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.