Heart rate monitor test, Pt 2

In yesterday's post I compared readings from the wrist-based Fitbit Surge and a Garmin chest-strap heart rate monitor connected to a motoACTV. The activity was a base mileage run, with a pretty steady effort that should have stayed around 155bpm, and mostly did according to both devices. Though the two weren't quite identical, the differences seemed more random than systematically biased toward one being always higher or lower than the other. There was one point in that activity, coming up a hill near the end of the run, where the Garmin read above 170, but the Fitbit stayed around 155bpm. That was potentially nothing, but also potentially an early warning that Fitbit would either be slower to recognize sharp elevations in my heart rate or wouldn't recognize them at all.

Today I repeated the experiment during a more intense workout: one mile warm-up, then two by two miles at about race pace with four minutes of rest in between, and a mile cool-down. 

The results suggest that the late hill yesterday was an early warning:

The readings are drastically different during both of the intense intervals (the taller plateaus in the middle). Not only are the Fitbit readings lower than the Garmin, they're even lower than the readings from the warm-up and cool-down. It's as if the more intense intervals completely discombobulated the wrist monitor. 

Trusting the Garmin readings as the truth, since they make more sense, HR went up to about 160-165bpm almost immediately during the intervals and then gradually inched up to about 170 over the two miles. But starting from the recovery low of about 120bpm, Fitbit takes about a quarter of a mile to get up to 140 during the first interval and almost the full two miles to get there on the second interval. And then never tops about 145. That's a difference of at least -40bpm during the first mile of the second interval, and never less than about -30.

Interestingly, the two devices are almost identical during the recovery from the second interval and the cool-down mile. There's a weird spike in the Fitbit during the walk to my car after the cool-down, but otherwise they were in sync there. 

The differences during the workout are massive and potentially (though not definitively) significant. I'd say this goes beyond "discrepancy" and into the territory of full-blown malfunction during highest-intensity exercise. But I'd say that only matters if you're interested in actively managing your heart rate during exercise, such as trying to hit a target zone or prevent over-exertion. That's it. In terms of calorie measurement, considering the short length of time you're likely to be in this peak HR zone, the readings won't make much of a difference for total daily caloric burn---for this workout it was a difference of only 20 calories. Considering the advantages I see in having a (slightly) lighter device and not needing to wear a chest strap, the potential to get an extra 20 calories of "credit" for the day isn't worth it. Plus, those 20 extra calories captured by a chest strap aren't going to be part of a daily total because no one is wearing a chest strap all day; anyone doing daily totals is already relying on the wrist watch. And to you, I say don't worry about the difference in readings. But again, if you have heart issues or are otherwise interested in fine-tuing your exertion during workouts, Fitbit is probably not reliable enough and you should wear a chest strap.

The rest of my training week is easy runs. Saturday I'll do a long run, and unless I do it super early it'll be a hot one. I'll do the same experiment for those runs but won't update this again until Sunday night.

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.