One of the questions I get asked the most is: "How accurate is the heart rate tracking during a Challenge 23 workout?" It's a fair question. If you're pushing through a session and making decisions based on your heart rate zones, you want to know the numbers mean something.
So I decided to put it to the test. Apple Watch versus the Polar H9 chest strap — side by side, same workout, same exact start time. No guessing, just data.
The Setup
I grabbed a second phone, paired the Polar H9 to it, and loaded up Challenge 23 on both devices. One running off the Apple Watch, one pulling data from the H9. I hit start on both at the exact same moment. The plan was simple: run the full workout on both and compare the heart rate data second by second. The H9 is widely considered one of the most accurate consumer heart rate monitors out there, so I was genuinely curious to see how the Apple Watch stacked up.
What Actually Happened
The first work session looked great. Both devices were tracking, data was flowing, and I was already getting excited about the comparison. Then came the first rest period. And somewhere during that break, the Polar H9 quietly disconnected.
No alert. No warning. It just stopped recording.
By the time I finished the full workout, I had a complete dataset from the Apple Watch and exactly one interval from the H9. The rest was just... gone. No overlap, no usable comparison data. Hours of planning and setup for a single data point.
What This Means Going Forward
I'm not writing off the H9 based on one test. Bluetooth disconnects happen — it could've been interference from running two phones side by side, a strap contact issue during the rest period when sweat dries, or just a random hiccup. I'll run this experiment again under the same conditions.
But here's the thing: if it happens twice, that's a pattern. And I won't recommend a device to our users that drops out mid-workout. The whole point of using a chest strap is to get better data. If the connection isn't rock solid, that advantage disappears.
What's Next
I'm going to run the comparison again soon. This time I'll make sure the H9 firmware is fully updated, wet the strap electrodes before starting (Polar recommends this), and keep the two phones a bit further apart to minimize any Bluetooth crosstalk. If the H9 holds its connection for the full workout, we'll finally have the data I was after — and potentially a great recommendation for users who want that extra accuracy.
If it disconnects again? Well, then we'll have a different kind of answer. And that's just as valuable.
Either way, I'll share the results here. Building Challenge 23 means being transparent about what works, what doesn't, and what we're still figuring out. This is one of those "still figuring out" moments.
Stay tuned.
Founder, Challenge 23