Does It Really Matter If Your Fitness Tracker Is Accurate?

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Last June, Teresa Black insists that her heart rate hit 160 beats per minute. Her Fitbit Charge HR, however, clocked her at a mere 82 bpm. Considering that Black was in the middle of a workout — "an intense part of a personal training session," according to the class action lawsuit filed against Fitbit earlier this year — there were two possibilities: Black might have been a bona fide superhuman, who clearly missed her calling as a gold-winning Olympian, or, more probably, her wearable activity tracker wasn't reading her heart rate correctly.

The question driving the current suit against Fitbit is whether the company has misled consumers about the accuracy of its PurePulse tracking technology. Fitbit touts this proprietary LED-based system in its marketing materials. One of its tag lines for the Charge HR: "Every beat counts." Black and the other plaintiffs in the suit contend that this heart rate measuring technology is "effectively worthless."

But what if both sides are essentially in the wrong? The problem might not be with Fitbit's approach to heart rate monitoring, but with casual, wrist-based monitoring, and the idea that most of us can do anything useful with a running tally of our bpm count.

"If you went to 20 people in a gym wearing heart rate monitors, and you asked them what range they should be in, I'm telling you, 85 to 90 percent wouldn't have a clue," says Don Saladino, celebrity trainer and owner of the Drive 495 health club in New York City. "Which makes it irrelevant. They'll look at a 145, and then a 155, and think that's better, because their heart rate got higher. But on that day, the best thing for them might have been to stay lower, because maybe they're overtrained or under-rested." Trainers like Saladino routinely use precise heart rate calculations to fine-tune workouts for a client, based on his or her specific goals and current physical state. But defining those target zones involves more than heuristic shortcuts, such as the common equation of determining your maximum heart rate at 220 minus your age. Zone-based training is complex and case-by-case, with numbers that fluctuate based on a wealth of different factors. It also involves using chest-mounted monitors, which are uncomfortable to wear, but whose results are incredibly precise.

Maybe this sounds like a snobbish point — that Fitbit wearers simply aren't hardcore enough to take advantage of heart rate monitoring. But according to Matt Fitzgerald, a coach and trainer who narrates audio workouts for heart rate monitor and app-maker Pear Sports, heart rate counts are inherently mystifying. "A lot of endurance athletes, runners, and cyclists who monitor heart rates, they don't even know really what to do with it. That's one of the limitations. Just seeing a number, there's a gee whiz factor to it. But what do you do with it? You either have to be working with a coach or trainer who can help you use heart rate data effectively, or you have to acquire that knowledge yourself. That's why so many heart rate monitors and fitness trackers end up in the bottom of a drawer somewhere."

To be useful, heart rate monitoring requires both the technological component, to measure actual beats per minute, and the expertise to put those numbers into specific context. Even if Fitbit or other wearable makers were capable of reliably matching the accuracy of a bulky chest-worn monitor, no smartphone app is going to take into account how hard you drank last night, or what your training goals are, when coming up with a current target zone. Deciphering bpm is, for now, more about human experience and judgment calls than one-size-fits-all software.

One potential result of the current lawsuit might be a dual realization — that wearables shouldn't bother collecting heart rate data, because most of us can't make real of use of the data. The most obvious benefit of fitness trackers has been to encourage sedentary users to get moving, whether by gamifying exercise, or by specifically badgering them to hit that default, 10,000-step daily goal. Heart rate monitoring might simply be too bulky, and too arcane, to be relevant to a device whose main priorities are convenience, and ease of use. "We're just not there yet with wrist-worn heart measurement," Fitzgerald says. "We'll probably get there, but at this point, they're just not accurate enough."

If it isn't obvious, this isn't an analysis of the legal merits or flaws of the suit filed against Fitbit, which leans heavily on the issue of safety. Black contends that exceeding 160 bpm would have been dangerous for a woman of her unspecified age. Another plaintiff claims to have been explicitly told not to exceed 160 bpm by his doctor. Whether alleged inaccuracies posed a health risk to these specific people is a question better addressed in court. But the less we confuse wearable fitness trackers with precision medical equipment, the better off we'll all be.