Training with DFAa1
In my last article I introduced DFAa1 What is DFA a1 and how could it be useful as a training tool?, how it can be used to determine your aerobic threshold, and some of the potential benefits to be gained by using it as a training tool. Assuming that you are onboard and keen to give it a go, in practical terms, how do you actually do this? In this article, I’ll be explaining what equipment you need to test your aerobic threshold using this method using an indoor bike trainer, how to effectively do the test, and how to look at and use the results.
Advantages
To briefly recap, DFAa1 (or ‘detrended fluctuation analysis alpha 1’) is a factor that describes the type of patterns that can be found in your heart rate variability and has been found (when it crosses a certain value) to closely equate to your aerobic threshold. This represents the point at which you are using your mitochondria to your fullest potential and burning the maximum amount of fat. It’s essentially homing in on the precise intensity to carry out endurance training in order to best improve your long-duration aerobic performance.
Limitations
Like any tool or technique, a key aspect to effective use is to understand its limitations. One criticism of DFAa1 that has been made is that while comparing mean results gives acceptably similar results vs lactate testing, individual results can vary. This is essentially saying that in studies comparing AeT (aerobic threshold) determined via DFAa1 vs a lactate measuring method, the group average values are similar, but for some individuals, the DFAa1 result may be different to the lactate-determined value. This was identified for comparing heart rate for the two methods, from a study using data gathered from triathletes at a training camp (rather than testing, in lab conditions), and then another study looking at elite cyclists
In my mind, this indicates a couple of things - that potentially, determining AeT using DFAa1 to determine a power-based threshold may be more accurate than using heart rate (this lines up with my own personal experience), and that if test protocols & controls are not strictly followed (possibly the case during a training camp with minimal guidance, vs lab tested) then perhaps method variation could alter the results. In addition, the studies carried out so far don’t always use the same control method - currently, lab testing for AeT may involve the use of gas exchange (wearing a mask as per a VO2 max test), or taking blood lactate concentration values. All this is to say that any testing methodology, especially if it’s new, is susceptible to a greater or lesser degree to slight changes in methods or equipment. So, how do you test correctly?
The following guidance will get you on the right track, however, I’ve not gone into a great amount of detail. If you are interested in learning more and really getting to grips with the theory and reasoning behind some of these test methods and equipment choices then this blog is a great resource. The author is one of the key researchers in this new area and offers practical advice as well as links to the main studies carried out so far.
Test Equipment
The first thing you will need is a heart rate chest strap. Wrist-based monitors won’t give the accuracy required for this type of measurement. Most chest straps will broadcast HRV data these days, so unless your strap is really old it should be fine. The gold standard for accuracy would be a Polar H10 model.
Then you need something to record the HRV data for you - either a sports watch that has this function or a phone app. Be aware however that most HRV functions built into watches are for recovery/readiness metrics, which is not quite the same thing. You need to be able to toggle on HRV data to be recorded during an activity (something that is not often set by default). If you don’t have a watch that allows this function already, then one of the following apps would be your best bet:
HRV Logger Phone App (around £10/10€)
This was the first app that was available as a stand-alone HRV recorder. As such it’s got less functionality than the Fatmaxxer app below, but will work with any Bluetooth heart rate strap
Fatmaxxer Phone App (around $2)
This app is the one I use the most, and find gives the most accurate results (more on that later). The drawback is that it only works with the Polar H10 chest strap
Alpha HRV Garmin App (free whilst in Beta)
This is a Connect IQ app, so is an add-on for your Garmin device. The app is currently free and is certainly the most convenient if you wish to view your DFAa1 data in real-time, or if your chest strap only broadcasts ANT data.
Lastly, in order to create the step test conditions, you’ll need a bike and smart indoor trainer which outputs power. Just a quick note - you may be wondering why this is all focused on cycling. These tests would also be possible while running on an indoor treadmill (and some of the validation studies were carried out this way), however, with nonlab-grade equipment, the accuracy of results seems much more variable while running. The cause for this is unknown, however, is possible to do with the rhythmic impact forces from running disturbed the sensor or the quality of the connection between chest and strap, which could be conflicting with the data analysis when trying to find the ‘real’ HRV patterns.
Testing
The test itself is very much like a standard VO2 max style ramp test, done in erg mode (fixed power output), with a couple of exceptions. The first is that as the aerobic threshold arrives fairly early on (compared to traditional VO2), the steps should be focussed around lower intensities. The second requirement is that long steps (4 to 6 minutes) are needed to ensure that the body has settled to a ‘steady state’ each time before moving on to the next step. I’ll typically use 10 steps 4 minutes long, going from 55% of FTP up to 100% (after a 10-minute, warm-up). It’s not necessary to go all the way up to FTP, however doing so means that you are guaranteed to have passed through your aerobic threshold (and gives you a decent workout as well!)
Quality of data
Unlike when ‘traditional’ heart rate data is needed (in order to simply show your heart rate), this HRV data and subsequent data transformation into a DFAa1 value appear to be quite sensitive to the quality of the information. As such, you should ensure the following:
A good connection between the heart rate strap and your chest (firm fit, moistened)
Ideally set the HR strap output to Bluetooth (rather than ANT), which gives a better signal output for this use
Data analysis
The last stage, once the test is complete and HRV data is collected, is the analysis (to transform to DFAa1). This is a key part of the process and the best tool to use is an HRV data analysis software called Kubios. It is possible to transform the HRV data to DFAa1 yourself in excel if you are so inclined, but the advantage of using a specific tool like Kubios is for the data filtering capability. Incorrect data can significantly affect the results if the largest errors are not filtered out and kept below a certain level. Essentially, a sample of HRV data is taken and transformed into DFAa1 value, which is then plotted against heart rate and power. The point at which DFAa1 crosses 0.75 is your aerobic threshold. Here are a couple of examples - DFAa1 is the orange line, and the grey dotted line represents the 0.75 value indicating aerobic threshold:
Interpretation
The two graphs above give examples of two very different aerobic thresholds - not just the values of the heart rate and power for each athlete, but more interestingly where that aerobic threshold arrives compared to their FTP. For the athlete in figure 1, it arrives at the end of the 60% FTP step, and for the other athlete at 85% of their FTP. These represent two fairly uncommon extremes; most athletes we have tested fall somewhere in between.
For athlete 1, the reason for the early crossover could be that they were fairly new to endurance sports - if it wasn’t already a high priority then this test result indicates that there are some big potential gains to be made by focusing on endurance work (specifically at the aerobic threshold). Athlete 2 was more experienced, but not necessarily as much as others tested with a lower AeT as a proportion of their FTP. So while some assumptions about training history can be made when looking for a reason for a given aerobic threshold, there are clearly other factors at work that can influence the outcome. For any athlete (and for any metric!) I would suggest that the aerobic threshold identified with this method is not taken as ‘the undeniable truth’, but used as a guide in conjunction with all the other information available to you. So as a validation, try out your identified AeT to see if it feels like a comfortable enough intensity where you feel you are working but could be kept up for a significant amount of time. Another thing to try is the ‘conversation test’ - can you manage to speak in full sentences (with a pause every couple of sentences, not continuous talking) without feeling like you are getting too much out of breath? These two ‘checks’ are not as accurate as the DFAa1 method in my opinion, but can be worthwhile just in case something has gone wrong with your test and it’s thrown out an inaccurate result.
You may have heard the quote, ‘if you can’t measure it, you can’t improve it’. Wherever the cross-over occurs, just measuring it and tracking its progress over time gives you a great insight into the effectiveness of your endurance training, as well as identifying your own optimised aerobic training intensity.
Kevin opened a B&B for cyclists in France in 2014, & then a year later decided to start a cycle coaching (level 3) qualification. This was mainly in order to be able to better support his guests (but also to make his own training more effective too). At the B&B he runs the odd training camp for cyclists but mainly offers coaching advice while guiding guests. An engineer by background, he happy diving into all sorts of training data, but also understanding that coaching is about much more than just the numbers!
After being introduced to triathlon by a good friend & then taking part in races for a couple of years he decided that a level 2 coaching course with British Triathlon was the way forward, completing this in 2019.
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