Apple Watch Heart Rate Accuracy: Part 2


Now that the Apple Watch is out, I figured I’d follow up my previous post on Apple Watch Heart Rate Accuracy.  Apple has recently posted a bit more information on their site on the heart rate functions on the Apple watch and in a nutshell “it kind of works, but if it doesn’t work during your specified activity, you can connect another heart rate sensor to the watch to get an accurate heart rate.”  So, they make it clear – the Apple watch is really good at measuring activity, it probably gives you a decent heart rate measurement every 10 minutes using a low power infrared sensor, and when there is more movement detected, the green LEDs fire up and give you a heart rate for exercise.  If you are not receiving an accurate heart rate, make sure your wrist strap is tight enough and if it still isn’t accurate, then it is probably best to just connect an external sensor.  In other words, it might not be very accurate.

Hmmm… I hate to say it (sort of) – but I told you so.  Let’s take a quick look at why it struggles. Full disclosure - As I stated in my original blog, I test for Scosche on their optical heart rate armband and Apple is having to jump similar hurdles to the ones we had to in the 1st generation of Scosche optical heart rate sensors 4 years ago!  

#1: All in one sensors are not great at doing any one thing.  

A specialized single function sensor will almost always outperform a one size fits all sensor.  A machete will cut through the forest better than a Swiss Army knife.  Not an easy task.  Similarly, the Apple watch is the Swiss Army knife of watches, attempting to cut through a thick forest one branch at a time.  It’s just not an adequate enough tool for the job.  In reality, to get accurate heart rate data, you need a specialized, specific sensor such as a chest strap or armband such as a Rhythm+ to do the job correctly – a machete.   We could have put more features in the Rhythm+ at the expense of a great heart rate experience, and I’m glad we didn’t.  

#2 Construction:

The Apple watch is a giant, heavy mass of a watch that moves with the inertia of its’ high center of gravity and fashionable, comfortable wrist straps.  The ideal watchband for an accurate heart rate monitor should be flexible and stretchy.  The flexibility and stretchiness of the watchband allows the sensor to follow the movement of the arm, expand and contract with your muscles under constant tension, and dampen vibration.  The Apple watch does not do this, and how could they?  It is impossible with a metal or rubber strap – they are simply not elastic.  Along with the high center of gravity there’s too much movement on the skin for an accurate measurement.  It looks pretty though.  We spent a lot of time on this at Scosche to keep a low center of gravity and test the right materials to get an accurate, consistent signal.

#3 Optics

Apple is correct about green light being absorbed by the hemoglobin in blood, however certain pigments in darker skin tones can impede green light.  The Valencell PhD’s (whose Performtek sensor technology is in Rhythm+) discovered yellow light is less sensitive to skin pigment.  The best optical sensor will have a combination Green + Yellow light for the most accurate signal on all skin tones.  

#4 Algorithms

It is now clear that the new Apple Watch uses dynamic algorithms to compensate for the design errors in points 2 and 3.  A dynamic algorithm changes the colors, intensity, and sampling rate based on the signal integrity measured by the photosensor(s).  The best algorithms in the world cannot make up for poor signal quality.  A fixed algorithm on the other hand uses the optimal intensity, sampling rate, and a color usage and these parameters are locked from the moment the sensor is powered on.  In my experience with Scosche’s testing, the engineers and other smart people in the room have all agreed using dynamic algorithms are a bad idea and prone to infinite loops like a dog chasing it’s taleThe human body can provide a dynamic response with endless parameters, which can change at the same time, all the time.  There are simply too many possible scenarios for the algorithms to figure out what is going and to be accurate in all situations, especially taking in to account the first 3 hardware hurdles mentioned above.  

I have still yet to fully test the Apple Watch, but after reading their latest information about the heart rate functionality, I have even less confidence in the ability for the Apple Watch to provide accurate heart rate data during most types of exercise.  It may even be difficult to export the heart rate data to compare accuracy versus other products!  As I said before, if anyone can do it, Apple can, and I hope they DO!  This product is still in its infancy, and it most certainly will evolve with generations.  For now, as far as heart rate training goes, we are left with a neat looking toy to send our significant others the pitter patter of our heart beat.  Awww.... how cute.  

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