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I went for a 4-mile run today and monitored my heart rate much more closely than usual. What I found was surprising. For the first 10-15 minutes of the run, which was gently uphill, my heart rate was only around 88-90 bpm. But, on the slightly downhill trip home (the last 10 min of the run), my heart rate was strangely high – in the mid-160s. Normally, my HR doesn't seem to break 170 even during bouts of intense cardio, such as HIIT on the stationary. Throughout the run, I maintained a pretty steady pace and perceived exertion (not intense, a bit faster than comfortable), averaging 7:45 min/mi.

I first suspected that the heart rate monitor was initially only picking up every other heartbeat, since the initial readings were roughly half the later readings. But I verified many of these readings by checking my pulse manually, so I know they're real.

Also, I've had something similar happen occasionally with other cardio activities. Once I manually took my pulse several times after challenging sets of burpees and was shocked to get languid pulses in the 50s given how hard I felt I was working.

If it's relevant, I am not a runner. I do steady-pace running only infrequently – say, 3-4X/month – although I am otherwise quite active. My resting pulse is around 45 bpm.

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There are times when your heart rate monitor will not measure the correct rate. For example if you chest is too dry or two wet or it is moving around and you loose connection. My Garmin HR monitor will often read my initial heart rate at 225 bpm for the first couple of minutes. I know my HR is not that high. (Actually physically counted it once when it read that amount).

I would try another monitor and compare the two. In addition I would think that 90 bpm is a little low for a run rate, especially on an uphill section.

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But I said I checked the readings against a direct manual measurement (both at my wrist and carotid). – half-pass Aug 26 '14 at 3:24

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