Alan Couzens

Define Zone 2; Balance Intensity with Volume over Time

Here's what I think... Interesting paper and I agree with several of its conclusions, especially the failure to properly define "zone 2" i.e. when I have conversations with people around "zone 2" we're often using different definitions/ talking about different intensities. That said, a few counterpoints on the general arguments against low-intensity training: • It largely evaluates L.I.T. as an isolated intervention rather than as part of a complete training system. I don't think many advocates are suggesting that's the *only* training that you should do. • Most of the evidence cited is acute (signaling studies) or relatively short-term (weeks to months), whereas endurance adaptations are accumulated over years. This is the big one. What works for high levels of fitness/performance long-term? And the real-world evidence here is clear. • It focuses heavily on intensity while underweighting the importance of volume, consistency and long-term training load. • Many comparisons equalize training time or volume, effectively removing the primary advantage of Zone 2: the ability to tolerate and accumulate more work. • Acute increases in AMPK, PGC-1α, etc. do not necessarily predict superior long-term adaptations. Studies looking at actual changes in mitochondrial content generally support low-intensity training. • The paper acknowledges that relatively few studies have actually examined true "Zone 2" training directly. So, it's really hard to have a worthwhile discussion/argument around it when it hasn't been properly defined. • It relies heavily on VO₂max as the primary outcome, while underemphasizing durability, fatigue resistance, metabolic flexibility and the ability to sustain high energy throughput. • The "general population" is treated as a fairly homogeneous group, despite major differences between sedentary individuals, recreational exercisers and trained athletes. • The question "Is Zone 2 the optimal intensity?" may be the wrong question. The more relevant question is: "What role does low-intensity training play within an optimal long-term training program?" My view remains that L.I.T. isn't valuable because it's magical. It's valuable because it provides a very high adaptation-to-recovery-cost ratio, allowing athletes to train more consistently, for longer, over many years.... And that's how you get fit in the real world.