Sunday, December 22, 2019

DFA a1 and Zone 1 limits - the effect of Kubios artifact correction

In several prior posts, the use of the non linear HRV index, DFA a1 was discussed as a marker of zone 1 to zone 2 transition.  This of course coincides with the VT1/LT1, however since both parameters can be subject to error, an accurate assessment of DFA a1 may be an even better training zone marker.  Also in a prior post, a warning was given about the effects of correcting artifacts in the RR data by the Kubios software.  Although the literature review was sound, the window length I used for deriving DFA a1 was not optimal.  The recommended window of measurement for this index should be 2 minutes.  I decided to do another set of comparisons using the proper window time (2 min).  To simplify the issue only 2 cycling power intervals were done, VT1 power and 15-20 watts higher.  In a perfect scenario, the DFA a1 would be about .7 at VT1 power and suppress close to .5 (white noise) with just another 15 or 20 watts added.  

The question is: Does the presence of even minimal "acceptable" artifacts create an erroneous DFA a1 result that would yield the wrong zone 1-2 demarcation?  I am using the latest version of Kubios premium and extracted the DFA a1 results computed every 20 seconds of the past 2 minutes of each activity.  



The first test was done on an indoor trainer with very steady power and little body motion.  The second was done outside on a smooth road.  In both cases, the Polar H10 belt was worn with the Hexoskin "Pro" shirt and plenty of conductive gel was used.

Here is a listing of the first test indoors with artifacts, DFA a1 values for the Polar H10 using both medium and autocorrection.  The free version will only use the medium correction method.


Which graphs out as such.
The first half is at 168 watts and the second is at 186watts:

  • The big issue is the shift to higher DFA a1 complexity with artifact correction.  This occurs with either Auto or Threshold correction methods in Kubios.  I listed the raw data so you can see for yourself.
  • The blue dots are the zero artifact values which stay remarkably stable in the .7 to .8 range at the VT1 power, with prompt suppression to .5 (white noise) just 20 watts higher. 
  • The red dots are the Polar derived reading with low artifacts.  Although there is a change in DFA from low to higher power, it is substantially muted and well above the .7 cutoff at all times.
  • The Hexoskin had zero artifacts, so no correction is needed.
  • If I was attempting to stay in zone 1, or was testing myself for the DFA a1 transition point of .7, the Polar (with artifact correction) would have misrepresented the true values.
  • In my opinion this is extremely important to know.  Inappropriate (high) exercise efforts will occur if one is relying on the DFA a1 to delineate zone 1 to 2 transition.  Reproducing these findings across sessions and individuals will not be reliable either.

How does outdoor look?
Six minute intervals were done at 170 then 190 watts on a smooth road.  This was an hour after the indoor session.
Raw data:


With the DFA a1 comparison below:
Blue dots are the artifact free Hexoskin DFA a1 values, red from the Polar with Auto artifact correction:

  • Again we see that the DFA a1 is shifted higher in the face of artifacts with Kubios correction.  This occurs with either type of correction method.
  • In the artifact free tracing, there is a sharp transition from correlated to uncorrelated (white noise) in the DFA a1 tracing with just a 20 watt increment of power (blue circle).  This does not occur in the artifact corrected tracing (red circle).

Summary:
  • Both of these examples serve as a warning.  Given the findings here, the current artifact correction methods of Kubios will increase the DFA a1 complexity to enough of a degree to misrepresent zone 1 transition and VT1.
  • I am not implying that the Polar H10 is artifact prone.  However in my situation, I rarely get clean tracings.  On the other hand, the Hexoskin is generally near artifact free, even at heart rates near 180 bpm.  
  • If one is relying on getting an accurate DFA a1 during dynamic exercise it seems essential to avoid artifacts at this time (and not rely on correction).  
  • Perhaps in the future, Kubios or other HRV software will be able to correct RR artifacts without substantial effects on DFA a1. 
  • Finally, the use of Kubios artifact correction can lead to situations where both a given individual's findings or research investigation into DFA a1 will yield erroneous results.

Part 2 - HRV artifact effects on DFA a1 using alternate software 

DFA a1 vs intensity metrics via ramp vs constant power intervals  

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