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Tuesday, March 3, 2020

Recent article on DFA a1 and Zone 1 demarcation

Over the past year, several posts have been presented looking at the relationship of certain HRV parameters and the first ventilatory threshold.  I decided to take a closer look at the usage of DFA a1 as a zone 1 marker for exercise prescription and write up it up as a case report .  As part of the investigation, incremental exercise ramps were done with and without beta blocker therapy.  The use of beta blockade allows us to change the stroke volume x heart rate proportions, without affecting the end result, the cardiac output.  One of my questions was whether absolute heart rate was the prime driver of DFA a1 change or was it cycling power/VO2.
The investigation used many of the devices discussed over the past years including muscle O2 as an index of A-V O2 extraction, Hexoskin for artifact free HRV recording as well as ventilation rates. 
Here is the link to full text and the abstract is below:

The introduction and discussion will be instructive from the viewpoint of the various pitfalls of LT1/VT1 determination for zone 1 limitation.  The use of beta blockade was quite interesting to explore.  For any of the readers out there who are athletes on beta blocker therapy, this should reassure you that no major impairment of endurance performance should occur.

In addition to the data presented in the above paper, I would like to add another piece of the puzzle.  Although I did 10 minute intervals in the published study, looking for stability of the DFA a1, longer cycling sessions near VT1 were not done. To address what happens with prolonged cycling just below VT1 for about 80 minutes of constant power cycling was done.

Below is a trace of DFA a1 over about 80 minutes, recorded with the Hexoskin (no artifacts), with DFA a1 readings every 7 minutes or so (raw data to the left).  The power was fixed at 8 watts below VT1 by the Elite Suito trainer.

  • What this is showing is that as long as power is below VT1 (by 8w - clamped by the trainer), DFA a1 stays above .6 (orange).  Even though there is some minimal cardiac drift in heart rate past the 40 minute mark (blue), DFA a1 remains quite stable at .6 to .7.  
  • For my purposes, this is comforting to see.  Power just below my VT1 does not lead to demonstrable suppression of DFA a1 despite continuous effort for long duration.  Since the goal is to spend large amounts of time in zone 1, undue neuro-vascular stress is undesirable.
  • Further testing with slightly higher power would also be interesting, looking for progressive decline, for example at 10 watts above VT1.  

Heart rate variability during dynamic exercise

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