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I've noticed a recurring pattern when I do sustained hard runs in which my heart rate increases in an expected, gradual way during a hard bout (e.g., running uphill for an hour or doing a 30-min set at my tempo pace), but then starts to increase much faster after the hard bout is over (e.g., running back downhill or returning to a moderate pace after the tempo set). Then my heart rate stays much higher for the rest of the workout, even though the hard bout is long gone.

For example, on a 2-hour trail run that is initially uphill, I might average 145-150 bpm on the way up at a fairly hard effort, but then average 170 bpm on the way back down. Or, here's an interesting heart rate graph I saw during a tempo run:

enter image description here

The red line is my heart rate. (Yes, the axis scaling is ridiculous, and I don't know how to change it.) You can see how, from about the 2-mile to the 6-mile mark, I had entered a main set of tempo running, and my heart rate steadily climbed from 143 to 157. After that, I went down to a moderately easy run, yet my heart rate quickly climbed to 193! (The fluctuations at the end are because I was alternating 5 min of easy running with 5 min of tempo-pace running.) And interestingly, those last very short sets of tempo running also felt really hard -- definitely harder than they would have felt if I had just extended the main set by 10 min.

Interestingly, I do not see this heart rate pattern when I do much shorter work bouts (e.g., V02 max intervals of ~3 min in length). Then, my heart rate goes up during the work sets (to, e.g., 155-160ish -- not to 190!) and then down during the recoveries. Also, if I do the same trail run but take the uphill at an easier pace (e.g., heart rate 135), then my heart rate goes down, as expected, when I start heading downhill again.

What would explain this pattern of delayed heart rate increase after a hard bout?

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Jason is correct in that the spikes into the 190's are probably artifacts and errors due to poor readings from the monitor.

However, also on longer efforts, you may be seeing the effects of cardiac drift. As you exercise, your body needs more blood to bring oxygen to the muscles and take away waste material. The heart naturally starts working harder to deliver this.

Over time, the stroke volume (amount of blood pumped in a minute) starts decreasing, so your heart naturally increases heart rate to compensate. Temperature can also affect this as your body temperature rises. This slow, steady increase in heart rate over longer workouts is cardiac drift. Since you mentioned this was towards the end of a longer 2 hour session, that may also account for the high spikes that you mentioned.

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The heart rate monitor is losing track of your heart rate. I suggest to look at the operating instructions to try and help it function correctly, and check the battery since it may be running low or sweat might be getting into it. Also suggest to look into other devices. I use a chest strap to get a good consistent result. Sometimes I see something like this when chest strap battery is dying and the wrist based monitor takes over on my running watch. Hope this helps.

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