QUESTION
Watch -t ype hear t-rate monitors are get ting bet ter all the time but they are far from perfect
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.
A mong the many upgrades that have appeared since the Casio PRT-1G P (the first GPS-enabled watch) is the optical hear t-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 work ing on sensor quality but can the data be really trusted?
How sensors work
According to a paper which investigated sources of inaccuracy in wearable technolog y, “ 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 var ious points in your body changes a little with each beat. Hear t-rate can be deduced from measuring the time bet ween these changes. Chest straps work differently: they detect electrical signals from your heart and tend to be more accurate.
The science
A lot of scientific work has been done on optical sensors. They ’re also used in medical settings, so understanding when they ’re inaccurate is important. The accuracy of wearables is hotly debated – brands like W hoop rely on optical sensors being accurate for their business model but some sports scientists refer to them as “random number generators ”.
The truth probably sits somewhere in the middle. Optical sensors are not per fect (no sensor is) but they are certainly not useless. A paper in the publication Nature addressed issues with wearable tech. It discussed potential inaccuracies in PPG which could “stem from three major areas, including diverse skin types, motion artifacts, and signal crossover ”. For cyclists, diverse skin types and motion artifacts are the t wo problems here.
The scientists involved in the paper obser ved that, “We saw no statistically significant difference in accuracy across skin tones, but we saw significant differences bet ween devices, and bet ween activity types, notably, that absolute error during activity was, on average, 30% higher than during rest.”
These findings lead to a couple of useful thoughts: the morning resting hear t-rate from a wearable is mostly accurate; and our activity type might also impact the accuracy of the
