This is my current understanding, which has evolved significantly over the past few weeks.
Aerobic threshold is a nebulous, albeit useful metric. We take it to mean the HR or pace below which we are predominantly using our aerobic energy system to perform exercise (note: the aerobic energy system uses both carbs and fats, while the anaerobic system does not use fats). There are different ways to define or determine one’s AeT. One reliable and reproducible method is to correlate AeT to LT1.
LT1, or lactate threshold 1 (not to be confused with LT2, or anaerobic threshold [AnT]), is the point where lactate starts to spill into the blood stream, often defined as a lactate level of 2. (There is always some lactate in the blood stream, the level of 2 was chosen somewhat arbitrarily as the cut-off point.) If you hold your pace constant at or just above your LT1 (or AeT) the amount of lactate in your blood stays constant, let’s say at 2: your body clears it from the blood as fast as it spills into the blood. (Note: at higher HRs, when lactate levels keeps rising despite holding a constant pace, this is defined LT2, or AnT—you can no longer clear the lactate from the blood as fast as its spilled into the blood.)
Keep in mind we don’t know exactly whats happening in the cell, let alone the mitochondria, during all of this. We just know what we see in the blood stream. I say this because we don’t know if lactate appearing in the blood truly means the anaerobic energy system has just bypassed the aerobic system (percent wise) with energy production. This is because a well developed aerobic system can also clear byproducts of anaerobic metabolism before they enter the blood stream. To some extent, the muscle cell is a black box. This why I believe AeT is a nebulous metric. Nevertheless, seeing lactate spill into the blood likely indicates that the aerobic system is ‘overloaded’ and you are no longer at an easy, ‘endlessly’ sustainable pace, ie, below AeT.
The theory behind a drift test being useful in estimating AeT seems to be based on a combination of the above lactate studies, logic, and the data crunching experience at Training Peaks. If your chosen HR is well below a lactate of 2 (ie your LT1), your pace will not drift down more than 3% over an hour. If you’re at a HR with a lactate much higher than 2, your pace will drift down more than 5%. If you establish a pace where your HR yields a lactate of about 2—that is, you’re at LT1—then your pace will drift down between 3-5%. Furthermore, if your AeT—as determined by this test—is within 10% of your AnT, it is thought that your aerobic energy system is well developed. These tests, however, may be confounded by the inherent variability of ones HR due to stress, environment, diet, sleep, and running economy. The reliability and accuracy of these tests have not been confirmed with scientific study, as far as I know. Nevertheless, the logic behind it appears sound, and both UA and TP have their respective expert opinions vouching for it.
Another simple way to determine your AeT (or the top of UA Zone 2) is to determine your Max HR, VO2max, or AnT and then take a population based, predetermined percentage of these HRs as your AeT. For example ~70% of your MaxHR estimates your AeT. The MAF method, takes this a step further and doesn’t require any individualized testing: your AeT is simply 180 minus age (+/-). Although these methods are simple and easy to use, they’re plagued with the inherent inaccuracy of predicting individual metrics based on population averages.
So which metric should one use? Honestly, it probably doesn’t matter too much because the goal is to train well below AeT, and not necessarily at AeT. An estimate of AeT (the top of Z2) based on population data using Max HR and/or AnT would be reasonable. When in doubt—based on your ventilatory rate and perceived effort—just slow down. If AeT rises with time, you don’t need to chase it, just continue running at the same easy pace—benefits remain the same. Maintaining an easy feeling during these sessions should be paramount. Training long slow distance (LSD) comfortably below AeT has been proven to build one’s aerobic energy system. This is why we spend plenty of time here. For example, if your MaxHR is 190 your aerobic (LSD) training zone would 60-70% of that (HR 114-133). If you initially choose to target the middle of this range (123) this would give you a 10 point buffer for possible inaccuracy of the AeT (133). One can then make small adjustments to this predicted aerobic training range over time based on how easy the running feels. Be honest with yourself. Chasing AeT should be avoided, however, because it often causes one to actually spend too much time in Z3 without knowing it, and if this occurs studies have shown that it negates the benefits of polarized training.
I will post again regarding Metabolic Efficiency Tests.
