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Zone 2 Training: What the Science Really Says About Building Your Aerobic Base

If you follow endurance sports online, you've heard it a hundred times: slow down, stay in "Zone 2," and your aerobic engine will transform. It's one of the most popular training concepts of the last few years — but a major 2025 review in the journal *Sports Medicine* takes a much more critical look at the evidence behind it. ## What Is Zone 2 Training? Zone 2 generally refers to low-intensity aerobic exercise performed below the first lactate threshold (LT1) — the point where blood lactate starts to rise above resting levels. In practice, most coaches and apps translate this to roughly **60-70% of maximum heart rate**, an effort where you can still hold a full conversation (RPE around 3-4 out of 10). The idea, popularized by endurance coaches and physiologists, is that spending most of your training time here builds mitochondrial density and fat-oxidation capacity without the fatigue and injury risk of harder efforts. ## The New Evidence: A 2025 Review Challenges the Hype A narrative review published in *Sports Medicine* (Storoschuk, Moran-MacDonald, Gibala & Gurd, 2025) examined the evidence behind Zone 2's popular claims. The authors found that **current evidence does not support Zone 2 as the single optimal intensity** for improving mitochondrial capacity and cardiorespiratory fitness in the general population. According to the review, this recommendation "contradicts substantial evidence supporting the use of high-intensity exercise for improving mitochondrial capacity and cardiometabolic health" — meaning moderate- and high-intensity training (including HIIT) can produce equal or greater improvements, often in less total training time. The authors specifically emphasize that "prioritizing higher exercise intensities is critical to maximize cardiometabolic health benefits, particularly in the context of lower training volumes" — a key point for people who can only train a few hours per week. ## Why Endurance Athletes Still Rely on Zone 2 None of this means Zone 2 is useless — context matters. For competitive endurance athletes who train 8-15+ hours per week, low-intensity work is what makes that volume sustainable. This is the basis of the well-documented **polarized training model**, where elite runners, cyclists and triathletes spend roughly **80% of their sessions at low intensity and 20% at high intensity**. Large aerobic volume at an easy pace lets athletes accumulate training stress with low injury, illness and burnout risk, while a smaller number of hard sessions drive fitness gains near race pace. ## Practical Guidelines

• Intensity: roughly 60-70% of max heart rate, or an effort where conversation stays comfortable.
• Frequency/duration: most easy-volume endurance programs use 3-5 sessions per week of 45-60+ minutes each.
• If your training time is limited: the 2025 review suggests prioritizing some higher-intensity sessions rather than relying on Zone 2 alone.
• If you're building endurance volume: Zone 2 remains a practical, low-fatigue way to accumulate training hours.

This is general scientific information, not an individualized training or medical prescription. Heart rate zones vary by fitness level, age, medication and health status — check with a physician or qualified coach before starting a new training program, especially if you have cardiovascular risk factors. --- *Source: Storoschuk KL, Moran-MacDonald A, Gibala MJ, Gurd BJ. "Much Ado About Zone 2: A Narrative Review Assessing the Efficacy of Zone 2 Training for Improving Mitochondrial Capacity and Cardiorespiratory Fitness in the General Population." Sports Medicine, 2025;55(7):1611-1624. https://link.springer.com/article/10.1007/s40279-025-02261-y This is general scientific information, not personal medical or coaching advice. Consult a physician or qualified coach before starting a new training program.*

Source: Storoschuk et al., Sports Medicine (Springer), 2025