The Monday Shakeout: Are Wrist-Based Heart Rate Monitors as Good as Chest Straps?
By Andrea Myers
In this week's Monday Shakeout, Andrea talks about a much-debated subject in the running world - that of watch-based vs. chest strap heart rate monitor. What's best? Are wrist-based accurate? Andrea looks to research in today's article to see what the science says.
Wrist-based heart rate monitors are increasingly common these days, found in everything from $60 fitness trackers to $1000 watches designed for multi-day ultra races. Many people assume that the heart rate data from their watch is just as accurate as a chest strap (and potentially more comfortable), but what does the literature have to say?
Wrist-Based Monitoring vs. Chest Strap
A person's heart rate (HR) is measured differently by a chest strap as compared to a wrist-based monitor. A chest strap uses electrodes to measure the electrical activity of the heart, and numerous studies have confirmed their accuracy in comparison to the gold standard for heart rate measurement, an electrocardiogram (ECG). The first wireless heart rate monitor was released by Polar in 1982, and since then, many athletes have used chest strap heart rate monitors to help quantify their level of effort in training and racing. Thankfully, the materials used have improved since 1982, with the chest strap improving from a relatively stiff plastic material to the more flexible and comfortable fabric material currently available from several manufacturers. The idea of a non-chest based monitor is appealing, especially for those who find even modern chest straps uncomfortable due to perceived ribcage restriction or chafing.
Wrist-based heart rate monitors are a much easier sell, as they are integrated into the watches that many of us already wear. These heart rate monitors estimate heart rate through optical sensors, which shine LED light onto the skin, and then measure the amount of light reflected back to a photodiode sensor. Heart rate is estimated based on the change in blood volume from each pulse at the wrist. The technical term for this method of measurement is photoplethysmography, which is the same technology used in pulse oximeters (the finger sensors that are often used in doctors' offices and hospital to measure HR and blood oxygen saturation).
What Does the Science Say?
It is known that there are three sources of inaccuracy in optical heart rate monitors, including due to different skin tones, movement artifact, and signal crossover. Bent et al investigated these sources of inaccuracy in wrist based HR monitors in 2020, looking at the Empatica E4, Apple Watch 4, Fitbit Charge 2, Garmin Vivosmart 3, Xiaomi Miband, and Biovotion Everion to the gold standard ECG. They found no significant correlation between skin tone and HR measurement error. This finding differs from findings of inaccuracy based on skin tone in pulse oximeters, which may be due to the use of green light in wrist based heart rate monitors, as opposed to red light in pulse oximeters. A study by Fallow et al in 2017 found that green wavelengths showed greater HR accuracy at rest and during exercise across skin types as compared to other wavelengths.
Bent et al also found that wrist-based HR monitors have increased error during activity as compared to at rest. Specifically, they found that the watches reported a higher HR as compared to the ECG during physical activity. Signal crossover refers to the tendency of optical HR monitors to incorrectly read the repetitive signal from rhythmic movement (such as arm swing during walking or running) and report that movement frequency as HR. They found that arm swing during walking resulted in higher errors in all devices except the Apple Watch 4, and that the errors resulted HR readings that were higher than actual HR from the ECG.
A systematic review done by Fuller et al in 2020 examined the scientific literature for studies on the reliability and validity of commercially available wearable devices. Reliability refers to the consistency of a measure, meaning the measurements are reproducible under the same conditions. Validity refers to the accuracy of a measure, meaning it actually measures what it is supposed to measure. The systematic review included 158 studies that examined nine different optical HR monitor brands, including Garmin, Fitbit, Apple, Mio, Misfit, Polar, Samsung, Withings, and Xiaomi. Most of the included studies were done in laboratory conditions as opposed to in the real world, which limits the applicability of the results. The studies only had sufficient heart rate validity data for Apple Watch, Garmin, and FitBit devices, and the authors found that all three brands measured HR within +/- 3% and were within acceptable measurement error. Three of the included studies assessed interdevice heart rate reliability, and found that Apple Watch had good reliability during treadmill walking and jogging (max speed 10 km/hr) and Garmin and FitBit had good reliability during treadmill and overground walking.
The Accuracy of Monitors for Fast Running
This website is called Doctors of Running, so what about studies that specifically look at the accuracy of optical HR monitors during faster running? Pasadyn et al in 2019 assessed 50 subjects while running on a treadmill at speeds of 4, 5, 6, 7, 8, and 9 mph. This study used Apple Watch III, Fitbit Iconic, Garmin Vivosmart HR, and Tom Tom Spark 3 and compared them to a 3 lead ECG and Polar H7 chest strap. The Polar strap was the most accurate, followed by the Apple Watch III. Agreement with the ECG was 0.98 for the chest strap, 0.96 for the Apple Watch III, and 0.89 for the Fitbit, Garmin, and Tom Tom. The authors' conclusion was that for athletes who are most concerned about accuracy, a chest strap or Apple Watch may be the best choice.
Summary
So what is the takeaway message here? The chest strap HR monitor is unquestionably the most accurate when compared to ECG, but if you do not want to wear a chest strap, the Apple Watch may be the most accurate for runners. If you are using an optical HR monitor for your runs, I would recommend taking its readings with a grain of salt, especially if you see outlier data that does not correspond to your perceived effort level. Of course, if you see outlier data that corresponds with unusual symptoms, such as shortness of breath, dizziness, or chest, jaw, or arm pain; you should speak with a doctor right away. Keep in mind that none of the above mentioned studies included the most recent versions of these watches, so it is certainly possible that accuracy of specific watches has changed (for better or worse) and I look forward to future research on this topic.
As always, the answer depends on the individual. I run with a Garmin Forerunner 245 paired with a Garmin HRM-Pro chest strap, and I also wear an Apple Watch 6 so I don't have to carry my phone. I have done many dual recordings of my runs, and have consistently found that the Apple Watch reports significantly higher HR than the data from my chest strap, to the point where it says my HR has stayed 10 beats above my max HR for 30+ minutes at a time. I would never rely on my Apple Watch for running HR data due to its consistent inaccuracy for me, but of course my experience may not be your experience.
Sources
Gillinov, S., Etiwy, M., Wang, R., Blackburn, G., Phelan, D., Gillinov, A. M., Houghtaling, P., Javadikasgari, H., & Desai, M. Y. (2017). Variable Accuracy of Wearable Heart Rate Monitors during Aerobic Exercise. Medicine and science in sports and exercise, 49(8), 1697–1703.
Bent, B., Goldstein, B. A., Kibbe, W. A., & Dunn, J. P. (2020). Investigating sources of inaccuracy in wearable optical heart rate sensors. NPJ digital medicine, 3, 18.
Fuller, D., Colwell, E., Low, J., Orychock, K., Tobin, M. A., Simango, B., Buote, R., Van Heerden, D., Luan, H., Cullen, K., Slade, L., & Taylor, N. G. A. (2020). Reliability and Validity of Commercially Available Wearable Devices for Measuring Steps, Energy Expenditure, and Heart Rate: Systematic Review. JMIR mHealth and uHealth, 8(9), e18694.
Pasadyn, S. R., Soudan, M., Gillinov, M., Houghtaling, P., Phelan, D., Gillinov, N., Bittel, B., & Desai, M. Y. (2019). Accuracy of commercially available heart rate monitors in athletes: a prospective study. Cardiovascular diagnosis and therapy, 9(4), 379–385.
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