July 12, 2024

F1000 Scientist

Health to Life

Heart Rate Monitor | How Exactly Does a Wearable ECG Fitness Tracker Work?

6 min read

Tracking your heart rate during workouts isn’t new, but heart rate tracking technology is rapidly advancing. Gone are the days of merely monitoring your beats per minute; the latest trackers can now alert you to an irregular heart rhythm, and some are even rolling out low-range VO2 max tracking.

With more younger people suffering from strokes, and more reports of cardiac arrests happening during exercise, it might be a good time to consider upgrading your tracker. We tapped experts to find out just how much you should rely on your new tracker’s heart-monitoring software.

How Do Watches Track Your Heart?

The latest smartwatches have the ability to monitor your heart via an electrocardiogram (ECG or EKG) app. An ECG records the electrical activity of the heart that is transmitted to the skin, explains Patrick Green, M.D., Fellow of the American College of Cardiology, cardiologist at UCHealth Heart and Vascular Clinic in Fort Collins, Colorado.

The ECG test on the Apple Watch can detect and notify you of low, high, or irregular heart rhythm that shows signs of atrial fibrillation (or AFib—an irregular or quivering heart beat that can lead to cardiac events, such as stroke and heart failure) or any underlying abnormalities in heart rate, according to Apple. The ECG app checks your pulses to get your heart rate and can then measure if the upper and lower chambers of your heart are in rhythm, according to Apple. If the two chambers are out of rhythm, that can signal Afib. The technology cannot detect a heart attack.

The Apple Watch Series 4, Series 5, and Series 6 (not 1, 2 or 3) provide an actual ECG tracing by recording your heart beat and heart rhythm. The Samsung Galaxy Watch 3 and, pending FDA clearance, the Fitbit Sense also have the technology.


The Frontier X is a heart rate monitor strap that can be used during exercise to detect heart strain, and can record ECG over the duration of your run or workout. Another device that you can use at home for an ECG reading is the Alive Cor/Kardia Mobile (but it’s not a fitness tracker).

When it comes to accuracy, studies showed most devices do a good job of correctly recording heart rate, though chest strap devices are more accurate than wrist-worn ones, Green says. A study published in the Journal of Sports Sciences found that the Fitbit Charge HR provided heart rate estimates that were equivalent to those given by a Polar chest strap monitor.

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All iterations of the Apple Watch are equipped with PPG light sensors and can notify you of high or low heart rate or irregular rhythm. Recent research published in the New England Journal of Medicine studied the accuracy of the Apple Watch PPG (photoplethysmography) light sensors. Researchers found that while rare (only 0.52 percent of participants received an irregular pulse notification), the Apple Watch correctly identified an irregular pulse 84 percent of the time.

All are most accurate at lower and medium heart rates and less accurate at high heart rates, according to Green.

When Should You Be Concerned About a Reading?

If your watch alerts that you may have atrial fibrillation, you should immediately see a health care professional for evaluation, Green says.

Additionally—focus on symptoms, rather than a number generated by a device. Be evaluated for palpitations—the sensation your heart is pounding or racing out of proportion to level of activity. Or, if you feel your heart rhythm is irregular, especially if this is associated with symptoms such as dizziness, lightheadedness, or fainting, Green says.

It’s important to note that one’s heart rate at rest should generally be 50 to 90 bpm. If resting heart rate is persistently greater than 100 to 110 bpm, this is an indicator you should be seen by a doctor. Or if you notice an abrupt change in heart rate during exercise, for example a sudden jump from your usual 160 bpm during a sustained effort to greater than 200 bpm, this could indicate an abnormal heart rhythm, Green explains.

“Generally one would notice a decrease in output if this were to occur.”

But if you notice a pattern of decreasing resting heart rate, Green says it could be for a few reasons. If you’ve been steadily increasing your workouts, these increasing levels of fitness would be associated with a slower resting heart rate.

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On the flip side, medical problems, such as hypothyroidism—an under-active thyroid—can also cause low resting heart rate. That would typically be associated with symptoms such as fatigue, weight gain, and constipation, Green says. Again, he recommends not focusing on a single piece of data in isolation.

Who Can Benefit From a Heart Rate Monitor?

Many people are data-driven and like the feedback that HR assessment provides. A tracker with added ECG function can help you keep tabs on heart rate and may be helpful for diagnosing Afib or showing heart beat irregularities. If a person has had heart health issues in the past, a doctor may suggest the person uses a tracker to keep tabs on heart rate.

“Athletes trying to maximize fitness may want to train in target heart rate,” Green says. “Others are just as well off by gauging level of exertion—mild, moderate, high intensity—based on how they feel, such as respiratory rate, breathlessness, etc.”

Is the Cost Worth It?

“These devices aren’t cheap. It really is personal preference and goals,” Green says. “I wear a [heart rate] monitor in spin class because I think it helps motivate me to work a little harder. I also run without it. It feels good to be untethered from technology.”

If you’re not ready to invest in a smartwatch or heart rate monitor, you can also manually check your pulse to track it throughout your workouts and at rest. The easiest way: Count the number of beats over 10 seconds and multiply by 6.

What about other trackers that don’t have ECG?

Nearly all trackers have some type of heart rate monitor, which can help you gauge the intensity of your workouts. These kinds of trackers are equipped with light sensors that measure heart rate, like a photoplethysmography (PPG) sensor. The light sensors shine a light through the skin to detect blood flow and measures how many times your heart beats per minute.

“The advantage of light sensor, is that once you turn on the feature, it sits in background, and intermittently checks for irregular pulses,” Marco Perez, Ph.D., associate professor of medicine at the Stanford University Medical Center tells Runner’s World.

And, while these trackers may not warn you of Afib, they are also equipped with light sensors that monitor your heart rates. Keeping tabs on your pulse with your Fitbit or Garmin watch, at rest and during exercise can help you get an idea of your typical resting heart rate and heart rate patterns during exercise, so you can more easily see if something seems irregular with your heart rate.

Another option is to pair your fitness trackers with other apps that are made for health data monitoring, such as the Cardiogram Heart Health app, which can help you track heart rate during activity and at rest over time to provide beneficial information related to your heart rhythm.

The biggest thing to remember is that you cannot rely on any technology to tell you when to see a doctor, warns Perez. You have to follow your symptoms.

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