PHYS-01 · Principle
Is a 3-minute interval anaerobic?
Mostly not. All three energy systems work at once, and even in an all-out effort the aerobic share passes half at about 75-80 seconds by most estimates, so a 3-minute rep is mostly aerobic.
How the body works, agreed across the reviews below; a study entry shows what one experiment measured.
Practical Application
Is a 3-minute interval anaerobic? Mostly not. All three energy systems work at once, and even in an all-out effort the aerobic share passes half at about 75-80 seconds by most estimates, so a 3-minute rep is mostly aerobic.
What it does not mean: that zones switch one system on and another off, or that "anaerobic" means no oxygen is used: the aerobic system matters even in short maximal efforts (REF-185).
Mechanism
Phosphocreatine and glycolysis regenerate ATP fast but in limited amounts; the aerobic system has enormous capacity but delivers more slowly; energy comes from every pathway in almost all activities, not one after another (REF-185). Carbohydrate fuels both anaerobic and aerobic metabolism in most Olympic events (REF-449).
In 102 studies (311 data points, 78% of samples men) the 50/50 point of a maximal bout was 78.6 s, with no running-cycling or training-status difference (REF-460, A-); a second group cites about 23% phosphagen, 49% glycolysis and 28% aerobic for a 30-s sprint (Baker 2010, full text).
Estimates depend on how the oxygen deficit is built (Noordhof 2010), so a 41/59% aerobic/anaerobic 400 m in 11 trained men is that study's number (REF-767, B).
Common misreading
"Anaerobic means without oxygen"; "Zone 5 trains the anaerobic system, Zone 2 the aerobic one": every effort draws on all three.