How Do Wearables Track Your Heart Rate?
If you own a Fitbit or Apple Watch, you know it includes a pretty cool feature: a heart rate monitor.
The device, worn around the wrist, can detect a person’s beats per minute (BPM). So, if you’re going for a morning run or squeezing in an afternoon workout, you can always keep an eye on your heart rate. This is particularly useful for people who need to monitor their heart health, such as heart attack survivors, but it’s also useful for healthy individuals. The device also tracks your resting heart rate.
So, how does it work? While this may vary by wearable (ie., Fitbit or Apple Watch), it generally functions via the same technology: photoplethysmography.
What is photoplethysmography?
Photoplethysmography is a technique that takes advantage of a scientific fact: Blood is red because it reflects red light and absorbs green light. If you’ve ever flipped a Fitbit or Apple Watch to look underneath it, you’ll notice it has a sensor that emits green lasers. When pressed against the wrist, these green infrared LEDs, along with photodiodes (a type of photosensor), are actually able to detect changes in blood volume (ie., the quantity of blood in a person’s wrist).
At a more granular level, here’s what happens:
When your heart rate begins to increase, so does your blood flow. The greater amount of blood flow, the more the LED green light is absorbed by your blood. In order to calculate how many times the heart beats per minute, the LEDs flash hundreds of times per second. For an accurate read on a slower heart rate (ie., resting heart rate), the device uses increased LED brightness.
Here’s an example of how photoplethysmography translates to data for users. I went on a hike last year during a trip to San Francisco. The graph below (from my Fitbit) tracks the various peaks in my heart rate during a hike through Muir Woods.
For reference, it’s important to know that you can calculate your maximum heart rate by subtracting your age from 220. For me, that’s:
220-30 = 190
That means that the average maximum number of times that my heart should beat per minute during exercise is 190. Notice the highest peak in the graph. I reached 180 BPM that day! That would explain why I felt like my heart was about to explode.
References:
https://support.apple.com/en-us/HT204666
https://www.ncbi.nlm.nih.gov/pubmed/17322588
https://www.wareable.com/apple/apple-watch-heart-rate-monitor-guide-340
https://www.mayoclinic.org/healthy-lifestyle/fitness/in-depth/exercise-intensity/art-20046887
