How accurate is the Apple heart rate monitor? In truth, measuring your heart rate is actually pretty easy. To calculate your heart rate, all you you need is a finger, a pulse, and the ability to count. Even devices used to measure your heart rate have been pretty accurate for decades.
So why all the controversy over the accuracy of the Apple Watch?
The kind of hyper-accurate equipment that hospitals use is impractical for day-to-day use. Not everybody wants to strap electrodes to their finger or chest just to know their heart rate with perfect accuracy. So most wearable manufacturers, including Apple, use an alternative method called photoplethysmography. This technique uses a combination of basic optics and biology to shrink heart sensing technology into something that can fit on the back of a smartwatch.
Measuring Heart Rate With Light
Photoplethysmography is based on the simple fact that blood is red, and therefore absorbs color on the opposite side of the color spectrum: green. The Apple Watch has green LED lights that flash against the back of your wrist, which are paired with photodiode sensors that measure how much green light is being absorbed.
On every heartbeat, the volume of blood that flows inside of your wrist is high, and therefore more green light is absorbed. Between beats, less green light is absorbed. The lights and sensors can measure the frequency of the change between high light absorption and low light absorption, which your Apple Watch then calculates into a heart rate.
While that system sounds all nice and sciencey, it’s a bit controversial. Fitbit, which uses a similar system, was sued last year because the plaintiff alleged that their fitness monitors didn’t measure heart rate accurately. (However, Consumer Reports tested the Fitbits in question and claims that it’s highly accurate.)
How Accurate Is The Apple Heart Rate Monitor?
Fortunately, we can answer this question with science. Based a on a study published late last year, the Apple Watch actually offers one of the most accurate wrist-worn heart rate monitors that you can find.
Marc Gillinov, M.D., of the Cleveland Clinic published a study that examined the accuracy of a handful of heart rate monitors. Specifically, the study looked at the Fitbit Charge HR, Apple Watch, Mio Alpha, and Basis Peak. Researchers tested the devices on 50 adults. Each was randomly assigned two different wearables. They tested the devices under several different conditions on a treadmill, collecting 1,773 heart rate measurements in total. To test for accuracy, researchers checked those results against a medical electrocardiogram.
The Apple Watch and Mio Fuse were closest to the electrocardiogram. They had a concordance correlation coefficient (the measurement of how statistically close a pair of observations are) of .91. The next best was the Fitbit Charge HR, which was measured at .84. Bringing up the rear was the Basis Peak, which was clocked at .83. (Incidentally, the Basis Peak was also listed as one of the worst wearables of 2016.)
However, all of these devices were blown out of the water by the Polar H7 chest strap, which measured a concordance correlation coefficient of .99.
According to the authors of the study, “None achieved the accuracy of a chest strap-based monitor. In general, accuracy of wrist-worn monitors was best at rest and diminished with exercise.”
The lesson? If you want one most the most accurate wrist-based heart rate monitors, then the Apple Watch is one of your top options. But if you want totally accurate measurements, nothing beats a chest strap heart rate monitor.