All these factors affect the relationship between how hard your body is working and the number that appears on your power meter. So far, every player in the market has chosen a slightly different way of quantifying running power (see “Surge Detectors,” below), but it’s not clear whether the differences matter in practice.

Austin O’Brien, a 27-year-old Iowa-based marathoner coached by Schwartz, says that his Stryd numbers—beamed wirelessly to his watch from a foot pod attached to his shoe—remind him to slow his pace during recovery runs when a headwind or hill might push his effort too high. But the biggest benefit, he says, is being able to compare how much power he sustained in training runs with varying routes and conditions. “I believe it’s the most accurate way to track improvement over time,” he says. That sort of post-run analysis is also how Alexi Pappas, who represented Greece in the 10,000 meters at the 2016 Olympics, uses Garmin’s Run Power app, which connects to a chest strap or a waist-mounted pod to monitor your body’s accelerations. “I look at the data later,” she says, “because I’ve always been told not to think too much about form and data during runs.”

The latest iteration of Stryd’s device, released last June, incorporates a wind port, analogous to the tubes used to measure airspeed on planes. (Garmin uses local weather forecasts and barometric sensors in the watch to estimate the impact of winds.) That new feature came in handy for O’Brien at the Indianapolis Monumental Marathon in November. Based on the power data collected during previous training and at races, Stryd’s algorithm had predicted he could sustain between 384 and 399 watts for 26.2 miles; his most recent marathon-effort workout averaged 393 watts. (Power numbers are proportional to both pace and body size, which is why O’Brien’s target was so much higher than Lane’s.) With three miles to go at the Indy race, his pace started dropping, but his power meter assured him that he was pushing at an appropriate intensity, and the pace drop was simply due to a headwind. He hung on for a time of 2:18:38, just under the 2:19 Olympic Trials standard, with an average power of 392 watts.

While Stryd won’t reveal sales figures, it reports that the number of runners uploading its power data from the six World Marathon Majors races around the world has doubled each year since 2016. Garmin estimates that about 60,000 people are using . But is there anything lost by outsourcing your pacing to an algorithm? In race situations, it might be better to ignore the power meter—even if that means pushing into the red zone temporarily. “Sometimes going with a move may be a tad over your head, but it’s also easier to hang on and be in the race than to come back later,” notes Steve Magness, a track and cross-country coach at the University of Houston.

Schwartz agrees that in a competitive context, you sometimes need to surge. He can even quantify it for any given runner and race scenario: you might be able to hold a three-watt surge for 90 to 120 seconds, for example, or a five-watt surge for 45 seconds. But the best outcome, he says, is when you’ve trained enough with the power meter that you internalize the feeling of the right effort level. “To me, one of the keys to success as a runner is self-awareness,” he says. “You learn to lock in. And then you don’t need to look down at the watch anymore.”