The 120% is just a guideline. As I understand it, there is no physiological “switch” that trips when you cross 120%. Anything above threshold is at least partially anaerobic. VO2max is a physical state (maximal oxygen uptake) rather than a power level. So you can get there via long intervals at lower power (say, 110%) or really short intervals at say 130%. I’ve heard people say you can theoretically get there by riding at like 102% for a long time, but realistically your legs will fatigue first. There’s also some individual variation. For me 60/60s have to be at more like 130% to elicit VO2max, for example.
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