Saturday, January 2, 2021

DFA a1 and exercise intensity FAQ

With the recent interest in using the DFA a1 HRV index to determine aerobic thresholds, follow exercise intensity and the use of real time DFA a1 output from HRV Logger, I've decided to put together a "frequently asked questions" list.  This is by no means totally inclusive and will be updated on a regular basis as new questions (and answers) come up.  Here we go...

I have an hour to listen to something but can't read the blog right now.

 

What is DFA a1?

  • Simply put, it's an index of heart rate beat to beat, fractal related self similarity.  Although your heart rate may be 60 bpm, the beats are not occurring exactly every 1.000 seconds.  The pattern of self similarity changes as exercise intensity rises, from values well above 1, moving down to .75 near the aerobic threshold and dropping even further above this exercise intensity.  See the articles below for details.
Can it be used to determine the aerobic threshold?
  • Yes.  In fact we have several articles describing this process.
  • https://www.frontiersin.org/articles/10.3389/fphys.2020.550572/full
  • https://www.frontiersin.org/articles/10.3389/fphys.2020.596567/full
  • https://www.mdpi.com/1424-8220/21/3/821 
  • https://blogs.bmj.com/bjsm/2021/02/13/from-laboratory-to-roadside-real-time-assessment-and-monitoring-of-the-aerobic-threshold-in-endurance-typed-sports/
  • Below is a YouTube video I did for a conference going over the advantages of DFA a1 over other indexes
 
 
 

 

How accurate is it?

  • Before answering, we need to think about how accurate the comparison "gold standards" methods are.  As discussed in the articles above, there are real issues in both lactate and gas exchange tests, making them subject to various errors and inconsistencies.  Some gas exchange results are so confusing that they are not interpretable.  Machine based gas exchange results are not always accurate.  From the limited study data so far, it seems the DFA a1 is a reasonable surrogate for the AT.  Below is the Bland Altman analysis and regression plot from our validation study:


 
  • As you can see, some folks had more or less agreement with the gas exchange AT, but for the most part the differences were small (several bpm)

What sports can this be applied to?

  • This is a very valid question.  So far, only running and cycling have been well explored.  Other activities such as those using upper and lower extremities (xc skiing, kayaking, rowing) may not follow the same relationship with the AT. 

What can affect the numbers I get?

  • A very wide range of factors.  Stress, caffeine, caffeine withdrawal, food, fasting and over-training are some of the factors before we even process the data.  Preprocessing algorithms, software settings are also critical.  Kubios may give different results from a python based method. We will need to do formal comparison testing between Kubios and python methods eventually.

Do I need clean, artifact free data?

  • A very important item that will affect the DFA a1 is artifact in the RR series.  Missed beat artifact is the most common, and if above 3% could, but if above 6% will affect the values you get.  A single APC may also dramatically drop the DFA a1 for that window of measurement.  Correction methods help with this but are now perfect.  One of the strengths of our Frontiers study was that we used ECG data with almost no artifact.  YMMV using a chest belt with artifact.
  • We recently had an article accepted at the journal "Sensors".  Below is the abstract from that study:
 
Recent study points to the value of a non-linear heart rate variability (HRV) biomarker using detrended fluctuation analysis (DFA a1) for aerobic threshold determination (HRVT). Significance of recording artefact, correction methods and device bias on DFA a1 during exercise and HRVT is unclear. Gas exchange and HRV data were obtained from 17 participants during an incremental treadmill run using both ECG and Polar H7 as recording devices. First, artefacts were randomly placed in the ECG time series to equal 1, 3 and 6% missed beats with correction by Kubios software’s automatic and medium threshold method. Based on linear regression, Bland Altman analysis and Wilcoxon paired testing, there was bias present with increasing artefact quantity. Regardless of artefact correction method, 1 to 3% missed beat artefact introduced small but discernible bias in raw DFA a1 measurements. At 6% artefact using medium correction, proportional bias was found (maximum 19%). Despite this bias, the mean HRVT determination was within 1 bpm across all artefact levels and correction modalities. Second, the HRVT ascertained from synchronous ECG vs. Polar H7 recordings did show an average bias of minus 4 bpm. Polar H7 results suggest that device related bias is possible but in the reverse direction as artefact related bias.

 

So what does this mean on a practical basis?  Anything with >6% artifact in the area of interest should not be trusted.  Since both the Kubios threshold correction method and HRV logger use similar techniques, 3% or less artifact containing data will provide reasonable HRVT accuracy.  There is also a chance that 3-5% artifact containing data series will be fine, but you may want to re test yourself.  The effect of missed beat artifact on DFA a1 is to artificially raise the computed value at low DFA a1 ranges (not high ranges).  For example, if the DFA a1 was .5 with no artifact, after adding 6% missed beats with correction (by Kubios), the software will output .65 +-.  

Here is a look at how that works out on a Bland Altman assessment


The solid line is the "average" difference between methods, notice how this process is dependent on what DFA a1 actually is.  There is minimal "bias" between DFA a1 of 1 and .5 which is important for the HRVT.  However, values below .5 are very much altered.

  • Also see below under recording devices.

What artifact correction settings do you recommend?

  • If you are using the Kubios paid premium version, use the "auto" method.  Free version Kubios uses the threshold method (similar to HRV logger).  The medium correction setting is the default and should work well (similar to the 20% setting in Logger).  The exception is with an APC where a sudden drop is seen.  Using the extra strong filter setting (or the "work out mode" in Logger) will filter out the APC but can also filter some physiologic beat to beat variation.  Get a feel if you exhibit frequent APC activity, and if so, use the more aggressive settings.

Does recording device matter?

  • This is something else we are looking at.  The above validation study was done with a research grade ECG.  It is very possible that a chest belt device will detect R peaks differently as well as be affected by preprocessing issues.  Interference with either chest wall or diaphragm related activity can change the ECG waveform.  Disturbance of the pattern of self similarity would then occur after the introduction of this type of distortion.  However, the Polar H10 results appear very close to accurate waveform ECG derived values.
  • In the Sensors study, we found that the Polar H7 "measures" DFA a1 as slightly lower values.  This is in the opposite direction as what missed beat correction induces, which is actually quite convenient!  The end result of a Polar H7 recording with 3-5% missed beat correction may yield values that are very close to those of an ECG.  Below is a figure from our article that shows this very nicely.  The Polar reads lower than the ECG, but the 6% artifact recording reads high - making for a "self correcting" effect.  If you had a Polar RR series with no artifact, yes, you might have some bias.  We are continuing to look into this.

    Time-varying analysis (window width: 120s, grid interval: 5s), DFA a1 for matched time series containing no artefact in one representative participant, ECG (solid triangle), Polar H7 (open circle), ECG 6% MC (open triangle).

     

I have the option of recording HRV either using ANT+ or bluetooth - is there a difference?

  • Although I initially couldn't believe this should make a difference, it apparently does!  Here is an experiment I just ran.  The first tracing shows a recording using a Polar H10 to my Garmin watch using Ant+.  The second is a recording of the same power/duration/conditions on another day using bluetooth (same H10, same watch).


  • There are clear differences - the bluetooth tracing has zero missed beats (one APC noted), but the ANT+ recording has many (the vertical lines).  What strikes me as interesting is that the ANT+ artifacts do increase with motion and intensity, which argues against a simple signal transmission issue.  
  • Bottom line - if you are seeing many artifacts using ANT+, try switching to bluetooth.  Thanks to Marco Altini for the initial anecdotal observation.

If I shouldn't use ANT+ then how can I get the RR data to both my Garmin watch/head-unit and HRV logger during an exercise session?

  • The Polar H10 has a nice feature that enables two different devices to simultaneously receive RR packets over bluetooth.  It is not enabled by default so you will need to do so.  The instructions are here.
  • Once enabled you can have your Garmin watch and the HRV logger (or other bluetooth device) receive data at the same time.  But remember, other nearby receivers may be able to pick up your data and see your stats. This applies to ANT+ as well.  Using the Polar Beats app, you can turn off multi device bluetooth and/or ANT+ at will. 
  • If the H10 is already added as an Ant device in the Garmi unit we need to get rid of it - first - delete the Ant device from "Sensors", Go to - add new external HR, but don't add the Ant, the Garmin will then ask to search bluetooth, say yes and add the bluetooth HRM.

How do I set up an aerobic threshold test scenario with HRV Logger?

  • I've devoted many posts on doing this in Kubios but lets look at a simple method in HRV Logger. Warm up 15 to 20 minutes then do 6 minute constant load efforts.  Make sure you line up the first effort with an even time number in the Logger.  Since the Logger spits out a value every 2 minutes, it's helpful to have the time under effort match up.  As an example, start the Logger, warm up 20 minutes then at 20 min exactly (on the Logger), start your first interval at a very easy level.  Throw out the first value after the interval start (it's not at steady state yet) but the values at 24 and 26 minutes will be valid.  At 26 minutes, boost your power or speed by a notch (still easy) and measure at 30 and 32 minutes (remember that 28 was non steady state).  Keep this progression up until you pass through .7 to .8 DFA a1.  That would have been your AT related intensity.  Do one more effort at the next stage higher to confirm the DFA a1 is indeed below .7. 

How do I reproduce your published study protocol?

  • Here it is:
  • The following procedure was used to indicate at what level of running intensity (as VO2 or HR) the DFA a1 would cross a value of .75: DFA a1 was calculated from the incremental exercise test RR series using 2 minute time windows with a recalculation every 5 seconds throughout the test. Two minute time windowing was chosen based on the reasoning of Chen et al. (2002). The rolling time window measurement was used to better delineate rapid changes in the DFA a1 index over the course of the test. Each DFA a1 value is based on the RR series 1 minute pre and 1 minute post the designated time stamp. For example, at a time of 10 minutes into the testing, the DFA a1 is calculated from the 2 minute window starting from minute 9 and ending at minute 11 and labeled as the DFA a1 at 10 minutes. Based on a rolling time recalculation every 5 seconds, the next data point would occur at 10:05 minutes (start 9:05 minutes and end 11:05 minutes).
    Plotting of DFA a1 vs time was then performed. Inspection of the DFA a1 relationship with time generally showed a reverse sigmoidal curve with a stable area above 1.0 at low work rates, a rapid, near linear drop reaching below .5 at higher intensity, then flattening without major change. A linear regression was done on the subset of data consisting of the rapid near linear decline from values near 1.0 (correlated) to approximately .5 (uncorrelated). The time of DFA a1 reaching .75 was calculated based on the linear regression equation from that straight section (Figure 1b). The time of DFA a1 reaching .75 was then converted to VO2 using the VO2 vs time relation, resulting in the VO2 at which DFA a1 equaled .75 (HRVT). A similar analysis was done for the HR reached at a DFA a1 of .75. First, ECG data from each 2 minute rolling window was used to plot the average HR and DFA a1. The HR at which DFA a1 equaled .75 was found using the same technique as above, a linear regression through the rapid change section of DFA a1 values of 1.0 to below .5, with a subsequent equation for HR and DFA a1 (Figure 1c). Using a fixed variable of DFA a1 equals .75, the resulting HR was obtained. The HR at DFA a1 .75 (based on ECG data) was then compared to the HR at VT1 GAS obtained from the metabolic cart data (based on the Polar H7).
  • Note should be made of the difference between Kubios "time varying" timestamps and the Logger.  In Kubios, a given time varying window timestamp is centered in the middle of the window (bold print above) whereas in the Logger the timestamp is at the end of the window.

How do I make sure I'm really doing a recovery ride on my rest day?

  • This is an ideal scenario for the Logger in real time.  Just watch the live read out and keep DFA a1 above .8 or even .9 (yes .75 is the cutoff, but there is individual variation and a small buffer is advised).  A single value that falls below .75 then normalizes where it started again was probably due to an APC.

Are my values going to be the same day to day?

  • Probably not.  Although they may be close, it's normal and expected to have some shifting in heart rate or power on a day to day basis.  This would be the case with gas exchange or lactate as well.  As stated above, other factors will change the index result, especially heat, skin temp and humidity.

Can the intensity of exercise where DFA a1 = .75 be used as a way of tracking fitness changes after training?

  • There is nothing published on this as of yet.  It is something we are currently looking at and I will update this when I am able.

I'm on beta blocker therapy, will this change the DFA a1 to intensity relation?


How do I match up timestamps in Kubios and HRV Logger?

The time stamping is tricky.
The times are all different in each "method".
Logger - time is at the end of a 2 min window - so a timestamp of 2 minutes is from 0 to 2 min elapsed.
Kubios free - the time is from the beginning of a 2 min window - so a timestamp of 2 min is from 2 min to 4 min elapsed
Kubios premium time varying download (enclosed) - the timestamp is in the window center - so a timestamp of 1 min is from zero to 2 min elapsed.
Therefore the logger will be different from Kubios either way.

Pre correction


Make sure you shift Logger 1 minute forward (the Logger at T=2 min equals the time varying Kubios at 1 min)

Post correction


Why are my DFA a1 values too high for the level of effort I am doing?

  • The most common reason would be the effects of high rates of missed beats in the RR sequence.  Although programs like HRV logger auto correct for artifacts, they don't tell you how many.  Kubios will give you artifact rates - you should not trust rates beyond 5% as per our Sensors study.

Can I use the DFA a1 as a way of checking my respiratory compensation point, MLSS, VT1, LT2?

  • On paper, that seems like a good idea since DFA a1 drops with intensity - but:
  • It is very difficult to evaluate that possibility because of several issues.  As we now know, missed beat artifact goes up with exercise intensity and the correction of these missed beats by the usual methods will lead to a positive bias in a1 (it falsely rises as we showed in Sensors above).  In addition there seems to be some negative bias in some of the chest belt devices where they may read a bit lower than ECG.
    Bottom line - there may be a second "threshold" buried in the low a1 zone but it is hard to look for and probably is not going to be practical for most users. 
  • Having a very low DFA a1 generally indicates high intensity but not with precision.  Better non invasive alternatives exist such as the FTP or muscle O2 desaturation.

Where does the DFA a1 value of .75 actually come from?  Why doesn't it vary person to person?

  • Initially, the .75 value was "guesstimated" from data showing that DFA a1 runs about 1 during very light exercise (representing very correlated/self similar patterns) but drops to .5 (corresponding to random beat patterns) at very high intensity.  Therefore an in-between point of .75, could represent a moderate effort.  Looking at previously published data by Gronwald, Hautala and Blasco-Laforga shows a DFA a1 of about .75 lying in the area where the AeT should be.  We went on to show that in recreational runners, an a1 of .75 is a valid surrogate (on average) for the AeT.  
  • A key advantage of the a1 is it's dynamic range - whereas other HRV indexes hit a nadir at the AeT, the a1 value is at it's midpoint and will continue to fall past the AeT. 
  • The other important consideration is that no calibration is needed.  These are dimensionless values, so my value of .75 represents a similar physiologic state as yours (partiality correlated) .  Of course since it represents net "organismic demand" and status of the autonomic nervous system, there can be day to day fluctuations and effects from fatigue, stress, temp etc.  
  • Heart rate nor power, although great metrics, can't be used for accurate zone assessment, unless one calibrated them to a lactate or gas exchange test.  The closest parallel example to a1 would be lactate (a measure of internal metabolic status), but even that has very wide variation in levels at the MLSS.
  • All these factors result in our ability to use the index for intensity assessment while exercising.

 I've noticed that I can't drop my a1 below .5 using the HRV logger, any ideas.

Does the HRVT (a1 derived AeT) change if I'm sick?

  • We really don't know.  But a recent observation I made might shed some light.  I did a 20 minute Zwift ramp (130 to 230w) the morning of my second Moderna Covid vaccine (Pre) and another the next morning afterward (Post).  Yes, I had the typical post vaccine sore arm, nausea, fatigue, muscle pain and was really "spaced out". Like the flu but no sore throat or congestion.  Did the HRVT change?  Very surprisingly it did not:

The time/power at crossing a1=.75 was just as it usually is 210-215 watts.  N=1 data certainly, but intriguing that a1 seems fairly well linked to exercise load, at least in the short time frame - I wasn't up to going on for another couple of hours to see what would happen.

Why does my DFA a1 seem lower (for a given HR) running vs cycling?

Several potential reasons:

  • Random differences and day to day variation - try to repeat the tests on a regular basis to see if it is real.
  • Potential loss of R peak precision. What I have found is that in certain people (a minority) some electro-mechanical factor creates some distortion of the R peak. This may be diaphragm related but more likely trunk musculature that is firing more strongly while running. If you wore the Movesense ECG you may see this:

instead of this:


 

  • Since the DFA a1 is related to "correlation" of beat patterns, having a loss of precision of those patterns by distortion of the R peak will reduce the value seen. This was nicely demonstrated by Dr Mourot's study
  • This will not affect the HR since the same beat count per time is present. It also is not noticeable at rest since those offending muscles are not firing. 
  • If you see a large discrepancy and don't have the ECG to conform why, trust the bike data over the run until we get more information on this problem - again, it occurs but not in everyone. 
  • More on a possible reason with interesting data

Can the index be used for monitoring endurance and HIT fatigue

Can I get a single lead ECG from a Polar H10 sensor?

  • Yep! - but there are several quality issues such as low sample rate (125 Hz).
  • Here is the guide 
 
 

Heart rate variability during dynamic exercise 


 

52 comments:

  1. What would be a good power between “steps” using HRV logger and the six minute long steps? 10 watts? 20 watts?

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  2. I think 20 watts should be fine. Once you get your approximate threshold, you can retest by just doing 3 stages, AT-20, AT and AT+20w.

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  3. Hey Bruce,

    Great blog! I am a nephrologist passionated about endurance training :-). I even did some of my training in Gainesville back in 2005-2007!
    I am going to try to use DFA a1 to estimate my LT1 tonight!
    Thanks for sharing the information.

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    1. Thank you for your kind remarks, glad this method may be of help to you. And yes, greetings from Gainesville!

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  4. Bruce, thank you for your work, so helpful. I'm using HRV Logger, Kubios-free and runalyze to establish my LT1, using your protocol: 20' wu, 6' steps with 0.3 km/h increments. I noticed that DFA cross .75 value, but then rises up, for 20/30 minutes (even if HR, speed and fatigue is high) Actually it seems that 133 bpm and 156 bpm are my aerobic threshold. I know it is very weird, can you hepl me to interpetrate ?

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    1. Could you send a dropbox link to the rr data so I can take a look?

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    2. No problem
      https://www.dropbox.com/s/rsdxkpxs3upegrn/2021-3-7_RR_Calcolo%20Lt1.csv?dl=0
      I have also all the files exported via HRV Logger

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    3. Thanks, ignore the early dip, analyzed and put on twitter

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  6. Hi Bruce, thanks for replying on Twitter, really appreciate it. However, 140 characters is just not enough.
    I'm really enthused by the science of DFA alpha 1 and have just listened to the Endurance Innovations podcast where I'm very pleased to say I understood all of what you said.
    One question came to mind while I was listening (and cycling), what would be the effect of wearing 2 HR straps simultaneously? Have you ever tried this?
    Also, I was going to do the testing sessions as workouts in TrainerRoad but now I'm thinking that I'll keep it simple and just send a workout to my Garmin and do it from there. This way it will keep the connections to devices to a minimum (Garmin, TrainerRoad, smart trainer, Android app would all need to be used otherwise). I assume you would concur with this approach.

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    1. Thanks for the kind words. Although 2 straps can be used, if they end up bumping together that could produce artifact which we want to avoid. Your best bet (and what I do) is use a Polar H10. You have 1 bluetooth to Garmin (that's what I use for kubios), 1 bluetooth to another device for accurate RR (I use HRV logger realtime), and unlimited Ant+ (trainer road, zwift, android ipbike etc).

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  7. Yes I wish I'd bought the H10 now for that reason but didn't give enough thought to how I'd connect and to how many devices etc. Ah well, I'll manage.

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  8. Good blog and information. I started using the hrv logger, 3 days ago and I think my LT1 is 133bpm. As I trained in the mountains I accumulated a lot of fatigue and I think that at 155 bpm the value of alpha 1 appears 1.20. I encountered the same problem. Can you help me?

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    1. Thanks, not sure I totally understand. But after HIT or long rides, all bets are off on a1 thresholds. It will seem lower than usual at a given power. Remember to discard data if artifacts are above 5%

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    2. Thanks 🔝
      I saw in hrv logger the artefacts are above 5%. In relation to my doubt it is in trail running, when I start on the flat the alpha 1 is 0.75 {133bpm} but when I go up, logically my hr goes up {155bpm} and my alpha 1 is 1.20. I think more beats per minute my alpha 1 goes down, if the exercise is more intense

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    3. Artifact correction can raise the a1, especially if the a1 is around .75 or lower. So if you hit a1=.75 at a HR of 133, then starting getting >5% artifact at higher HR, the a1 would rise erroneously from the artifact correction effect.

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    4. Now I get it!
      what is your opinion about a protocol before a 50k test with 2500d +? In the warm-up before the race some run with increment?
      Other things, I train a lot my MFO {max fat oxidation), the intensity of a1=0.75,is the point of fat max? thank you very much from Portugal - Porto, send my our address and I sent to you a bottle of port wine

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    5. Ha, thanks, my reward is seeing you get the hang of this. As far as training, that's a tough one. I'm not a coach and I'd hate to give you the wrong info.

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  9. Great work on all of this! I am trying to work on the difference between my a1 running and cycling. I see your comments above. How would walking come in to this? I noticed today on a run, that when I was walking up a hill, to keep my heart rate the same as when running, a1 went up to what seemed a more appropriate level (generally it is too low running - around 20 BPM below cycling). Would the difference between running and walking fit with your theory as to why there is a discrepancy?

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    1. Yes, good observation. Walking up a steep grade or even a stair stepper would be a good way to estimate your running threshold. It seems that the act of foot strike/body impact creates some electro mechanical artifact causing loss of R peak precision, resulting in a low a1 (loss of "correlation" patterns)

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    2. Great thanks. I have a stepper so will give it a go!

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    3. Please let me know how it goes.

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    4. I did a test today on a stepper (actually it is an elliptical trainer so uses arms as well). I did 4 min warm up and then 4 mins ramps trying to keep HR at 115, 120… to 145. I then did exactly the same test on a treadmill. The a1 results seem very different and a bit strange!. For the stepper, a1 never got down to 0.75, with the lowest being 0.9. For the treadmill, a1 was never above 0.75 (except for the warm up which was partly walking). Using a Polar H10, data doesn’t look too messy. I am looking at the data in Runalyze and Kubios (free) so difficult to do direct time comparison other than in the 4min blocks. So, for me definitely seems to be a problem looking at a1 when running. Not sure why I wasn’t seeing a1 going below 0.75 on the stepper. You are welcome to the FIT files if any use.

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    5. That's exactly what I'm looking for. If you could send me a dropbox link to the fit files I can do a formal comparison and post that here. I did a podcast a couple of days ago and part of the discussion was on the issue of early a1 drop in some runners. Thanks.

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    6. I listened to the podcast, very interesting. Link is https://www.dropbox.com/sh/0xtctaiyyglg6sw/AACflqzlutKBxeBwuEj_0UdYa?dl=0

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    7. In case it is useful for comparison, I have also uploaded a bike test I did a couple of weeks ago. This was 10 min warm up , then 5 bpm heart rate ramps of 6 mins, to 120 bpm to 150bpm with 8min warm down

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    8. Got them, will work on a formal comparison and let you know shortly - thanks! Yes, the bike is very helpful.

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    9. Here we go - http://www.muscleoxygentraining.com/2021/03/dfa-a1-problems-during-running-why.html

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    10. Great thanks! Really interesting. I am still surprised that the Stepper results seem higher than I would expect. I have been doing MAF training for the past 5 years or so and my MAF is around 130 which always felt right and I had assumed that things would align around there somewhere. Guess I just keep exploring! Keep up the good work.

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    11. I got an H10 and cannot get a stable DFA a1 while on treadmill (seems too low, I have to walk to stay above threshold) While on a smart trainer I cannot Get it under 0.90 until I am at 160bpm, 91% of my HRmax (i'm 48)
      What can I do to determine my LT1 ?
      1 - treadmill but with bruce protocol for ergometer test (3% incline every 3 minutes) ?
      2 - trust the bike readings of DFA a1=0.75 ? (seems too high for me)
      3 - non of the above, just run with the talk test ?

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    12. I understand that there are a lot of protocols for treadmill tests for athletes: Bruce, Ellestad, Balke, each one with thei own modifications.
      To avoid early DFA a1 decline, what would be a treadmill protocol suited to get true readings ?

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    13. See my comments in the other post about why the run measurement may be an issue. Try either an elliptical, stair stepper or bike to get the HR at a1 = .75. If the bike seems too high, do it a couple more times to confirm. Lastly, some people may simply not get perfect agreement.

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    14. I already saw it, and for me it is mandatory to find hte correct AeT
      I will follow your protocol on bike one again using Zwift and Elite
      Could beta blockers be an issue ?

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    15. Let me know how you do. Beta blockers are probably not the issue, I originally did a case report on/off Atenalol to see if a1 behavior changed (it did not). Some unpublished data also suggests that they are not a factor.

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    16. I just tried another little test. I did a walking 4 min HR ramp up a hill. The data looks like it agrees with bike/elliptical data. I need to find a steeper and longer hill to try and get to a1 = 0.75 as I couldnt get HR high enough. Looks like it could be another option for anyone who has incongruous running data and may not have access to other equipment.

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    17. Good idea, thanks for the feedback

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    18. spookymuggs, I did quite the same as you: treadmill, fixed speed at 5.5km/h, incline at 0.5% to start, then +0.5% every 6 minutes. DFA went down to 0.80 but then rose up again after 75 minutes with a 8.5% incline and I stopped

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  11. I was trying to investigate a bit further and found there are a couple of apps that connect to the H10 and give an ecg trace. I wondered if they would give any useful information if I tried to capture when running? I have uploaded a screen recording in the dropbox folder above. Let me know if you think it could be useful or not.

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    1. Now that's cool! Although exciting, I looked at a related app specs, and the sample rate is only 130 Hz - way too low for wavefrom analysis (need at least double that, 500 Hz is even better). If you are worried about an arrhythmia it seems useful, but then you need to be trained to read a rhythm strip. I am going to look at it further - thanks again. Will report back.

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  12. This is really interesting research. I've tried multiple straps and apps (Kubios & KRV Logger) but it's very challenging to get my a1 below 1.0

    I got it below 1 during a very tough Vo2 interval session. But Tempo power seems to be around 1.4 for me. Do you think some folks might have a different metric? (I've used a Polar & Garmin strap)

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  13. Which Polar are you using, and are you using bluetooth for the recording?

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    1. I've used both Polar H10 & Garmin HRM-Duel, both via Bluetooth for sure - with HRV Logger.

      It might have used Ant+ for the analysis I did in Kubios - that was recorded via a Garmin Watch (paired with the Strap). Not sure what the Garmin watch uses.

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    2. You need to delete the ant+ hrm in Garmin settings and re pair as bluetooth only. Record a ramp to almost max, send me the fit file and I'll be happy to review it with you. Please use the H10.

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    3. Thanks! I'll give that a shot this week and put on dropbox.

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    4. Looks like my watch (Fenix 3) only supports Ant for the HR strap. Would recording via the HRV Logger be OK?

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    5. Yes, the HRV logger is fine. Use the workout mode for artifact correction, 2 min windows and you should be good.

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    6. Think I'm there now. I did 4 min steps and increased 10w each step. FTP is about 200, max HR is around 190. I was a bit nervous so HR felt a tad high at the start, then was about right when I hit 200w. I've included the HRV Logger detail as well as the Zwift FIT file with power data & HR.

      https://www.dropbox.com/s/0n7mf5cmdk020tu/HRV_data_010421.zip?dl=0

      If I had to guess based on breathing I would say LT1 is around 140w, but interested in what you think! Thanks again for looking at this data.

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    7. See my latest post for details - I get much higher than your numbers.

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    8. Wow, thanks. Appreciate that. Very interesting

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    9. This could well be a dumb question as am getting out of my depth.... but I looked at this data in kubios (free) as I am still trying to get my head around all this. When the data is divided into the 4 minute ramps, around ramp 6, the poincare plot starts to show strong linear patterns that get more distinct as the ramps increase. I havent noticed this type of thing before and wondered if it had any significance?

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    10. I know what you are describing - I'm not sure what it represents, but I see that commonly. It could be a restricted set of values, so they are segregated together.

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