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FatMax tested at five different intensities, between 55-95% of anaerobic threshold and showed similar levels of fat metabolised<ref name="k385"/>. This suggests that it's not worth being too focused on a specific intensity beyond going slow enough that you can keep going for a long time.
=Measuring FatMax=
The best way to measure FatMax is using an incremental stress test while measuring oxygen consumption and carbon dioxide production to get respiratory exchange ratio. That's part of a standard [[VO2max|V̇O2max]] test, but not something that's easily or cheaply available to most runners. Another alternative is to look at [[Lactate]] levels. There is some correlation between the initial rise in Lactate and FatMax<ref name="a585"/>, and while not perfect, I think it's good enough. Lactate meters are becoming cheaper and more widely available, with the promise of continual lactate monitoring coming soon. The initial rise is Lactate is different from what is normally meant by [[Lactate Threshold]], which is the transition from moderate to hard exercise (aerobic to anaerobic). This initial rise is sometimes call called the "First Lactate Turn Point" or "Aerobic Threshold". Definitions vary, with some studies using any rise from resting lactate levels, some using 0.5 or 1.0 rise from resting levels, or a fixed 2.0 level of lactate.
[[File:Three Phase Model of Exercise.jpg|center|thumb|600px|A three phase (zone) model of exercise intensity<ref name="k156"/><ref name="e211"/>.]]
A simple approach is to check if you're at or below FatMax is to recite the alphabet. If you have to take frequent breaths, say every 8 letters or less, then you're above FatMax. If you can recite the alphabet easily, then you could increase your effort until you fail, at which point you have a sense of your FatMax<ref name="talk test"/>.
There has been some interest in using Heart Rate Variability to calculate "DFA A1" (detrended fluctuation analysis alpha 1), but the evidence doesn't support this approach<ref name="DFA A1"/>.
=The intensity gap=
The most obvious problem with FatMax training is that you have to train at a much lower intensity the most people are comfortable with. Some runners will struggle to even run with their heart rate low enough to be in FatMax. Runners may find that FatMax intensity falls into the gap between the lower intensity of walking and the higher intensity of running. It's often said that the most common problem in recreational runners is their easy runs aren't easy enough, and their hard runs aren't hard enough. The two problems are related in many runners, where their easy runs or too hard to allow them to fully recover and run hard on their hard days.
=Heart Rate Drift and FatMax=
Prolonged exercise will result in an increased [[Heart Rate]] for a given effort (pace or power), something called [[Heart Rate Drift]]. This drift means that FatMax (as measured by VT1) occurs at a lower power output and a higher heart rate after 2 hours just below FatMax (90% of VT1)<ref name="f565"/>. This indicates that you may need to adjust your pace or power downwards during prolonged training, but also accept a rise in heart rate as well.
=Practical Advice =
My practical advice is that FaxMax training is worthwhile, but don't get hung up on the exact intensity. Instead of looking at heart rate zones, simply ensure you go slow enough that you can keep it up longer without feeling exhausted. The pace should be slow enough you can hold a conversation, which seems to be one of the best intensity metrics, even now , with so much available technology. How long will depend on your fitness, but longer is better. Decades ago, coaches recommended "2-3 weekly interval sessions, a weekly long run, and as much low intensity as you can handle<ref name="Bowerman"/>", which still seems to be true.
=Tangent – Maffetone's MAF Method=
Philip Maffetone developed his "Maximum Aerobic Function" or MAF method<ref name="uMAF"/>. While this is based on similar concepts to FatMax, it uses exercise intensity based on an assumed [[Maximum Heart Rate]], which isn't valid. Calculating your Maximum Heart Rate from your age is like calculating your weight from your height. There's a corelation between height and weight, but no one would think that calculating weight from your height makes sense. The MAF subtracts your age from 180, then modifies the result. So, if you're 21, your MAF heart rate would be 159. If your true max HR is 214, that 150 would be 74% of max HR, but if your true max HR is only 164, it would be 97%! (A study of elite athletes showed a range of max heart rates between 164 and 214<ref name="x643"/>.)
<ref name="k156">Ronald K. Binder, Manfred Wonisch, Ugo Corra, Alain Cohen-Solal, Luc Vanhees, Hugo Saner, Jean-Paul Schmid, Methodological approach to the first and second lactate threshold in incremental cardiopulmonary exercise testing, European Journal of Cardiovascular Prevention & Rehabilitation, publisher Oxford University Press (OUP), volume 15, issue 6, 2008, ISSN [http://www.worldcat.org/issn/1741-8267 1741-8267], doi [http://dx.doi.org/10.1097/hjr.0b013e328304fed4 10.1097/hjr.0b013e328304fed4], free !!doi-access!!, pages 726–734</ref>
<ref name="e211">James S. Skinner, Thomas H. Mclellan, The Transition from Aerobic to Anaerobic Metabolism, Research Quarterly for Exercise and Sport, publisher Informa UK Limited, volume 51, issue 1, 1980, ISSN [http://www.worldcat.org/issn/0270-1367 0270-1367], doi [http://dx.doi.org/10.1080/02701367.1980.10609285 10.1080/02701367.1980.10609285], pages 234–248</ref>
<ref name="talk test">The "Aerobic Threshold" (AeT) Explained For Cyclists, https://www.highnorth.co.uk/articles/aerobic-threshold-cycling, High North Performance !!website!!</ref>
<ref name="DFA A1">Updated view on HRV analysis during exercise and DFA alpha 1, https://www.marcoaltini.com/blog/updated-view-on-hrv-analysis-during-exercise-and-dfa-alpha-1, Marco Altini !!website!!, en !!language!!</ref>
<ref name="f565">Julian D. Stevenson, Andrew E. Kilding, Daniel J. Plews, Ed Maunder, Prolonged cycling reduces power output at the moderate-to-heavy intensity transition, European Journal of Applied Physiology, publisher Springer Science and Business Media LLC, volume 122, issue 12, date 2022-09-20, ISSN [http://www.worldcat.org/issn/1439-6319 1439-6319], doi [http://dx.doi.org/10.1007/s00421-022-05036-9 10.1007/s00421-022-05036-9], free !!doi-access!!, pages 2673–2682</ref>
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