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"Zone 2" is everywhere—but are we even talking about the same thing? Dr. Marl Humbert and Dr. Cameron Garber sit down with performance physiologist Daniel Crumback to cut through the noise and reframe training zones around what actually matters: ATP production and substrate use. Instead of leaning on arbitrary percentages of HRmax or VO₂max—or single-point lactate snapshots—they unpack how real-time O₂ and CO₂ data reveals where your metabolism shifts, why inflection points beat "crossover" shortcuts, and how to translate those shifts into smarter prescriptions for everyday athletes, patients, and pros.

What you'll learn

• Why "Zone 2" has multiple definitions—and which one holds up physiologically

• How to use substrate utilization (fat vs. carb) to define zones 1–3 with confidence

• The pitfalls of percentage-based zones and lactate-only methods (and when they mislead)

• How to spot true zone boundaries using trendlines and slope changes in VO₂/VCO₂

• Practical coaching takeaways: Zone 1 as real recovery, Zone 5 for mitochondrial density, and how to "train to train" when limitations exist

• Why respiratory mechanics can be the hidden limiter—and how to address it before piling on intensity

• How to individualize plans so beginners avoid injury and advanced athletes actually progress

If you want zones that match your physiology—not an algorithm's guess—this one's your blueprint.

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Listen to the Metabolic Mastery Podcast:

Apple Podcast: https://podcasts.apple.com/us/podcast/metabolic-mastery/id1812972674

Spotify: https://open.spotify.com/show/2sgLMFv1Z8DwEz87yUpapI?si=29a177a6c6ca4ed4

YouTube: https://www.youtube.com/@korrmed/podcasts

YouTube Music: https://music.youtube.com/playlist?list=PLVCRWXBDdFo_S0OeaAsPv8xTnfajTLc75&si=d4wVUA0jqajkbukB

Amazon: https://music.amazon.com/podcasts/d76834b7-c11a-485f-8010-32b490d49e57/metabolic-mastery