You've read about it. Now run it. One protocol. Done right. Nothing else.
10 minutes warmup. 4 rounds of push-and-recover. 5 minutes cooldown. That's a complete cardiovascular transformation in less time than a Netflix episode.
Real-time heart rate monitoring from your wrist. No chest strap needed. Just tap to connect. The watch becomes your coach — haptic taps tell you when to push harder and when to ease off, even if you can’t see the screen.
The app coaches you through every phase. Push harder. Ease off. You just follow the cues.
Watch your VO₂ max trend upward over weeks. Every session recorded. Every improvement visible.
One workout. Forty years of research. A structure so simple it fits on a napkin.
Enter your age to see your personalized heart rate targets.
Helgerud et al., 2007 demonstrated that the 4×4 protocol significantly increased maximal oxygen uptake — the gold standard of cardiovascular fitness — in just two months.
Helgerud et al., 2007 — Medicine & Science in Sports & Exercise
The NTNU Cardiac Exercise Research Group has built the largest body of evidence for any single interval protocol. From elite athletes to cardiac patients, across ages and fitness levels.
NTNU Cardiac Exercise Research Group (CERG)
The HUNT Fitness Study — one of the largest health studies ever conducted — validated the connection between VO₂ max and all-cause mortality across tens of thousands of participants.
The HUNT Fitness Study — NTNU, Norway
The science behind the 4x4 — explained by the people who study it, practice it, and live it.
Why VO2 max is the key to longevity
Dr. Rhonda Patrick
FoundMyFitness
The Norwegian 4x4 explained
Chris Heria
THENX
How to improve cardiovascular fitness
Andrew Huberman
Huberman Lab
VO2 max and all-cause mortality
Tim Ferriss & Peter Attia
The Tim Ferriss Show
Two taps = push harder. One long tap = ease off. Your wrist becomes the coach when your eyes are elsewhere.
Targets recalculate the second your zone shifts. The watch adapts in real time — no manual adjustment.
Leave your phone behind. The watch records the full session and syncs back when you reconnect.
Optimized for the latest haptic engine. Battery-aware mode keeps a full session under 6% drain.
Free to try. Three sessions a week is all it takes.