Apple Watch Heart Rate Accuracy


I have been working with Scosche Industries as a consultant, assisting Scosche in testing and the development in their optical heart rate monitoring starting with MyTrek, then Rhythm 1.5, and now our latest and greatest optical heart rate monitor, the Scosche Rhythm+.  Over the 3 generations of Scosche heart rate monitors I have gotten a pretty good idea of what is needed to make optical heart rate monitors accurate, and equally important what makes accuracy fail.  I intend to give Apple the benefit of the doubt when it comes to their development of an accurate wrist solution for continuous heart rate monitoring in the Apple Watch, although I see some potential “hurdles” to overcome.  If anyone can overcome these “hurdles” it will be Apple, and I can’t wait to see how they deal with these potential issues.

Hurdle #1

The Apple watch is a substantial device in weight with a high center of gravity - especially compared to the Rhythm+    

I’m using the Rhythm+ as a benchmark for optical heart rate accuracy since it is the most accurate product on the market that I’ve tested.  I do know this from my testing - More weight and a high center of gravity = more movement on the skin which will affect the accuracy of the signal generated from the photosensor(s) on the bottom of the Watch.  This will not be a problem when you are sitting still at the desk, intimately “texting” a live heart beat to your girlfriend (aww), but it will certainly be an issue when you are exercising.  For example, every bump you hit on the bicycle or movement of the wrist can move the heavy watch and throw off the photosensor’s accurate signal and thus impact accurate data collection.  

Hurdle #2

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Will a magnetic wristband be tight enough to keep light out?

After testing products like the Mio Link/Alpha which are wrist monitoring optical heart rate monitors, I know you need to get the wrist monitor very tight - and nearly cut circulation off at your wrist to get a good signal.  Tim Cook was actually wearing the Apple Watch above the wrist bone similar to where Mio recommends.  Even above the wrist bone there’s not much meat, and lots of little bones that distort the photosensor’s clear signal and create noise, thus throwing off the algorithms.  Bottom line is, that and optical heart rate wrist band needs to be tight, especially because of Hurdle #1!  I have gotten great results testing the lightweight, low center of gravity Rhythm+ on the wrist, but on the forearm or upper arm is the most accurate location due to a clear signal that is generated on a meatier location on the body.  Less noise in the signal = more accurate heart rate #.  The forearm also has a larger diameter and we have had great success with the Rhythms’ breathable, flexible velcro strap for both accuracy and comfort.

Hurdle #3

2 Green LED’s vs. 2 Green + 1 Yellow LED

In our testing at Scosche with Valencell, we found that to get a good signal on ALL skintones Valencell added a Yellow LED which penetrates deeper into the skin than green LEDs.  Green lights are most effective for measuring just below the surface of the skin and have been proven to NOT work very well when used alone with darker pigmented skin. When used in combination with the yellow LEDs, the green and yellow tones combine to accurately read through all skin tones.  From what I can see, the Apple watch only has 2 green LED’s so it is unclear to me if it will be accurate with all skin tones.  

Hurdle #4

Keep Ambient Light/Sunlight OUT!

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As described above, it is really important to keep sunlight and ambient light away from the photosensor’s sight.  The Rhythm+ uses a “gasket” that presses the photosensors and LEDs into the skin .5mm to bury the photosensor into the skin along with the LEDs.  That way, the photosensor is reading a signal that is illuminated by the LEDs, not the fluorescent light bulbs in the gym, or the sunlight outdoors.  The bottom of the monitor must also have a sufficient footprint to block ambient light, therefore thinner optical monitors have shown to be less accurate.  I have not had the chance to test the Apple watch, and it is unclear to me if they have figured out how to block ambient light efficiently enough to gather good data.  

Is the Apple Watch Designed to be a Continuous Heart Rate Monitor for intense exercise?

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The way I saw it, In Tim Cook’s presentation, the Apple Watch was shown to be an excellent Activity/Life monitor, where movement, lifestyle, and general health were emphasized, and not heart rate training.  I could be wrong, but I think Apple knows heart rate is not going to be super accurate during exercise with the Apple Watch, therefore they built the Watch around making a great interface for activity monitoring - IE: how often you stand up throughout the day, pedometer information, etc.  This will surely play out to be a thorn in the side of FitBit and other activity monitors, but I don’t think it poses a threat to serious heart rate training athletes.  

Data Centralization

I think Apple knows their job is to centralize the data collected from accurate sensors, not to necessarily create the sensor itself - except for activity monitoring.  For example, Health Kit will allow all connected apps to push data that has been collected from different sensors directly into Apple’s Health App.  I don’t see Apple getting into the Blood Pressure monitor or blood glucose monitor business any time soon, but apps that monitor blood pressure and blood glucose can push their data to Apple Health.  I DO like the way the Apple Health App will centralize that information into one health app, along with other health related metrics.  It makes sense to have all that data in one centralized location.  The bottom line is that you should be able to use other heart rate sensors to do your workout and gather accurate data, you won’t be confined to only using the Apple Watch to collect accurate heart rate data.

Benefit of the Doubt.

As I stated in the beginning, I am willing to give Apple the Benefit of the Doubt.  If anyone can do it, Apple and their unlimited resources can solve these potential issues.  For one, the Apple Watch might have 2 photosensors.  There appears to be 4 circles on the bottom of the Apple watch - 2 Green LED’s and 2 photosensors?  Along with super fast processing power the Watch might be crunching more data in “stereo” via an extra photosensor.  They certainly could have hired enough talent to write some super sophisticated algorithms to get an accurate heart rate number.

I don’t know what they have up their sleeve (pun intended) but I do think they know it is not going to be a great heart rate monitoring solution for serious athletes.  Only time will tell.  For the time being, I'll stick to my Rhythm+.

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