Zone 2 Training: the Latest Fad

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In recent years Zone 2 training has become the latest fad. It seems like everyone has a different definition or interpretation of the infamous Zone 2 training. When discussing training zones, it is important to understand a couple of key points. Training zones are typically set up to approximate metabolic anchors but not exact points and some of these anchors may not even exist. Training zones are simply bins where we put training so we can assess the training loads and adaptations to training. When we discuss training zones, we need to understand how the people discussing these zones have established those zones and what they are. We are all usually speaking a different language here.

Many point to Inigo San Millan’s developmental work in cycling as the origin of Zone 2 training. His work has providing clarity in this space, and I agree, to a point. However, the importance of intensity control in training has been around for a long time, but this renewed focus has provided some needed focus and clarity. For a lot of athletes, we need to make this whole zone conversation clearer and a whole lot simpler. In his previous lab work at CU Boulder, Inigo has arrived at Zone 2 definition based on lactate accumulation rates, peak fat burning (FatMax) and VO2/CO2 kinetics. For most athletes, this level of lab testing is expensive and not readily available. Honestly, without additional field testing, there are no assurances that what one learns in the lab test applies in daily training.

There are certain predictable benchmarks that Inigo has observed over the years. LT 1 (Lactate Threshold 1) VT1 (Ventilatory Threshold 1) usually occurs at 68-75% of maximum heart rate and a lactate of 2 mm or less. Given the day-to-day variation in these metabolic markers—and variations due to training and recovery status—it seems to me that sophisticated lab testing, while very interesting and nice to assess, may not tell us much more than using general markers that are easily available.

We know that aerobic threshold can shift a bit day-to-day. The idea that you can precisely anchor it with a fixed lactate number is at best misleading, unless you’re doing frequent spot checks every workout. Here’s a case from a test done in Inigo’s lab at CU. Granted, it’s just one athlete, but I have another test done there on a different athlete that confirmed similar zone definitions.

In the CU test, Inigo defines Zone 2 between 132–140 bpm. In contrast, I would set aerobic threshold closer to 1.7 mmol with a corresponding HR around 144 bpm. His Zone 2 is where lactate levels are stable—below the aerobic threshold. And frankly, that’s the best definition of Zone 2 I’ve come across. In my experience, that’s the optimal intensity for endurance development. The challenge in defining Zones is how these anchors are determined. In the above lactate example different coaches and physiologists would use different methods. Some would use a fixed lactate level of 2 mmol, which in this case would be about right while others might use an arbitrary point of a .5 mmol increase in lactate above baseline. I wonder why when we try for precision through testing when we use arbitrary anchors? In this case, it is clear to me that LT 1 is right at the top of Zone 2 as Inigo has defined it.

In a future article we will discuss lactate thresholds, how they are identified and if they really exist. Here’s a hit toward that future discussion: if we were to account for the possible range of error of lactate analyzers—and install error bars on the graph described above—we would be hard pressed to say, in this case and many others that I have seen, that the increase in lactate is in fact linear with the increase in the rate of work.

Inigo’s Lab Test Assessment

Zone 1—I agree with Inigo—it’s mostly for recovery. Zone 2, however, is the real engine builder. It’s not hard, and it’s not moderate. It’s steady, aerobic work. For most of us, this lands around 70–72% of max heart rate. Here is the critical point of clarification: what Inigo defines as Zone 2 training, in the five-zone model I use would be defined as Zone 1. In the Zone system I use, Zone 2 is about 73-82% of maximum heart rate and a training zone that is the culprit for much of the overtraining I have seen. Whether one subscribes to the polarized model of training first proposed by Selier, or a pyramidal approach, the Zone 2 debate has been good as it has brought the proper focus to the majority of our training time and the need to control intensity effectively.

High-Intensity Zones

So now that we’ve cleared up Zones 1 and 2, what about the rest?

Here’s where it gets messy again. Some coaches and physiologists anchor higher zones around anaerobic threshold ( a questionable concept), lactate threshold, ventilatory threshold 2, O2/CO2 crossover points, you name it. But to me, those are arbitrary and probably not real metabolic anchors. They try to pinpoint the same thing: the exponential rise in metabolic cost with increased effort. Fair enough, but it doesn’t make them useful anchors.

Taking Functional Threshold Power or Critical Power, both define sustainable outputs over time. Great for tracking fitness progress, but the durations they’re tied too often don’t match race demands. And that old “hour-long effort = anaerobic threshold” rule of thumb? Let’s be honest, it’s unvalidated and unreliable.

I think it’s time we throw most of these out with the morning trash. There’s no solid metabolic validation behind them, and no reason to assume they reflect actual physiological functions. What we do know is that for every athlete, there exists a maximal sustainable pace, power, or intensity over a specific duration. So, let’s use that.

Practical Zone Setting Based on Race Durations

In the endurance sports I coach, events typically range from three minutes to two hours. So, the logical approach is to assess the intensity athletes can hold across those durations—and then use that to define training zones. Based on Véronique Billat’s research, I use a six-minute max effort as an anchor. It aligns well with the duration needed to hit VO2max. That six-minute effort sets the ceiling for most high-intensity work, especially for athletes racing for longer durations.

What I’ve found? The difference between that six-minute max and race pace for events lasting 1–2 hours isn’t huge—typically in the 5–7%, maybe up to 10% range.

I also use maximal sprint tests to determine speed/power reserve. That’s a different conversation, but important. Your ability to improve performance is tied to how much reserve capacity you can develop.

In Summary

There’s no need to chase arbitrary lab metrics ; we should simply start focusing on what matters: duration, intensities, and sustainable efforts that align with race performance. Zone 2 or Zone 1 under my model, is your foundation. The higher zones? Anchor them around efforts that reflect the demands of your sport, not some lab threshold with shaky definitions. Again, some will ask whether lab tests are more accurate? NOPE, I have done serial lactate testing over several days, multiple times. The daily variation is as large or larger than how we estimate zones based on these simple heart rate tests.

Simple. Practical. Effective.