A new study has quantified the difference in accuracy between wrist-worn wearables and heart rate chest straps. Plus, it seems not all fitness trackers and smartwatches are created equal.
Essential reading: Best heart rate chest straps
A chest strap detects your pulse electronically and sends that data to your smartphone or a wrist-worn tracker. This is similar to what you can see in hospitals with patients who have sensors strapped to their chest. These type of monitors tend to be very accurate.
Fitness trackers and smartwatches, on the other hand, monitor your heart rate differently. They achieve this feat by shinning a light on your wrist. When your heart pumps, the blood moves through your veins at a quicker rate, causing less light to be reflected back. By analyzing the light reflected by blood vessels passing through your veins, the wearable is able to work out your heart rate by using an algorithm.
While wrist-based heart rate monitors are more convenient, they are also less accurate. Especially at high levels of exertion. This is why it’s important to wear such devices snuggly when working out.
Now a new study has quantified the difference in accuracy between heart rate chest straps and wrist based heart rate monitors. It was published by the Cleveland Clinic in Cardiovascular Diagnosis & Therapy.
The study recruited 50 healthy, athletic adults (34 men, 16 women) with a mean age of 29. They all ran on a treadmill for 12 minutes with a three lead ECG and Polar H7 chest strap monitor and two randomly assigned, commercially available, wrist-based heart rate monitors. One was warn on each wrist. These included the Apple Watch Series 3, Fitbit Ionic, Tom Tom Spark 3 and Garmin Vivosmart HR. The speed of the treadmill was increased gradually, starting from 4 miles per hour to a final 2-minute stretch at 9 miles per hour (at zero incline).
The researchers then pitted the results from different heart rate monitors against the ECG. Unsurprisingly, the Polar H7 chest strap had the greatest correlation coefficient with the ECG (rc=98). This was the case at all paces. The Apple Watch was next (rc=96), followed by the other three wrist-based heart rate monitors which all had a similar level of agreement (rc=89). Each of the four watches was assessed a total of 25 times.
Whats more the study found that as runners went faster, accuracy of wrist monitors tended to suffer. The Apple Watch Series 3 fared best, once again. At 9 mph it was only off by 1.5 bpm, as compared to nearly 3 bpm for the Garmin.
However, at rest the difference was negligible. This would suggest that it’s perfectly okay to use such devices to monitor heart rate at low intensity activity and to keep tabs on resting heart rate which is perhaps one of the most important indicators of health and fitness of an individual.
“This study demonstrates a moderate to high level of accuracy of four watches for monitoring HR across many treadmill speeds.” researchers wrote.
“If accuracy is imperative, a chest strap or the Apple Watch Series 3 may be the best choice”.
This was not the first such research by Cleveland Clinic. An earlier study looked at accuracy of wrist-based wearables while participants were using a treadmill at moderate speed (up to 6 mph). The agreement with ECG was as follows: Polar H7 chest strap (0.99), Apple Watch (0.93), Garmin Forerunner 235 (0.92), Tom Tom Spark Cardio (0.88) and Fitbit Blaze (0.76).
It’s worth noting, these conditions do not exactly replicate running outdoors or on a different terrain. Furthermore, because they are running on a treadmill participants were in some instances recorded when gripping the treadmill handrail.
So what can we take from all of this?
While fitness trackers and smartwatches provide a more convenient way of tracking your ticker, the accuracy can never compare with chest or arm band style monitor. Particularly for high intensity workouts. If you are very serious about heart rate training a chest strap is still the way to go. Another great option is an upper/lower armband monitor which tend to be just as accurate.
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