Can you trust data from wrist-based heart-rate sensors?

Watch-type heart-rate monitors are getting better all the time but they are far from perfect

- Tom Epton 

Once the preserve of runners, GPS watches have become popular with cyclo-cross riders, mountain bikers, triathletes and now road cyclists. The latest generation not only do the job of a head unit but are fitness trackers and more.

Among the many upgrades that have appeared since the Casio PRT-1GP (the first GPS-enabled watch) is the optical heart-rate sensor, which is designed to eliminate the need for a chest strap.

But many athletes and coaches have found some of the data from these sensors to be less than accurate. Manufacturers have been working on sensor quality but can the data be really trusted?

How sensors work

According to a paper which investigated sources of inaccuracy in wearable technology, “Heart-rate measurements from wearables are derived from photoplethysmography (PPG), an optical method for measuring changes in blood volume under the skin.”

Put simply, when your heart beats, blood moves around the body in pulses. These pulses mean that blood volume at various points in your body changes a little with each beat. Heart-rate can be deduced from measuring the time between these changes. Chest straps work differently: they detect electrical signals from your heart and tend to be more accurate.

The science