Training VO₂max on purpose: the HIIT and cardio engines

We have written before about why VO2max is such a powerful predictor of all-cause mortality. The good news that gets buried: unlike your age, VO2max responds to training. The question is which training.

Why intervals

High-intensity intervals spend time near VO2max, which is precisely the stimulus that raises it. Misi’s HIIT engine structures work and rest intervals with heart-rate and pace targets so you are training the right system, not just getting tired.

The role of steady-state

Zone-2 steady-state builds the aerobic base that makes the hard intervals productive and recoverable. The cardio engine schedules both so they complement your lifting rather than competing with it.

Measured, then re-tested

Misi tracks your VO2max and fitness age over time, so you can see the number you are training actually move — and compare it against age and sex norms.

What actually counts as HIIT

High-intensity interval training is not just hard cardio — it is repeated bouts near your maximum, long enough to spend real time at a high fraction of VO2max, with recovery between them. Classic VO2max-building intervals run three to five minutes at a hard but sustainable pace, repeated four to six times; shorter, all-out efforts train different qualities. The stimulus that raises VO2max is time spent near the ceiling, so intervals that are too short or too easy simply make you tired without moving the number.

How much of each

A common evidence-based split is roughly 80% of your cardio time in easy zone-2 work and 20% in hard intervals — the classic polarised model. In practice that might mean two to three easy aerobic sessions and one or two interval sessions per week, scaled to how much lifting you are doing. The exact numbers matter less than the principle: most of your volume easy, a small deliberate dose hard, and almost nothing in the grey middle where you are too tired to recover but not working hard enough to drive adaptation.

Protecting your lifting

The fear that cardio kills gains is mostly a scheduling problem. Separating hard intervals from heavy leg days, keeping most aerobic work low-intensity, and fuelling around sessions largely resolves the interference effect. Done well, a stronger aerobic base actually improves your recovery between sets and across sessions — which is why Misi schedules the two together rather than treating cardio as an afterthought.

You cannot out-lift a poor aerobic engine. The longevity upside of VO2max is too large to leave to chance.