January 20, 2023
I run with a Garmin with an optical heartrate sensor. It gave me fits for the first 2 months of zone training. It showed my heartrate bouncing around more than I thought possible and caused me to run erratically: I’d walk when my heartrate jumped, and speed up when it tanked. I finally joined a gym in December so as to go inside during the worst that fall and winter have to throw at us around Bozeman. On an early treadmill run I gripped the heartrate sensors and I found that the readout showed my heartrate to be quite stable while the heartrate on my watch jumped up and down. I let go of the sensors and slid my watch from my wrist about an inch up my forearm. My heartrate stabilized. When I grabbed the sensors once again, the readout showed that the treadmill measure of my heartrate closely tracked that of my Garmin: they sometimes differed by plus or minus 1, but that is plenty accurate for me. My guess is that all the bones in our wrists make it difficult for the sensor to get an accurate reading.
Since then I have confirmed that this solution generally works well, but is not foolproof. On occasion, the heartrate on my Garmin still jumps for no apparent reason. I have tried taking my watch off and cleaning it on my clothes thinking that maybe a drop of sweat on the sensor is causing the issue. That helps sometimes, but not always. More generally, I have found that after a minute or two the “Excuse me while I lose my mind” moment ends and the heartrate monitor resets itself and produces the output I expect.
There is one situation where the heartrate readout on my watch that genuinely perplexed me for a while. When trail running up mountains I often find myself breathing hard while my heartrate sits comfortably in Zone 1. I can go long stretches, a half mile or longer, without my breathing and heartrate syncing so it’s not just that increases in heartrate lag breathing. In Training Log 5 I talked about the weak correlation between breathing and heartrate. What I experience on trail runs seems to be an extreme case of this.
But I think I’ve figured out why this occurs. When planning to run in Zones 1 or 2, I found that I settle into an easy pace. I’d even say that the pace I settle into feels natural for me. When I’m on a flat track, I tend to run in Zone 2. It’s just the pace that feels effortless. When I’m slogging up hills, be they mountains, or a long hill on a local road, I tend to settle into a Zone 1 pace if I am not pushing myself, if I just let the pace happen naturally. My breathing is hard, but my heartrate stays in Zone 1. Note this only happens if the trail is not too steep. One of the trails here that I run regularly climbs at a rate of about 550 feet per mile for the first 1.4 miles, then it starts climbing at more than 1000 feet per mile. In the first 1.4 miles, I have no problem jogging uphill at an 18-20 minute mile pace in Zone 1. Once I hit the steep section, I switch from a jog to a hike but I start moving into Zones 3 and 4 rather quickly. I’m engaging more muscles, fighting gravity harder, and my heartrate responds.
My heartrate activity on downhills also confused me for a while. I thought that since I was jogging downhill, my heartrate should decrease. But two things happen when going downhill: we engage different muscles, and we naturally pick up our pace. On moderately steep trails, I actually find it easier to maintain Zone 1 on the uphill sections. I do not want to fight gravity on the downhills by constraining my speed, so I naturally go faster and my heartrate increases.
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