I've been tracking my heart rate while running and have noticed that, every single run, my heart beat has a small, but noticeable, drop between 1:50 and 2:10 minutes into the run (after stretching and a 18minute fast walk warmup). This is on a treadmill at a constant pace, so it isn't me slowing down without noticing. I would have thought it was just a random fluctuation, but again this happens every time. What could cause this?enter image description here

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    My first thought is simply a recording error in the device. Especially since it immediately shoots up. Does the pattern look similar in the other recordings?
    – DeeV
    Jul 30, 2020 at 2:52
  • 4
    I think it would make more sense to try recording with a different monitor before looking for a biological reason for this. The left and right side of the anomaly match up pretty nicely, so I think any educated guess would be an error with the HR monitor.
    – Alec
    Jul 30, 2020 at 10:47
  • @DeeV and Alec , I thought when I first noticed it it was probably a recording error, but it seems odd that it happens at the same time everytime, is there anything that would cause that? The pattern looks similar in other recordings, but there is a fair bit of variance in how large the effect is, I've added two other recordings. I unfortunately don't have access to another monitor at the moment. Jul 30, 2020 at 12:02
  • It's an interesting thing, for sure. I'll be curious to see if anyone has any worthwhile thoughts on it.
    – Alec
    Jul 30, 2020 at 16:17
  • What is the timer on your display? Does it go to standby after 2 minutes?
    – Karlokick
    Aug 18, 2020 at 14:00

1 Answer 1


I would suggest to setup hypotheses and test them systematically.


Do you shift from walking to running very abrupt? Perhaps you should consider it as a spike just before the drop rather than just a drop.

Reason: Your body accommodates to a higher load than needed as a reaction to fast change in load.

How to test: If you walk say at 6km/h then very slow increase the speed to 8km/h. Then start to run at that speed, and slowly increase to the final running speed.

Alternative reason: You feel stressed when you know you have to run in the next 30 minutes.

How to test: Promise yourself to stop running after 30s.


Do you changes running style or step length as a response to increased speed?

Reason: At a certain level you shift to a more anaerobic running style reducing the need for oxygen.

How to test: Make a video of yourself running in the treadmill and compare to heart rate data.


Do you alter your respiration at a certain point? E.g. to make it fit your running rhythm.

How to test: Control your respiration and try to keep it calm the first minutes running.

  • 1
    Perhaps also try a shorter or longer warmup (it's long enough that you could trim 2 minutes off for testing): Is the glitch 2 minutes after starting to run, or 20 minutes after starting the routine? Also (similar to point 1) try a fast walk warmup (say 16 minutes), step up to a light jog for 4 (for example) minutes, then up to the run. You should see 2 steps in the HR. At which does the glitch show (or both)
    – Chris H
    Aug 9, 2022 at 11:05

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