Stephen SeilerPhD · exercise physiologist · Professor, University of Agder
No: the adaptations overlap, and what matters is the distribution of stress rather than one magic band
What it rests onExpert panel commentary in the International Journal of Sports Physiology and Performance, 2025, consistent with his own finding that threshold and high-intensity sessions delay recovery similarly while easy work does not
Source: ebi.ac.uk ↗
Iñigo San MillánPhD · applied exercise physiologist, University of Colorado School of Medicine
Yes: it is a biochemically defined equilibrium and its adaptations are not reproducible elsewhere
What it rests onMechanistic argument from redox balance, mitochondrial flux matching and the Brooks lactate shuttle, published on his own newsletter, 13 Mar 2026
Source: inigosanmillan.substack.com ↗
San Millan writes for a span from professional cyclists to metabolic syndrome patients and grounds the claim in cellular measurements; Seiler's evidence is training records of elite endurance athletes and recovery responses in trained subjects.
A vocabulary trap sits under the disagreement: in Seiler's three-zone model, Zone 2 is the region between the two thresholds that he argues people overuse, while San Millan's zone 2 sits below the first threshold, which is Seiler's Zone 1. So check which system a number is using before treating these two as disagreeing: much of the gap is vocabulary rather than physiology.
Both hold that the large majority of endurance training should sit below the first lactate threshold, and both regard threshold-heavy training as the common mistake.
A trial comparing matched training volumes just below versus modestly above and below the first threshold, measuring mitochondrial and performance outcomes rather than acute substrate use.
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