Heart rate monitor test, Pt 1

Since last week's news of a class action lawsuit against Fitbit for inaccuracies in its heart rate monitoring, I pulled out my old Garmin chest strap for the first time in about eight months to do a comparison. I'd gotten a Fitbit Surge (as a gift; thanks Anne!) back in November. With its wrist-based monitoring, I effectively retired the much less comfortable chest strap, so it took some digging around to find where I'd put it.

I've become pretty invested in the constant monitoring data the Fitbit offers, even spending time trying to deduce whether my slowing resting HR was the result of my heart strengthening as I increased my mileage throughout the spring, or part of my metabolism's effort to keep or regain the pounds I was shedding as a result. If the monitor's inaccurate, it calls all of that into question. 

As I've learned more about the suit, I'm less worried about the resting HR accuracy since the claims are focused on discrepancies between Fitbit and ECG reading during moderate-to-high-intensity exercise. I haven't read beyond what's in the popular press, but I wonder if there's a constant error percentage, and the larger differences in terms of bpm during exercise are simply the result of the larger multiplier provided by elevated heart rates. And using my HR as a check against overexertion, especially as we get into warmer temperatures, is probably a much more important concern than whether the resting HR data supports a new pet theory. 

So, as a first test, I ran five miles at a normal effort for my non-workout days with both the Fitbit Surge and the Garmin chest strap linked to my motoACTV (maybe still the best fitness watch out there, and it's now six years old). I also let each record a walking cool-down of about half a mile to capture how they measured my HR as it fell back down to normal. Downloading the GPX file from Fitbit and the TCX file from motoACTV, I linked up the times and associated HR readings, added identifiers for which device the reading came from, and then sorted them all on time. Graphing HR against time, but identifier variable gives this comparison:

There are differences throughout the activity, but not a consistent pattern in terms of one being either higher or lower than the other. The Garmin reading has more drastic swings, so there's a chance the Fitbit is slower to recognize changes in HR. I've definitely noticed that it's slower in recognizing changes in live pace readings during runs, so it's possible. 

In the first quarter of the run, Fitbit is consistently higher than Garmin, usually by around 10-15bpm. For most of the rest of the run they stay pretty close. There is one point where Garmin starts registering a HR over 170, and the Fitbit is still around 155. That's worth flagging because a 175bpm reading is around the point where I take the cue to start consciously backing off; I would have missed that cue with only Fitbit. 

On the recovery, it's the opposite of what I would expect based on the rest of the chart--Fitbit seems to recognize my steady "walking HR" of about 115 before the Garmin does and is 15bpm slower for that first part of my cooldown.

Nothing really stands out to me here overall. I don't consider any of the differences large enough to be concerned, but I may also be predisposed to simply not worrying about it. The spike Garmin caught that Fitbit didn't might be something to watch for, though.

Tomorrow I'll run the same test during a more intense workout. It's a 2x2mile with a rest in between, so I'll be pushing my HR higher on both intervals and have a warmup-workout-rest-workout-recovery cycle to check out. 

If you use a Fitbit to track your HR during exercise, and especially if you've noticed a difference going from another monitor to Fitbit, let me know about your experience.