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Watch Me Go

By Greg Rienzi
Wearable trackers are a billion dollar industry with devices capable of monitoring everything from sleep to heartbeats. Editor—and marathon runner—Greg Rienzi took one for a spin to see if a high-tech gadget really makes you more fit.

The first buzz on my wrist sounded at roughly 10:15 p.m. while I was lying in bed. The mild vibration and accompanying two-second noise, like a windup toy’s whirr, startled me. My wife, who had been on the brink of sleep, rolled over. “Oh, this is going to be fun,” she said with a tall pour of sarcasm.

The jolt emitted from my new toy, a GPS-based fitness tracker watch with a baby-blue band. It was telling me to “Move!” Walk. Run. Rollerblade. Just do something. At that hour, I ignored the prompt (and wondered how to adjust the factory settings to stop buzzing me at regular intervals). But I couldn’t complain. I had signed up for this electronic nudging.

I’ve worn a wearable fitness device before, but my previous model was essentially a sleek-looking stopwatch and a mile counter that I used to record runs. Now I was training for a marathon. I’m an active 47-year-old with five marathons under my belt, but I wondered whether a wearable could help me get faster and shave a few minutes off my personal record. Of course, losing a few pounds and sleeping better would be welcome, too. I was joining the 24-7 world of fitness monitoring—wearing a device to record my every step, heartbeat, snooze, and fitness activity—for it to tell me just how great (or substandard) I was. And, yes, prod me at all hours.

Some might say I was late to the game. One in five Americans now owns a fitness-tracking device. More than 100 million of these gadgets were shipped in 2016 alone, and today just about every electronics and sporting goods manufacturer has joined the wearables market, which is a $31 billion industry. The technology has come a long way since wearables were first introduced in the early 2000s. For a few hundred dollars, a band or watch can not only tell us how many steps we’ve taken and calories burned but simultaneously measure our heart rate, VO2 max (maximum oxygen uptake during intense exercise), and this thing called basal metabolic rate (how much energy our body burns). They can assess how well we slept. In the end, though, what do all these numbers mean? And can we trust them?

To help me along my journey, I enlisted experts in the fields of cardiology, sports psychology, neuroscience, and kinesiology to demystify the technology and help interpret the data feedback. Was wearing this gizmo on my wrist truly going to help me get fitter, lose weight, and improve performance?


Like many modern relationships, the wearable-human variety started with an online getting-to-know-you phase. Through a mobile interface, I entered my age, gender, height, weight, and fitness goals. I chose to set my daily steps at a relatively modest 8,000, improve sleep, and manage my weight. The first morning, I eagerly checked my status. I discovered I had slept seven hours and 50 minutes, with two hours and 21 minutes in deep sleep. My resting heart rate read 56 beats per minute. Later, I went for a 5K run. I burned 315 calories and ramped my heart rate up to 150 bpm. Great. But how does the device know this? The short answer: sensors and algorithms.

Fitness trackers measure motion. Most of today’s wearables come with a three-axis accelerometer to estimate orientation and track movement in every direction; a gyroscope to measure rotational velocity; and an altimeter to determine altitude. That’s how the watch knows you’re hiking up a mountain or climbing stairs, and thus exerting more energy, versus lying in bed. The data is then converted by the device’s proprietary algorithms into the desired metrics, such as steps taken and duration of deep sleep. These algorithms mostly work, but they also make assumptions that can lead to errors, especially when estimating calories burned. They don’t know individuals well. You can tell the device you’re a 175-pound, mid-40s male, but what’s your muscle mass? When did you weigh yourself, and is the scale accurate? These devices can’t do a real-time scan of your physiology (at least, not yet).

Seth Martin, a cardiologist at the Johns Hopkins School of Medicine who has studied mobile health technology the past seven years, says that most wearables on the market today are reliable and accurate for counting steps. A 2015 University of Pennsylvania study found that many smartphone applications and wearable devices varied only slightly— both underestimating and overestimating by a small fraction—from observed step counts. The devices are smart, too. They know when you’re driving, compared to walking, because your wrist and hands are not swinging. But, Martin says, fitness tech is still not an exact science. Some trackers might dismiss a wrist movement or not realize you’re pushing a cart at a grocery store. “Sorry. You won’t get the credit for those steps, as your hand is not bouncing,” he says.

As for heart rate, most wearables use photoplethysmography. Green LED lights flash hundreds of times per second on the wrist to detect the amount of blood flowing. Martin says heart rate accuracy depends on the device. Meaning some are better than others, and typically you get what you pay for. “The most accurate readings still come when you combine the wearable with a chest strap,” he says, referring to a separate piece of wearable gear that you could purchase. “But no doubt today’s bands and watches are more accurate than they used to be.” A 2017 Stanford University study into the accuracy of seven popular wristband activity monitors showed that six out of the seven devices measured heart rate accurately within 5 percent.

With some confidence that my watch wasn’t lying to me, and might only be cheating me out of a few steps, I plunged ahead with my marathon training. I logged every run and indeed every stride I took. In the steps department, I overachieved and averaged more than 8,000 daily, surprised how quickly a walk to my office garage and frequent trips to the kitchen added up. The “Move!” prompting was less intuitive, in that it was a pre-set interval alarm that I couldn’t adjust to my lifestyle. It often buzzed me at awkward times, like when I was driving or in a meeting. Although I could disable the alert, I chose not to because I did appreciate the reminders.

My running miles also increased. I began to devour all the data I was getting back. Within a few weeks, I noticed that my resting heart rate had gotten a few points lower and my VO2 max had inched up higher to read 52, which the device’s mobile interface told me was “excellent” and in the top 10 percent of my age and gender group. Somewhat dubious, I checked in with Ryan Roemmich, a human movement scientist at Johns Hopkins and Kennedy Krieger Institute. I got good news, with a caveat. While no watch can match a treadmill stress test given in the doctor’s office for VO2 max accuracy, Roemmich said that wearables do track change, meaning the 2 to 3 point jump in my VO2 max meant real progress, though 52 was likely not my real number.

I would also record my fastest measured mile to date while using the watch. One day, I pushed through the pain and lactic acid burn to achieve the under- 7-minute mark and have my watch tell me I had a new personal record. Every time I hit a milestone or overachieved on goals, I earned “badges,” which became an overstuffed trophy case on my device’s web interface. My stats also included a race-time predictor. The more I ran and the harder I trained, the faster my predicted finish time was for my future marathon. I ate it all up. In fact, the data started taking over my life.

The more I ran and the harder I trained, the faster my predicted finish time was for my future marathon. I ate it all up. In fact, the data started taking over my life.
Chasing carrot on a stick

Angie Fifer, a sport psychology consultant for Breakthrough Performance Consulting and the scientific program chair for the Association for Applied Sport Psychology, says that fitness technology is highly regarded in the running community because it can accurately measure these performance gains and serve as surrogate coach, telling us, “Good job!” when a target is reached. When I told Fifer about my growing addiction to my stats, she said she wasn’t surprised.

“The data sucks us in,” says Fifer, a marathoner, ultra-marathoner, and three-time Ironman finisher who uses a fitness tracker. “When you first start using a tracker, it can help you keep on pace, but over time your expectations get higher. Maybe you push harder to finish that extra mile, take those extra steps, or up the tempo to go just a bit faster than you did last time. Before you tracked runs, you might have had no idea where the mile turnaround was, or your pace.”

It’s not just marathon runners who become hooked on the data. The metrics a fitness tracker provides have value even to a beginner, according to Amy Haufler, a senior scientist in neuroscience at the Johns Hopkins University Applied Physics Laboratory and a kinesiologist with the University of Maryland. “You want to get better at something or be fitter. Well, what does better look like?” Haufler says. “This tech and the data you get back allow you to pursue that objective with measured observation, just like a weight scale does.”

Fifer warns, though, that some people become so obsessed with the numbers that they don’t listen to their bodies. They push beyond what is normal because the data becomes hard to ignore, a mindset that can lead to injuries. She recommends running a few days sans electronics, especially for recovery and “easy” runs.

I could see myself slipping into this mindset. I asked myself: Does a run or walk count if not being tracked? I felt demotivated at times when my watch battery died. Instead of just running without, I waited until I could get home and recharge the device. Fifer told me I was missing the point, and verging into an overreliance on technology.

“Who cares how fast you’re running? Just ask yourself, do I feel good, am I enjoying this exercise?” she says.


Not everyone becomes as enamored with their device’s stats as I did, especially when they don’t have a defined goal. For many people, the novelty quickly wears off. Once you know a walk around a lake is X-number of steps, do you really need a watch to keep telling you that?

And while trackers are smart, they can’t control for an owner’s expectations. Many set unrealistic goals, Haufler says, such as wanting to lose 5 pounds or log 100 miles on a bike in one week. “Some things are hard to achieve unless you start out in pretty good shape,” she says. Many expect the fitness tech to help them, she says, and when they fall short of their goals, or don’t see rapid progress, they give up. “A wearable is not all you need to succeed, but people can make that assumption,” she says. Weight loss, for example, requires other lifestyle changes, such as diet. A watch logging your steps simply isn’t enough. And this is where motivation comes in.

Neuroscientist Caroline Zink, an investigator at the Johns Hopkins–affiliated Lieber Institute for Brain Development, says that motivation is key to many positive health, fitness, and clinical outcomes. “People think if you want something enough, you’ll be motivated to get it. But we know that is not at all true,” she says.

Fitness tech often fails or succeeds, Zink says, based on “expectancy theory,” which states that our behavior results from our conscious choices among alternatives to maximize pleasure and minimize pain. Zink says my watch likely motivated me because it showed I could run faster, and that if I continued to train harder, my times would go down. “You might think at first, I could never do that. But then the device tells you otherwise. Wow, I did walk 10,000 steps every day this week and I lost weight. It works. A connection is made,” she says.

The more I used my device, the more I found my confidence growing. A week before my race, my watch predicted my marathon time would be 3:34, a full 25 minutes faster than my previous PR. This number would stay in my head through-out the race as I counted off the miles, bombarded by the constant flow of data via my watch and through my Bluetooth headphones that reminded me of my current pace. While I didn’t finish in 3:34, the watch’s prediction was only 9 minutes off, as I crushed my PR by 15 minutes.

Many expect the fitness tech to help them, and when they fall short of their goals, or don’t see rapid progress, they give up.

Did my device know me better than I know myself? Zink says yeah.

“That is what these devices can do. They activate our motivation circuitry,” she says. “Before, there was something in your head telling you that you couldn’t run this fast, but the watch was telling you: Yes, you can, and here’s some hard, biological evidence.”

While I did achieve my PR goal, I didn’t lose much weight. Using the device, I became a more confident runner, but at the same time it turned me into an overconfident eater. Seeing the 1,000 or so calories I burned on a long morning run made me feel I had the license to have a doughnut later that day. Basically, I rewarded my active days with treats, and padded my daily caloric intake.

According to various studies of devices, the results are mixed when it comes to outcomes like weight loss. In 2016, JAMA published a two-year study from the University of Pittsburgh on the effect of wearable technology on long-term weight loss. Participants were given a standard behavioral weight loss intervention, including being placed on a low-calorie diet and told to increase physical activity. Later on in the study, members of a randomized group were provided with a wearable device. Both the randomized group and the standardized group had significant improvement in body composition, fitness, and physical activity, but on average, the wearable group lost less weight (7.7 pounds) than the standardized group (13 pounds). The reason? Like me, many participants ate more than they otherwise would have because their devices told them they had kicked butt in the steps department. And sometimes the recording of calories burned was off. In a 2017 Stanford study, none of the seven devices studied counted calories accurately, with even the most accurate device off by an average of 27 percent.

Fitness tech often fails or succeeds based on “expectancy theory,” which states that our behavior results from our conscious choices among alternatives to maximize pleasure and minimize pain.

Martin says using a fitness tracker alone may be insufficient in facilitating behavior change. In a clinical trial with cardiac patients, Martin found that patients responded better when the prescribed tracker was used in conjunction with other motivators like automated text messages to eat better and get a good night’s rest. The device, while a useful tool, is not a panacea. True fitness requires a more holistic approach.

As for any quirks in calorie counting and wearable inaccuracies, Zink expects the error margins to go down in the near future. “They want to make sure they provide accurate data that is going to make someone healthier or achieve their fitness goals, and they are considering science heavily,” she says.

I sure hope the data is accurate. Today my watch predicted my next marathon time would be a speedy 3:15:00— and I’d love to prove it right.

Runner on mountain of fitness devices
Illustrations Doug Chayka

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