Friday, March 26, 2021

DFA a1 problems during running - why?

I have previously noted DFA a1 decline associated with poor Movesense ECG waveform and have had multiple "complaints" of DFA a1 reading too low at easy intensity levels during certain forms of exercise in certain people.  The most common example being running. The scenario is usually as follows :

"My DFA a1 drops while running at trivial effort, way below what it should be and much different than with similar heart rates when biking."

My guess was that some type of core/trunk/diaphragm electro mechanical interaction was distorting the ECG waveform, making the peak of the R wave less able to be measured precisely. Since DFA a1 is an index of correlation properties and fractal patterns, a loss of true R peak timing will result in an artificial reduction in the complexity of these patterns.  Kind of like a dirty fingerprint on your camera lens - a loss of detail occurs and the picture is not going to be sharp. But perhaps the answer is from a different type of interference.

Why would running give us a distorted R peak?  Here is a clever and informative view of  chest wall motion during running:

As you can see there is quite a bit of motion and sensor bounce going on.  Whether this is part of the issue is unclear, but it does make one wonder.  In addition, the volume is rapidly changing, which could lead to variation in the ECG signal.


The question is what happens if we reduce the "pounding"?  Would the DFA a1 measurement normalize to where it should?

Thanks to a blog reader who kindly did some tests, we have some confirmation that this might be true.

He did a series of intensity ramps under three conditions, all using the same HRM (H10)

  • Bike
  • Stepper/elliptical
  • Running on a treadmill

What I have done is simply plotted the heart rate and DFA a1 over the course of the intensity ranges.  Normally, if your DFA a1 is in a .75 range cycling at a HR of 150 for example, we would expect similar behavior while running with the same HR.

  • The Run test in red clearly shows the early DFA a1 drop at low HR.
  • The green (stepper) and black (cycle) show similar and more realistic DFA a1 behavior.
  • The stepper/elliptical machine is using similar motor kinetics as the run, just without the impact.


His HRVT on the bike:

  •  The HRVT HR is about 146 bpm - a very reasonable figure with good R correlation


  • Early DFA a1 decline when running with a typical HRM chest belt occurs is a subset of individuals.  However, most do not have this issue. 
  • It is not an issue on the bike.
  • In this case, simply decreasing the ground force impact totally eliminated the effect.
  • It seems to be due to some sort of electromechanical interference related to impact.  Since there is no significant impact on the bike or elliptical stepper, the effect on DFA a1 does not occur.
  • Can this be compensated for?  I can't say, but the HRM designers and engineers may be able to adjust their algorithms to eliminate some of this.  The first step (no pun intended) is to confirm that the phenomena exists (done), why it happens (maybe) then hopefully find the solution.
  • In the meantime, trust the bike results but an alternate solution is using an elliptical trainer, stepper, staircase as workarounds.


  1. Bruce, do you think that a forehead sensor, like the Moov sweat, would suffer the same problem ?

    1. Unfortunately the optical HRMs just don't have the RR accuracy for a1 - but yes, that would be a good thing to have tried otherwise.

    2. Hello Bruce,
      I did everything I could, I took many test, varying the ramp, the power, the speed and the grade. In cycling I get consistent results, I can clearly see DFA declining and going up, with Kubios, and I know now where is my LT1.
      In running it is impossible for me: I also did a couple of brick workout, bike+run: LT1 at 132-135bpm, but when I go on the treadmill, at 121 my DFA is already at 0.60.
      You identified the issue, now: do you think that Exoskin should suffer the same problem when running ? I was thinking of buying one, if there are no other sensor that could help me.
      Or my I trust the bike LT1 and assume it is (more or less) the same for running ?

      thank you very much for all your work

    3. You certainly have done a good job with this. I would trust the bike data for running, it should not be much different. The hexoskin actually doesn't do the best at a1 either with it's lower sample rate. So just go with your bike threshold for running and future progress in your fitness.

  2. I've been puzzling over this exact issue for over 2 weeks. At first, I thought it was due to running injury, but I could never explain why my cycling VT1 HR was so much higher than my running VT1 HR using DFA. This makes so much more sense. Thank you!