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I am doing intervals at 90% max. heart rate 2x per week (trying to replicate this study [PDF]) However, on some days the heart rate does not go up with intensity. I always run up the same hill - on some days the heart rate goes up quickly to 170 and I can keep it there with sub-maximal effort (if I push really hard, it goes pretty quickly to 185). But some days it barely gets to 160 and I cannot push it higher even if the effort feels harder.

At first, I thought that it could be symptom of tiredness or upcoming illness but it does not seem to be the case - sometimes I feel great after a few days of rest, I slept well, looking forward to pushing it hard... and the heart rate just does not go up. I assume that if this was caused by some heart condition, it would be like that all days. But it fluctuates seemingly randomly - one training it goes up easily, the next week it does not (in which case I stop exercising after the first interval and just take a walk). Any idea what could be the cause?

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There are many factors that can effect heart rate response. Including, but not limited to:

  • Tiredness
  • Time of day
  • Hydration level
  • Caffeine intake

This is why in the cycling world the power meter has become the device of choice as it gives much more reliable measurements. I believe foot pods are now available to give similar metrics for running, but I do not know much about them.

To try and rule out fatigue as the cause you could try tracking your resting heart rate (when you wake up in the morning), and if it is elevated above your normal baseline you may need more rest.

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So these are Clinical Concerns with Aerobic Activity:


  • HR doesn’t rise in proportion to exercise intensity
  • When SBP doesn’t rise during exercise or exceeds 200 mmHg
  • ↓in SBP of 20 mmHg during exercise
  • ↑in DBP of 15 mmHg during exercise or if it goes above 110 mmHg
  • Angina: Chest pain SOB, dizzy, fainting, excess sweat, angina, LE pain, cyanotic, change mental status, cough/weeze…

It's likely nothing but to be safe I'd have your primary care doc check it out -- a quick ECG can keep you safe in the off chance it is something you can take care of it now.

IMO I'd see what your doc recommends after talking with you and auscultation.

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  • Thank you, I scheduled a check up. – honzzz yesterday
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Is it possible that your heart rate monitor isn't working properly? Try making sure that the battery is good, the contacts are moist enough (or moisten it with salt water), and that you're wearing it properly. I have an issue with its electrodes drying out so I try to use one that uses LEDs. The arm bands tend to be more accurate. When it's too dry, it can still display a reading but may update too slowly.

If you can't fix it, you can try another heart rate monitor.

  • Thank you, this was the theory worth testing but it does not seem to be the case - I believe now that the heart rate monitor is working accurately. – honzzz Apr 23 at 16:35

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