It's All About Accuracy
A heart rate monitor has become a feature that separates serious fitness trackers from basic step counters. Your heart rate is a telling metric, providing insight into how hard your heart has to work both while resting and during exercise. For serious athletes and people who want to train using target heart rate zones, a heart rate monitor's accuracy makes a huge difference.
Given the dispute regarding the accuracy of Fitbit's optical heart rate monitors—where some studies claimed that the Charge HR was off by as much as 22 beats per minute — we visited with Dr. Suzanne Steinbaum, a cardiologist and director of Women's Heart Health at the Heart and Vascular Institute of Lenox Hill Hospital in New York City. Our mission: test several wrist-based fitness trackers, using a Quinton Cardiac Science Q-Stress test machine (a medical-grade ECG machine) as a control, for comparison.
Based on our testing, most fitness wearables do a fine job of monitoring your heart rate while you're resting or briskly walking, but they can struggle as you start to run and sweat. The Fitbit Charge HR was 4 beats per minute behind the EKG machine, but the Blaze was off by only 2 bpm. Other wrist wearables were in the 2- to 3-bpm variance range.
How They Work
Heart rate sensors typically come in one of two forms: a strap that wraps around your chest or a sensor that's embedded into a wristband. However, there's a lot of controversy around the accuracy of heart rate monitors, especially those in wrist-worn fitness trackers.
Chest-worn monitors, such as the Polar H7, typically detect your pulse via an electronic signal and send the reading to your connected device, such as a smartphone or a wrist-worn tracker.
Most wristbands (such as those in the Fitbit, Apple Watch and Garmin fitness trackers) use optical heart rate monitors, which send bright LEDs through the skin and then measure the amount of light that bounces back. Blood absorbs light, so variations in light that the sensors detect can be used to determine your pulse rate.
There are pros and cons to each type of sensor. Chest straps can be uncomfortable and a pain to get on, but they tend to be more accurate because they are right up against the skin and close to the heart. Wristbands, on the other hand, are more comfortable to wear but need to be carefully placed and secured to ensure the most accurate results. "If the bands are not on exactly where they should be — tight to the wrist, so the sensors can work — then the reading will be inaccurate," Steinbaum said.
How—and What—We Tested
Dr. Steinbaum hooked me up, literally, sticking electrodes on my chest, arms, stomach and back, and connecting them to a Quinton Cardiac Science Q-Stress test machine. Then, I got on a treadmill and started a stress test, in which I walked and ran on a machine-operated treadmill.
For our 2016 test, I wore the Fitbit Charge HR ($149), the Fitbit Blaze ($199), the Garmin vivosmart HR ($149), the Garmin Forerunner 235 ($329), the TomTom Spark Cardio + Music ($249), the Apple Watch (starting at $349) and a Polar H7 chest strap ($79). I compared their accuracy when sitting still, walking at a brisk pace, and running.
(We performed a similar test in 2015, also supervised by Steinbaum, with the Apple Watch, the Mio Alpha 2 ($199), the Microsoft Band ($99) and the Basis Peak ($199), along with a Polar H7 chest strap.)
What I found was that these optical heart rate devices were fairly accurate, with some caveats.
The most accurate heart rate monitor was the Polar H7 heart rate strap. Whether I was walking, running or sitting, it was always within 1 or 2 beats per minute of the EKG machine. What's more, it picked up changes in my heart rate almost instantly.
While I was resting, every device was within 3 beats per minute of the EKG machine, and picked up my heart rate quickly. My heart rate was in the 60s or 70s during this test.
Next, I got on the treadmill and started walking at a New York City pace (look out, tourists!). At this point, my heart rate had picked up to around 120 to 130 beats per minute. Again, I saw little variation between what was reported on the EKG machine and what the trackers showed. Here, too, they were also fairly quick in responding to changes in my heart rate.
Then, Steinbaum increased the speed and angle of the treadmill so that I was running on an incline. At this point, my heart rate jumped to 160 to 170 beats per minute, my arms were pumping and I started working up a real sweat. And that's where I started to have issues with almost all of the optical heart rate monitors.
While the chart may make it seem as though most of the trackers were fairly accurate — though less so than when I was sitting — I encountered two problems common to all of the optical heart rate monitors. First, if there was any sweat on my wrist, it would interfere with the sensor, resulting in readings that were far off from what the ECG machine and the Polar chest strap reported. As I swapped out each band on my wrist, I had to wipe down my forearm to make sure there was no moisture.
During this test, the Fitbit Charge HR was the worst performer, varying 4 bpm from the EKG machine, and only the Polar H7 was within 1 to 2 bpm. The Fitbit Blaze was the best-performing wrist wearable, with a variance of 2 bpm.
The other issue was that the optical heart rate sensors were much slower to display an accurate heart rate. After strapping one on my wrist midrun, it would often take upward of 20 to 30 seconds for it to show me an accurate reading. When I slowed down, most of the optical sensor readings would similarly lag a few seconds behind the EKG and Polar chest strap.
Athletes who want the most precise heart rate measurements should stick with a chest strap. It may be somewhat uncomfortable, but you'll get the best results.
Based on this test and others I've done, optical heart rate monitors will provide a fairly accurate reading, but you have to make sure that the wristband is strapped to your wrist properly. It has to be in the correct location (just below the wrist bone) and fastened securely; there can't be a gap between the sensor and your skin, and the tracker shouldn't be able to slide around on your wrist. Lastly, you have to ensure that both the optical sensor and your skin are clean and free of perspiration.
Although its heart rate monitor was so close in performance to its competitors, I still recommend the $149 Garmin vivosmart HR as the best fitness tracker for casual gymgoers, thanks to its large display and extra features. For more serious runners, the $329 Garmin Forerunner 235 is packed with goodies, such as a customizable display, long battery life and fast GPS acquisition times.
It's not only serious athletes who should be concerned with heart rate. Steinbaum said heart rate provides a good glimpse into your overall health. "Keeping an eye on your heart rate over time is a great thing you can grab onto that says, 'I'm in shape' or 'I'm not.'" So no matter the type of monitor you choose, your heart rate is a metric that's worth keeping track of as you work to reach your fitness goals.
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