Weird metrics from today’s run 🏃‍♂️

I haven’t been running enough, so I went for a run today. The metrics are kinda funny, by which I mean my Fitbit thought it was an insanely hard run. I thought it was interesting enough to post about.

Fitbit calculates heart rate zones based on your “heart rate reserve,” which is your maximum heart rate minus your resting heart rate.

Your resting heart rate (roughly what you’d get if you checked your heart rate right after you woke up, before you started moving around) the device actually measures. My resting heart rate, according to the Fitbit, was 56 bpm. (It actually hit 44 bpm at some point while I was asleep, but your lowest resting heart rate is a different number.)

Your maximum heart rate, though, isn’t measured. Instead, it’s estimated as 220 minus your age. I’m 66, so that comes to 154. So my heart rate reserve is 154 minus 56 equals 98. Then my various zones are calculate as a fraction of the reserve plus the resting rate. Zone 5 (peak activity) begins at 85%, so my zone 5 begins at (0.85 ✖️ 98) + 56, which comes to 83 + 56 = 139. All the parts of my run shown in red in the map above were run at a heart rate at or above 139.

In fact though, my maximum heart rate is way higher than that estimate. I have in the past been somewhat dubious of the maximum readings shown by my Fitbit during a run, because all the wrist-worn devices sometimes sync up at your foot-strike rate, so you get anomalous readings around 180 (a common foot-strike rate). But I also check my heart rate doing other exercises, such as kettlebell swings, where foot-strike rate doesn’t matter. Plus, I get heart rate readings from my Oura ring, which is not wrist-worn, and which doesn’t seem to have the same syncing-with-foot-strike problem. So I know my max heart rate is much higher.

On this run, for example, the maximum heart rate as measured by my fitbit was 169. My Oura ring thinks the peak was 166 (but it averages over 5-minute periods, which smooths out the peaks quite a bit).

Anyway, if you take 166 as my actual maximum heart rate, then my heart rate reserve is 110, 85% of it comes to 94, so my zone 5 range ought to begin at a heart rate of 150, rather than 139.

I find that a lot more plausible. If the Fitbit is right, then I just spent 36 minutes in zone 5, which seems very unlikely. It was kind of a hard run, because I haven’t been running enough, but I not only could have talked while I was running, I actually did sing, which is one of the markers for being in zone 1. (I was listening to and singing along with some Kpop songs.)

So, I think much of that run, even some of the bits shown in red above, were in zone 2 or 3, not zone 5.

Whatever the heart rate metrics, it was a rather slow, rather short run: 3.15 miles in 58min 10s.

Much better than not running.

Updated next morning: I slept great after my run, and woke up feeling great. Legs not sore at all. Overnight heart rate right back down to my current baseline.