Treadmill - low heart rate

I have a Garmin Footpod which has been calibrated via outside running. It tracks reasonably well with the displayed speed on the treadmill (+/- 0.4 kph). And I would also add that the RPE feels about right for the speed being reported.

However, my HR seems low compared to the same run speed outside. For example 4:45/k will be 126bpm on the treadmill (1% grade) and 133bpm outside (flat/windless/tarmac). Does this seem right?

I’m struggling to reconcile the ‘other’ factors that could be at play. Hills, weather, soft ground, etc.

And if this HR variance is right then the next question is, should I be running quicker indoors to compensate?

My experience has been that treadmill running always seems a bit easier at any given speed, both RPE and HR. Figured it was some mix of better cooling/airflow, a completely consistent running surface and gradient, poorly calibrated treadmills, or just the fact that I’m not actually having to move my weight. I set the gradient to 1% as well, but always feel have that feeling that it’s possible to “cheat” the treadmill a bit compared to running outdoors.

And yes, I tend to just bump the speed up a bit to compensate. I rely on HR a lot more for running than I do for cycling anyway, since the efforts are much steadier and you don’t get the power spikes that you do with cycling. And I only really run on treadmills when I’m travelling (which is sometimes quite a lot) so there’s a lot of equipment variation. So makes sense to go by HR more than anything else.

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Unless you have many runs’ worth of data and it is consistently lower I would say that 7bpm difference is negligible. Heart rate varies wildly day to day, and any number of small things can cause a 7bpm swing. My guess is your brain is more stimulated outside.

If it really is significantly lower over many data sets and you’re training by heart rate, bump your treadmill speed a bit I guess. I wouldn’t over-think it though. RPE is your best tool to compare inside and outside run speed.

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I would say I have 20-30 runs on the treadmill to compare with 100s of comparable runs outside.

Whilst most runs outside that I am talking about are ‘pretty flat’ they do have some small rollers. I wonder if that is more of a factor than anything.