The fitness number that predicts how well you’ll age.
VO2 max is the most oxygen your body can use in a minute — your engine size. It’s one of the strongest predictors of long-term health researchers have found — and unlike most of them, it responds to training.
Why bother raising it?
In a Cleveland Clinic study of 122,007 patients, being unfit carried a mortality risk comparable to — and in some comparisons greater than — smoking, diabetes, or heart disease, and researchers found no upper limit where more fitness stopped helping. It’s a big part of why VO2 max anchors Peter Attia’s longevity framework.
It’s also refreshingly measurable. Your Apple Watch already estimates it — it’s the “Cardio Fitness” number in the Health app — so you can watch it move as you train. And it will move: eight weeks of intervals raised it 7.2% in one landmark trial, and it still responds to training in older adults.
A big easy base, topped with a little hard work.
Most of your week stays easy — that’s Zone 2, which grows the aerobic engine the intervals draw on. Then, once or twice a week, you go near the top. The classic protocol is the Norwegian 4×4:
The hard part is pacing, and that’s what Zone2 does.
Interval training fails in two predictable ways: you start the first rep too fast, or you don’t recover enough between reps. Build the session in Zone2’s Workout Builder — each step with its own zone — send it to your watch, and your wrist tells you when you’re in the right range and taps you when you’re not.
Decide the workout before your ego does.
Mix any activities and zones in one session — an easy 30-minute run, a hard 10 on the rower, a long Sunday ride. Build it on your iPhone, send it to your watch, and let your wrist keep you honest.
Raise the number that matters.
Zone2 is free. Build your first interval session in a minute, send it to your watch, and see every hard rep land where it should.