How do fitness trackers measure your heart rate?

Heart rate tracking is increasingly common among fitness enthusiasts, and understandably so. Whether it is a rugged outdoor smartwatch or a discreet smart ring, smart wearables can help you determine if you exercised hard enough, rested well during sleep or stressed too much during the day — simply by listening to your heartbeat.

Some of the best fitness trackers can even use machine learning to give you detailed exercise recommendations based on how your health metrics change over time — not to mention that smart wearables can be invaluable tools for endurance athletes. But how do fitness trackers measure your heart rate, exactly? And how accurate are these measurements? We asked the experts.

Most fitness watches and smart rings depend on a technique called photoplethysmography (PPG). While its name may sound scarily complex, this technology is based on a relatively simple concept — light absorption by the body tissues.

How does photoplethysmography work?

PPG uses optical sensors that detect heart rate by measuring changes in the volume of blood flowing through tiny blood vessels in the skin and underlying connective tissue, Dr. Peter Sogaard, professor of cardiology at Aalborg University in Denmark and chief medical officer at VentriJect, a company which has developed a novel device for measuring cardiorespiratory fitness, told Live Science by email.

With each heartbeat, the heart muscle contracts and relaxes, to force the next portion of blood into circulation. Contracting causes a temporary spike in blood volume and increased pressure on the artery walls, and it is often referred to as the systolic phase of the cardiac cycle. When the heart relaxes between beats, the blood flow decreases — this is known as the diastolic phase. PPG sensors measure these changes in blood volume and pulse pressure (the difference between systolic and diastolic pressure), and then convert these metrics into heart rate measurements, Sogaard said.

PPG sensors detect these changes in blood movement by emitting light at specific wavelengths into the skin. Blood components like hemoglobin, the protein in red blood cells, absorb some of that light. The tracker's photodetector then measures the amount of light reflected back. The more light absorbed by the blood, the higher its volume in the blood vessels — and this is what fitness trackers interpret as a heartbeat, Sogaard said.

"The most commonly used light sources are infrared and green LED," Sogaard said. That is because these wavelengths of light tend to be the most effective at penetrating the skin tissue, according to a 2022 review published in the Frontiers of Physiology. Generally, the accuracy of PPG increases with the light wavelength, the review authors noted. Some optical sensors even combine several wavelengths to improve accuracy.

That is the theory. But how accurate are optical sensors in practice? The answer is less clear.

How accurate is photoplethysmography?

"Multiple factors can affect the accuracy of PPG, from tracker placement and body temperature to skin thickness, motion artifact [distortions caused by movement] and cardiovascular disorders like arrhythmia and peripheral vascular disease [a condition in which the arteries outside of the brain and the heart become blocked or narrowed]," Dr. Masaki Nakamura, a cardiothoracic surgeon at Baptist Health Miami Cardiac & Vascular Institute, told Live Science by email.

For example, optical sensors tend to provide more reliable heart rate measurements during rest and sleep than they do during exercise, according to a 2020 meta-analysis in the Journal of Sports Sciences. Scientists compiled 44 studies that compared the accuracy of PPG fitness to more precise ECG (electrocardiogram) and chest-strap heart rate monitors and found that the biggest discrepancies could be observed with weightlifting, cycling and other high-intensity sports. While most modern fitness trackers use accelerometer-based algorithms to account for rapid movements, motion artifacts can still affect their tracking accuracy.

Skin contact is another factor that can have an impact on PPG measurements, Sogaard said. For example, a device that is too loose or too tight will not allow the optical sensors to get a good reading. Skin temperature can also interfere with PPG. "If the patient has a low temperature, vessels are contracted and measurements become inaccurate," Sogaard said.

Moreover, optical sensors may struggle with detecting heart rate in people with darker skin tones or tattoos. That is because both melanin, the dark pigment in the skin, and tattoo ink can absorb light and reduce the amount of light reflected back to the tracker, according to a 2023 review published in the Journal of Racial and Ethnic Health Disparities.

People with obesity may also struggle with obtaining accurate heart rate measurements. Excess body fat can lead to changes in skin thickness and blood flow, which is why it may affect the accuracy of optical sensors, according to a 2021 review published in the journal Biosensors. The good news is that scientists are working on developing new technologies that would make PPG more accessible for these population groups.

While not without limitations, PPG fitness trackers can be useful for monitoring heart rate in people with certain cardiovascular conditions, according to Nakamura. "Continuous heart rate and blood pressure monitoring can be crucial for cardiac patients to prevent future events like a heart attack, stroke or acute heart failure. The current industry gold standards of blood pressure and heart rate assessment are the manual sphygmomanometer (BP cuff) and the electrocardiogram. However, both these exams can only be measured intermittently and can be affected by factors such as white coat syndrome [a condition when a person's blood pressure is high only when measured in a health care setting], caffeine or stress," Nakamura said.

That said, experts agree that PPG-based heart rate readings should never be regarded as a substitute for clinically validated tests conducted by healthcare professionals.

This article is for informational purposes only and is not meant to offer medical advice.