When runner Starla Garcia, 31, was marathon training, she noticed the data provided by her Garmin Forerunner 235 watch often seemed skewed, giving her very high heart rate readings during easy efforts. While there are many factors that can affect heart rate including heat, research suggests that Garcia, a Mexican-American runner, isn’t the only one who has experienced inaccuracies.
In a 2017 study published in the Journal of Personalized Medicine, researchers found that while most wrist-worn devices adequately measured heart rate in laboratory-based activities, device error was higher for males, darker skin tones, and those with higher body mass indexes. Those training at altitude or in extreme climates may also notice inaccurate data because external factors such as temperature, elevation, and humidity can also affect your heart-rate readings, says Neal Henderson, head of sports science at Wahoo Fitness, which uses an optical heart rate sensor in its watches.
So while having all your heart rate data available on your wrist is convenient, it may not be the most effective way to monitor or guide your training. To understand how we got to this hazy place of heart rate tracking today, we need to first understand how it works.
How Heart Rate-Tracking Devices Work
When it comes to tracking your ticker, there are two popular options: chest straps and wrist watched or arm-worn devices. Most chest-based monitors work by measuring the electrical conductivity of the heart, whereas most wrist, arm, or forearm devices measure blood flow. This is why these monitors are able to give you a heart-rate reading from the top of the wrist versus the bottom, where you would normally take your pulse, explains Melissa M. Markofski, Ph.D., an assistant professor in the department of health and human performance at the University of Houston and an ACSM-certified exercise physiologist.
“Every time the heart pumps, there’s an increase in blood flow, and in between the pumps, there’s a slight decrease,” she says. “We can measure where these peaks are and how often these peaks [occur], then you count that up, and that’s the heart rate.”
This method is known as photoplethysmography (or PPG) and is used by brands such as Polar, Coros, Apple, Garmin, and Fitbit to measure your heart rate via an optical heart rate sensor in your wrist- or forearm-based device. PPG is based on the idea that because blood is red, it reflects red light and absorbs green light. So when your heart beats, green light absorption is at a maximum with the increased blood flow that occurs in your wrist, whereas green light absorption is low when there is less blood flow in between heartbeats. By flashing the green LED lights hundreds of times per second to measure that absorption, these watches can calculate a close approximation of the number of times the heart beats each minute, which translates to your heart rate in beats per minute (bpm).
“Every time the heart pumps, there’s an increase in blood flow, and in between the pumps, there’s a slight decrease,” she says. “We can measure where these peaks are and how often these peaks [occur], then you count that up, and that’s the heart rate.”
This method is known as photoplethysmography (or PPG) and is used by brands such as Polar, Coros, Apple, Garmin, and Fitbit to measure your heart rate via an optical heart rate sensor in your wrist- or forearm-based device. PPG is based on the idea that because blood is red, it reflects red light and absorbs green light. So when your heart beats, green light absorption is at a maximum with the increased blood flow that occurs in your wrist, whereas green light absorption is low when there is less blood flow in between heartbeats. By flashing the green LED lights hundreds of times per second to measure that absorption, these watches can calculate a close approximation of the number of times the heart beats each minute, which translates to your heart rate in beats per minute (bpm).
Just how close is that approximation? Close, but not as accurate as a chest-strap monitor, according to a 2017 JAMA Cardiology study. It showed that wrist-worn, optically-based monitors didn’t match the accuracy of chest-strap-based monitors, with wrist-based monitors being best at rest and diminished in accuracy during exercise. The study results also showed that heart-rate data produced by the Apple Watch was more accurate compared to a Polar H7 chest strap than the Fitbit Charge HR, Mio Alpha, and Basis Peak (since recalled) were.
Similarly, a 2019 study in Cardiovascular Diagnosis & Therapy showed that the Polar H7 chest strap had the closest readings to an electrocardiogram (ECG), followed by the Apple Watch Series 3, while the Fitbit Ionic, Garmin Vívosmart HR, and Tom Tom Spark 3 all had equal agreement.
“Newer research like this study is showing that as manufacturers are able to put more sensors into the watches, they do seem to be getting to increasing accuracy,” Markofski says. “But if you’re really concerned about getting the most accurate reading and don’t have access to a lab where you could do lactate threshold testing, then you’re probably better off using a chest strap like Polar.”
The Limitations of Wrist-Based Heart Rate Tracking
Although wrist-based technology is generally more convenient and comfortable than a chest-strap monitor, being that it’s still relatively new, it’s not without issues, especially when accuracy is concerned.
For one, in order to use heart rate data effectively, you need a current set point to base your training around. Many commonly-used methods utilize maximum heart rate, but in order to use a watch to estimate that accurately, you have to be able to safely conduct a maximal effort test on your own. This is challenging and medically inadvisable for some populations such as those new to the sport or those with preexisting heart conditions.
Even then, it depends on how much physical and physiological data your device is using, says Kaitlyn Bishop, an ACSM-certified exercise physiologist at New York University Langone Health’s Sports Performance Center. “If you’re also inputting your resting rate, your age, your sex, your lean mass, body fat, weight, all of these measures are going to make it more accurate. But most of the time, because it’s a consumer item, it’s not that advanced.”
Another shortcoming of many wrist-based wearables with green-light technology like the one in Garcia’s Garmin Forerunner 235 is that research has found they don’t accurately measure heart rate data for all demographics nor all activities. This is because the majority of wrist-based heart-rate monitoring devices rely on optical sensors to monitor your blood volume via light absorption, says Mikael Mattson, Ph.D., an author of the study and a senior researcher in the department of cardiovascular medicine at Stanford Medicine.
“The reasoning [behind using] green is that it’s more stable and easier to get a good reading when you’re moving,” says Mattson. “However, this green light has a shorter wavelength than the more powerful infrared lights hospital-grade trackers use, and it is also more readily absorbed by melanin, a naturally-occurring pigment often found in people with darker skin tones.”
Other circumstances such as tattooed skin or thick arm hair, high humidity, altitude, pregnancy, or even biomechanical factors that vary when you’re running outside versus on a treadmill can also result in inaccurate readings, adds Henderson.
Alternative Ways to Use Heart Rate Effectively
While tracking your heart rate is flawed outside a lab, that doesn’t mean it’s completely worthless to your training. In fact, heart rate training can be an incredibly effective tool to personalize your training and improve your performances, so long as you understand it, customize it for your own body in practice, and not let it rule every run. Here’s how.
→Training
If you do want to track heart-rate information to guide your training, your best bet is to use a chest strap for the most accurate data. Then, take that data with a grain of salt and understand its limitations, suggests Kaitlin Goodman, a four-time U.S. Olympic Trials qualifier and running coach based in Boston who uses heart-rate tracking for her own training.
“We, as runners, are data-driven and love to have data to inform our training decisions,” she says. “It’s key to understand that it will be useful to a point, but you should still be doing a body scan and assessing how you’re feeling. If your watch says that your heart rate is 180, and you feel like you’re really running very comfortably, trust your body.”
“YOUR LACTATE THRESHOLD HEART RATE IS A BETTER BASELINE THAN YOUR MAX HEART RATE.”
This advice aligns with a unique method that David Roche, a running coach in Boulder, Colorado, two-time USATF trail 10K national champion, and coauthor of The Happy Runner: Love the Process, Get Faster, Run Longer, and his wife Megan, a five-time USATF trail national champion in events ranging from 10K to 50K, use.
Rather than base their training on a maximum heart rate or use age-based equations to find it, the Roches use a simple field test to find their lactate threshold heart rate, the level at which the intensity of exercise causes lactate to accumulate in the blood at a faster rate than it can be removed. This is the border between low- and high-intensity work. Your lactate threshold heart rate is a better baseline than your max heart rate, which is used in other methods, says Roche, because two runners with the same maximum heart rate can have widely varying lactate thresholds due to genetic or training differences.
Then, they find their zones from that number, and after they get a feel for those specific efforts, they hang the heart rate strap up for awhile, and only use it for spot checks throughout their training.
→Recovery
The Roches also use heart rate to keep tabs on their (and their athletes’) recovery. David is a fan of using Whoop and similar tools that measure heart-rate variability (HRV), the variance in time between the beats of your heart, as a proxy for recovery state. The higher your HRV, the more recovered you are. “It’s a great tool to gauge overall stress,” Roche says.
If your device doesn’t measure HRV, you can use your resting heart rate (RHR), which is measured quite accurately by wrist-based devices and chest straps. Take note of your normal RHR when you first wake up before sitting up in your bed. When it’s elevated five to 10 or more beats per minute above normal, that could be a signal you need more recovery. When it’s normal or lower, you’re ready for the next workout.
“Checking your resting heart rate every day is a great way to check in on your nervous system and on how you’re recovering from your training,” Bishop says. “A chronically high resting heart rate can be an indicator of sympathetic overdrive.” If you find that, it’s a signal to evaluate your stress levels, training intensity, or other components like alcohol intake that might cause a change in your recovery status, she adds.
→Nutrition
Garcia, who is also a registered dietitian, wanted to ensure she was getting accurate data to keep her hydration and nutrition levels in check while training and racing. After noticing the data her watch provided seemed off, her coach recommended she switch to a heart rate strap, which she did.
During Grandma’s Marathon, the temperature rose, elevating her heart rate with it. Garcia was able to recognize that she needed more electrolytes (specifically sodium and potassium) not just for hydration, but to keep her heart rate levels from spiking since her effort felt controlled. “I’d rather know that my nutrition is at 100 percent than at 80 percent especially when going for a time-based goal like that,” she said.
Where Does Heart Rate Tracking Go From Here?
There’s no sign of runners moving away from relying on GPS watches for their training, which is why some manufacturers acknowledge that they’re continuing to research and develop improved technology, including heart-rate monitoring.
“It is a common industry challenge to provide accurate heart rate for users with darker skin tones [as well as with tattoos, arm hair, etc.] but it is something we are interested in,” says David Song, a senior product manager at Coros. “We have tested our products on users with darker skin tones and plan to continue to do so in the future.”
Speaking on background, representatives from Apple and Garmin also confirmed that they have worked to develop and design optical heart rate technology features that have been validated all across the spectrum of skin tones to ensure that the technology is truly inclusive in nature. Mattson concurred with this, noting that earlier and cheaper watch models didn’t adjust for this at all, but that more of today’s advanced product offerings are improving.
Which is a good thing because both experts and runners agree that heart rate data is valuable when understood and used appropriately, like in Garcia’s case. Although she missed an Olympic qualifying time at Grandma’s, she continued using her heart rate strap to monitor her nutrition, training, and recovery in the summer months leading up to her next build-up to once again target the OTQ at the 2020 Houston Marathon. As a result, she was confident that a qualifying time was possible no matter what type of weather Houston threw at her.
“Even though it was colder in Houston seven months later, I knew I still needed to take in more electrolytes during the race than I did at Grandma’s,” she said. So she did, while wearing her Garmin chest strap, and ultimately meeting her goal of qualifying for the Olympic Trials with a time of 2:43:56.
