Heart rate monitors and fitness trackers like Fitbit have become trendy wrist accessories, but heart doctors say they often aren't accurate.
In a recent study of 50 healthy adults published October 12 in JAMA Cardiology, researchers assessed four popular wrist-worn heart rate monitors—Apple Watch, Fitbit Charge HR, Mio Alpha, and Basis Peak—under varying levels of physical exertion to assess how precisely they measured the user's heart rate.
Researchers found that that the Basis Peak overestimated heart rate by about eight beats per minute during moderate exercise, while the Fitbit Charge HR underestimated heart rate by about six or seven beats per minute during vigorous exercise. And in contrast to the electrocardiogram, the Apple Watch and Mio Fuse were within 27 to 29 beats over or under the actual heart rate 95 percent of the time, while the Fitbit's range was between 24 and 39, and Basis Peak's between 39 and 33.
"We found variable accuracy among wrist-worn HR [heart rate] monitors," the researchers wrote. "In general, accuracy of wrist-worn monitors was best at rest and diminished with exercise."
Heart rate monitors strapped to the user's chest, they said, were always more accurate than their wrist counterparts. These should be used instead of the wrist accessories, if it's imperative that a medical patient track their heart rate, the authors suggested. The study suggested that wrist-worn monitors are more often used recreationally and had "suboptimal" accuracy during even moderate exercise.
According to co-author Dr. Marc Gillinov, cardiac surgeon at the Cleveland Clinic, the accuracy of wrist-worn heart rate monitors can be impacted by motion, skin tone variance, and ambient light. "They should not be used to diagnose or treat anything," he said. "You can't count on them in that context."
Even Aaron Coleman, CEO of Fitabase, Fitbit's research partner, admits that the farther a heart rate monitor is from the heart, the more vulnerable it is to variance.
To assess the accuracy (or inaccuracy), of the wrist-worn heart rate monitors, the researchers studied 50 young and healthy adults with the devices as they used treadmills at different paces: between two and six miles per hour. For comparison, the research subjects also wore chest strap monitors, in addition to a different kind of wrist monitor on each wrist.
The researchers collected heart rate data when the subjects were at rest, as well as after at least three minutes of moving at the various treadmill speeds. The heart rates ranged from 49 to 200 beats per minute (bpm).
While wrist-worn heart rate monitors can be inaccurate, they still might be enough to at least get people moving and thus, more healthy. Clinical psychologist Joshua Klapow, Ph.D. told Shape Magazine that "if you want to stick to a new habit, monitoring is one of the best ways to make a change."