Is your cardio actually building fitness — or just burning time?
Most recreational cardio lives in a grey middle zone: too hard to build an aerobic base, too easy to drive real adaptation. Your heart-rate distribution tells the truth.
Plenty of people do cardio several times a week and see their fitness barely move. The usual reason is not effort — it is distribution. Too many sessions sit in a moderately-hard "grey zone" that is uncomfortable enough to feel productive but too easy to build a real aerobic base and too hard to recover from properly.
The polarised model
Endurance research keeps pointing to a polarised split: roughly 80% of your cardio time easy, in heart-rate zones 1–2, and about 20% genuinely hard in zones 4–5. The slow work builds the aerobic engine; the hard work sharpens the top end. The middle, done relentlessly, builds mostly fatigue.
A weekly zone budget
Misi aggregates your time in each heart-rate zone across the week and compares it against a Norwegian-method zone-2 target — broadly 150–180 minutes of aerobic base work — and scores how polarised your training actually is. When your distribution drifts into the ineffective threshold zone, it flags it.
Slower, on purpose
For most people the fix is counter-intuitive: slow your easy days down so you can genuinely push your hard ones. Seeing the zone breakdown in black and white is what finally makes that discipline stick.
If every run feels moderately hard, you are probably training the one zone that builds the least.
Ready to put this into practice?
Misi BioLabs turns your bloods into a meal plan, training block and AI coaching — in one place.
Start freeFree tools
- PhenoAge Calculator — Biological age from a standard blood panel (Levine 2018).
- VO2max Calculator — Cooper 12-minute test → VO2max + fitness age + mortality hazard.
- TDEE & Macro Calculator — Mifflin-St Jeor maintenance calories + protein/fat targets.