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Here's How Accurate The Fitbit Alta HR Actually Is

We pitted Fitbit's new ultra-slim wristband against a chest strap to see just how accurately it could measure beats per minute.

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Nicole Nguyen / BuzzFeed News

There are a lot of reasons why you’d want to get a fitness tracker. Maybe you’re trying to gather insights about your sleep, or get fitter, or lose weight. Whatever your goal, one thing is true: A fitness tracker is useless if it can’t accurately measure whatever it is you’re trying to track.

Fitbit recently debuted the Alta HR, an ultra-slim wristband with a new feature: heart rate tracking. But just how accurately can the wearable track your beats per minute? When we did a first impressions review of the Alta HR last month, our preliminary tests suggested that the tracker’s heart rate technology wasn’t always on point. So we spent the last two weeks working up a sweat while wearing the new band — and as we originally suspected, the Alta HR struggled to keep up with exercises with a lot of movement and high intensity bursts. It did, however, reliably measure resting heart rate.

Experts say most people don’t actually need to know their exact heart rate during workouts, so this may not matter to you. But accuracy does matter for some, namely people with heart conditions and endurance athletes. Fitbit also heavily markets the heart rate tracking capability of its latest devices, like the Charge HR, Blaze, and Surge — and last year, it faced a class-action lawsuit over its allegedly inaccurate technology. (The company has called the allegations “baseless” and contested the lawsuit, as well as noting that the devices are “not intended to be scientific or medical devices.”)

With all that in mind, we set out to answer the question: Should you still consider the $150 Alta HR? Here’s what we found.

How We Tested

Unfortunately, we didn’t have access to an electrocardiograph, a medical device that’s considered the gold standard for heart rate measuring. Instead we used the next best thing — a chest strap monitor, which multiple studies have shown to be significantly more accurate than wrist-based heart rate monitors — as our control. And we enlisted Open Lab Fellow/chart master Lam Vo to help us sort through all of the data.

For each run and bike ride, we wore a Polar H7 chest strap, in addition to the Fitbit Alta HR and Apple Watch. Nicole wore the devices on different wrists, while Stephanie wore the devices on the same wrist.

Nicole's Long-ish Run: A Close Look at the Data

Nicole Nguyen / BuzzFeed News

During my first impressions workout, a quick 17-minute run with short, intense uphill sprints, the Alta HR had trouble measuring my heart rate. So, this time around, I went for a longer-interval run, switching between a light jog and uphill sprints, for about 40 minutes. The terrain was a mix of trails and concrete on rolling hills.

After the run, I checked each device’s workout summaries. The Polar Beat app measured a 160 beats per minute (bpm) average. Compared to that measurement, the Apple Watch overestimated the average by 1 beat (161 bpm), and the Fitbit Alta HR underestimated by 3 bpm, which is pretty impressive and consistent with Fitbit’s claim of average absolute error of less than 6 bpm versus an EKG (electrocardiograph) test strap.

I extracted the heart rate data from each device, and Lam created an interactive chart, so you can see exactly how the wearables performed throughout the duration of my run (try clicking the names of the device and hovering your mouse over the graph!).

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Lam Vo / BuzzFeed News

The Fitbit and chest strap appear to agree for most of the workout, especially during the more moderate sections, where my heart rate is between 140 and 160 bpm. When my heart rate begins to pick up, however, (around 13:30, 13:35, and 13:51) the Fitbit band is slow to catch up with my increasing heart rate. In some cases, the difference is 20 beats per minute off from the chest strap.

What’s more surprising was how close the Apple Watch’s readings were to the Polar H7. Towards the end of my run, however, the Apple Watch tended to overestimate my heart rate by about 4 bpm.

How the Alta HR Performed During Other Activities

Stephanie Lee / BuzzFeed News

Before my runs, I recorded my resting heart rate, and the Fitbit Alta HR and chest strap readings were identical four out of five times (62 bpm) and only one beat off on the fifth run (63 bpm).

On a second, much shorter run at a faster pace, the chest strap and Apple Watch produced the same result, a 165 bpm average heart rate, while the Fitbit’s was six beats off, at 159 bpm (which is still within Fitbit’s claim of +/- 6 bpm versus your actual heart rate).

I wanted to test how the wearables performed on San Francisco’s bumpiest roads, so I wore them on my bike commute to work, which has plenty of poorly patched potholes and construction zones along its route. The difference between the chest strap (128 bpm) and Alta HR (96 bpm) reading was 32 beats per minute. The second ride, which was a bit more intense, yielded similar results. The Alta HR again measured a 96 bpm average, and the Polar recorded 141 bpm, a 45 bpm difference. Moving aggressively and unpredictably was tough on the Alta HR.

Stephanie saw very different results during her workouts. On the first run, the Polar read a 140 bpm average, while the Alta HR measured 163 bpm. On a second run, the Fitbit and Apple Watch had very close average heart rate readings, 157 bpm and 156 bpm, respectively, while the Polar strap measured 141 bpm.

After we shared our results with Fitbit, a spokesperson acknowledged that for workouts “where the wrist is moving vigorously and non-rhythmically, such as high-intensity interval training, P90X, or boxing, the movement may make it harder for the sensor to consistently detect an accurate heart rate through the wrist.” They also noted, “This is true of any wrist-based heart rate sensor technology.” They suggested wearing the Alta HR higher up on the arm, where there is less movement, during bumpy bike rides.

To sum up: In our experience, the Alta HR’s heart rate readings consistently matched readings from other devices when we were at rest, but they tended to be less consistent during high-intensity periods of exercise. We wouldn’t rely on it to be accurate on a beat-by-beat basis during grueling workouts.

Does Heart Rate Even Matter?

The truth is, however, most people don’t need the Alta HR, or any other wearable, to be quite that exact, experts say.

Knowing your precise heart rate during workouts is probably most useful if you fall into one of two camps: You have a heart condition, or you’re a hardcore athlete. In the first case, you’d want to avoid pushing yourself beyond your safety zone, and in the other, you’d want to make sure your heart was pumping at a high-enough intensity to adequately prepare for, say, a marathon.

Otherwise, “if you’re young and healthy, you don’t need to know your heart rate,” Marc Gillinov, a cardiac surgeon at the Cleveland Clinic in Ohio, told BuzzFeed News. “You might find it interesting — it’s not bad to know your heart rate — but it has no likely health benefits to you or me.”

That said, experts say a strong indicator of your heart health is your average resting heart rate, which is the number of times your heart beats per minute when it’s at rest (like before you get out of bed in the morning). And the Alta HR, in our experience, handles this just fine.

“If someone measures your resting heart rate as 110, that’s probably indicative that something is not good,” Gillinov said. “Alternatively, if you measure your resting heart rate as 60, it’s probably good news.”

This figure can be especially motivating for novice gym members. “As you continue to exercise more consistently, you might find your heart becomes more efficient and your resting heart rate is lower,” Heather Milton, senior exercise physiologist at NYU Langone’s Sports Performance Center, told BuzzFeed News. “It’s an objective measure of your fitness level.” Conversely, athletes can also use this knowledge to see if they’re training too hard without sufficient recovery time, leading their resting heart rates to be too high.

Chest Strap vs. Wrist-based Heart Rate Monitors: Which Is Better?

There’s a pretty obvious reason why even the best wrist-worn heart rate monitors usually aren’t as reliable as chest straps: they’re not right on top of your heart, measuring its electrical activity.

But a Fitbit or similar wristband, which is “a little bit downstream from your actual heart rate,” instead measures your blood flow as a proxy, Gillinov said. Its sensor shines a light into your skin, and the amount of light reflected back is proportional to the amount of blood flowing in and out, which varies with your pulse.

In a recent research letter in JAMA Cardiology, Gillinov and his colleagues had 50 healthy adults wear various wearables while running on treadmills at different speeds. In an experience similar to ours, the Cleveland team found that the Charge HR (which shares the same heart rate tracking technology as the Alta HR) tended to underestimate heart rate during more vigorous exercise.

Another reason a wrist-based reading might be off is because if your wrist moves during a workout, the band can slide up and down your arm and miss some beats. And if your heart is working at a very high intensity, it might produce some irregular beats that would go unnoticed by the tracker, Milton said.

So, Which Tracker Should You Buy?

It ultimately depends on your goals.

If you’re a cardiac patient, an endurance athlete, or just someone looking for the most accurate reading possible, consider getting a chest strap. Milton’s personal preference is a Polar chest strap paired with the Polar Beat or Polar Flow apps for tracking progress. The Polar H7 ($80) and newer H10 ($90) are what we used for testing the Alta HR’s heart rate accuracy.

In terms of wearables, Milton says she trusts Garmin watches. “Anecdotally, every client’s reading has been spot on," she said. "I measured their VO2 Max, and the watch is within 1 millimeter per kg per minute." VO2 Max is the maximum amount of oxygen your body can use during exercise and is typically measured with lab equipment, but wearables like Garmin can estimate your VO2 Max using heart rate data provided during a workout and other statistics like age and gender.

From our own testing, it seems the Apple Watch performed very closely to the chest strap for most runs. Still, like the Alta HR, the watch was less accurate during high-movement workouts, like bumpy bike rides.

All that said, if you’re someone who’s just getting into exercise, Fitbit’s new wearable might be what you want. A wearable like the Alta HR is a sufficient measure of your average resting heart rate, which is an indication of your overall heart health, and the intensity of a workout, which is important to keep up if you’re trying to burn fat.

Nicole Nguyen covers products and personal technology for BuzzFeed News and is based in San Francisco.

Contact Nicole Nguyen at nicole.nguyen@buzzfeed.com.

Stephanie Lee is a senior technology reporter for BuzzFeed News and is based in San Francisco.

Contact Stephanie M. Lee at stephanie.lee@buzzfeed.com.

Lam Thuy Vo is a reporter for BuzzFeed News and is based in New York.

Contact Lam Thuy Vo at lam.vo@buzzfeed.com.

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