Apple Watch delivers impressive heart rate accuracy

Yesterday I completed a 30min indoor cycle to compare Apple Watch Series 3 and it's built in optical heart rate monitor with a heart rate chest strap connected to a Wahoo bike computer.  Here's how I got on.

Getting Setup

This is my setup:

  • Apple Watch 3 4G/LTE using built in heart rate monitor and Apple Workout app 
  • Wahoo Elemnt Bolt bike computer connected to Wahoo Tickr X heart rate band
  • Wahoo Kickr Turbo
  • The Bolt was connected to the Kickr for Power in ERG mode, as well as my Di2’s (not relevant here) and a Trek DuoTrap for cadence

The session was 5 min warm up in zone 1, then 20 mins zone 2/3 with a few short harder spins towards the end, then another 5 mins cool down.

The Goal

My goal here was to compare the accuracy of the Apple Watch built in optical heart rate monitor to the chest strap.  Even though this is a single scenario test, I have been impressed with HR accuracy so far while using Apple Watch for all the activities so far - even for Pool Swimming!

To provide context and some background, optical monitors determine heart rate by shining a light through the skin and measuring the changing light refracted by the blood flowing through your veins.  This is typically not as accurate as a chest strap and is slower to react to changes. Chest straps  measure the small electrical impulses given off by your heart as the muscles contract which is more accurate provided your sweaty enough to conduct (not ever an issue with me).  By this metric Apple Watch should not be as accurate or responsive as the chest strap.


After the sweating, here’s how the session looks in the Activity app on the iPhone recorded by Apple Watch:


This looks good, but unfortunately this limited display in the Activities app is really only useful for an overview and though you can see a few bumps at the end it’s not easy to see how hard I pushed then and what my HR did.

Happily you can export or visualise this data using a variety of apps (usually at a small purchase or in-app cost) For example HeartGraph produces the following (you need to pay £2.99 to enable the   Premium features) which is better, shows HR zones and you can zoom in and out on screen in the app.  (This app has various other features such as comparing workouts that you may also want to explore for your £2.99)

HeartGraph App with zone banding

HeartGraph App with zone banding

You can also export the workout using RunGap or HealthFit to a format such as FIT or TCX and then import into other tools.

So that is what I did, then got the same FIT file from the Wahoo log and uploaded both into the analytics tools provided by the incomparable DcRainmaker.  Bear in mind that I had issues at the start of this where the original Garmin Strap I was wearing died, and so I replaced with the Tickr X, that meant the recorded start for watch and the Tickr X were at different times.  No worries the DCR Analyzer sorts that out and aligns the logs based on the actual time - just look at 10 mins onwards in the chart - and that is a pretty impressive result from Apple Watch, closely tracing the TickrX strap with a little lag (as expected with an optical HR)

Wow - that's pretty accurate tracing between the two.  Apple Watch is the blue line that starts at about 5 mins.


The more I use Apple Watch the more impressed I am with the built-in HR accuracy.  This test (be it a single test on a turbo) has shown it is a match to a typically more accurate HR strap when using the Workout app.  

At this stage I'm not sure if this is purely related to Apple Watch Series 3 that I used here, or watchOS 4 (I have read some comments suggesting HR accuracy has improved on AW Series 2 with watchOS 4 too, which is good because it was fairly dismal before) but I'm guessing the increased processing power for series 3 has certainly helped.

I find it interesting that Apple appear to be optimising heart rate based on the type of activity - they have a "High Intensity Interval Training" workout for example, that offers no scheduled plan, or additional data, but instead is simple tuning to recording HR accurately for that sort of activity (and I'm guessing calorie calculations too but that is a different story)

Overall I am perfectly happy training with Apple Watch based on Heart Rate accuracy - so far it has been great, and has actually outperformed my Garmin Forerunner 935 in accuracy when I have them next to each other.  As I continue to test, I'll report on any anomalies I find, but so far I'm impressed.