N4x4 — The Norwegian 4×4. 33 minutes, once a week.

Origin

Born in Trondheim

In 2007, researchers at the Norwegian University of Science and Technology showed that four four-minute intervals at 85–95% of max heart rate raise VO₂ max more than steady training. Fire ganger fire. Four by four. It needs no gym, no gear, and barely half an hour.

Red rorbu cabins in Reine, Lofoten — Reine, Lofoten — Norway

33 minutes, all in

1× per week

4×4 the intervals

The Protocol

A mountain tarn in Norway, clouds mirrored in perfectly still water

01

Warm up

5 minutes · easy pace

Ease the engine up to temperature. Five easy minutes lift your heart rate gently, deepen your breathing, and make sure the first interval doesn't arrive as a shock.

White water thundering down a Norwegian rock face

02

Go hard

4 minutes · 85–95% max HR

Four minutes past the edge of conversation — hard enough that you couldn't hold one. N4x4 watches your live heart rate and tells you to push or ease off. Your only job is to keep moving.

A white rowboat resting on a mirror-still Norwegian lake

03

Recover

3 minutes · stay moving

Three minutes of easy movement. Recovery isn't a pause in the training — it is the training. Let the heart settle so the next effort counts as much as the first.

Walkers on a rocky Norwegian headland above a white-sand bay

04

Repeat ×4

then 3 minutes cool-down

Four efforts, three recoveries, one easy cool-down home. Thirty-three minutes after you started, you're done — and you don't owe anyone anything until next week.

On your wrist

Apple Watch

Your wrist does the counting.

Live heart rateStraight from the sensor, every second of every interval.
Push or ease offOne cue in the middle of the face. The number turns green the moment you're in the zone.
Haptic phase changesFeel the switch from work to recovery without looking down.
Controls under your thumbStart, pause and skip mid-stride. The whole workout lives on the wrist.

No Apple Watch? Any of the heart-rate sources below works just as well.

The Sensors

Bring your own heart rate

Works with what's already
on your wrist. Or in your ears.

If it can broadcast heart rate over Bluetooth, N4x4 can coach from it. Pair once; your live BPM drives every interval.

Apple Watch Series 4 or later. Install the watch app and heart rate streams automatically — nothing to pair.

AirPods Pro 3 The earbuds you already wear double as your monitor. Enable once in Settings (iOS 26).

Garmin Forerunner, fēnix, epix, Instinct, Venu, vívoactive — switch on Broadcast Heart Rate.

WHOOP WHOOP 4.0, 5.0 and MG. Turn on HR Broadcast in the WHOOP app and you're connected.

Polar

H10 · H9 · Verity Sense · OH1 · Vantage · Grit X · Pacer · Ignite (heart-rate sharing on)

Wahoo

TICKR · TICKR X · TICKR Fit

COROS

Heart Rate Monitor armband · PACE · APEX · VERTIX (broadcast mode)

Suunto

Race · Vertical · 9 Peak (broadcast mode) · Smart Sensor

Straps & armbands

Garmin HRM-Dual · HRM-Pro · Scosche Rhythm24 · Rhythm+ · Powerbeats Pro 2

Everything else

Any monitor speaking the standard Bluetooth heart-rate profile connects the same way.

The in-app device guide has exact, per-model setup steps — and flags older watches that only broadcast over ANT+, which iPhones can't receive.

The evidence

What the science says

Take their word for it,
not ours.

None of these people has any affiliation with N4x4. They're quoted because they say it better than we could.

“If you compare someone of low fitness to elite, it is a five‑fold difference in mortality over a decade.”

Peter Attia, MD — The Drive, AMA #27, 2021

“Cardiorespiratory fitness (measured by VO2 max) is one of the best indicators of improved healthspan and lifespan. One of the best protocols for improving VO2 max is the Norwegian 4x4 protocol.”

Dr. Rhonda Patrick — FoundMyFitness, 2023

“4×4 is a simple method that is easy to remember, and experience has shown that most people can manage 4 minutes.”

Ulrik Wisløff, co-creator of the protocol — NTNU / Norwegian SciTech News, 2024

A glacial river running through a lush green Norwegian valley — Norge — where 33 minutes a week earns views like this

The founding study

Helgerud et al., 2007 — Med Sci Sports Exerc. Four 4-minute intervals at 90–95% of max heart rate improved VO₂ max significantly more than moderate continuous training at matched total work.

The mortality data

Mandsager et al., 2018 — JAMA Network Open, 122,007 patients. Higher cardiorespiratory fitness was associated with lower all-cause mortality, with no upper limit of benefit — roughly a five-fold gap between the least fit and elite.

Watch & listen

Peter Attia — Why VO₂ max is the greatest predictor of lifespan (YouTube) FoundMyFitness — These 3 workouts are guaranteed to increase your VO₂ max Huberman Lab × Dr. Andy Galpin — How to build physical endurance

Quotes are drawn from public podcasts and publications, attributed and dated. They are commentary on VO₂ max and the 4×4 protocol generally — not endorsements of this app. Cardiorespiratory fitness is associated with long-term health; it is not a promise of any individual outcome.

The app

iPhone + Apple Watch

Built to be looked at once,
then never again.

No feeds, no upsells, no account. N4x4 does one thing: coach you through the protocol, cleanly, every week.

N4x4 workout screen with interval ring, live heart rate and target zone — **The workout**One ring, your live BPM, and a push-or-ease cue. Nothing else.

N4x4 home screen showing weekly streak and VO₂ max trend — **The trend**Your VO₂ max charted over months and years, plus the weekly streak that keeps you honest.

N4x4 history screen with workout calendar and milestones — **The proof**Every session on the calendar. Every interval charted against its target zone.

Coaching

Voice cues tell you when to push and when to recover. Live Activities keep the interval on your lock screen and in the Dynamic Island. A reminder nudges you when the week starts slipping.

Milestones

A weekly streak that keeps you honest, and milestone celebrations at 1, 5, 10, 25, 50 and 100 sessions. Consistency, made visible.

Any cardio

Run, ride, row, swim, ski, climb stairs. Your heart doesn't care what the machine is called — if the rate is in the zone, the protocol is working.

No catch

No ads. No sign-up. Your data stays on your device. Free — a gift from one health-obsessed developer to the world.

33 minutes.
Once a week.
Train like a Viking.

Warm up

Go hard

Recover

Repeat ×4

Your wrist does the counting.

Works with what's already
on your wrist. Or in your ears.

Take their word for it,
not ours.

Built to be looked at once,
then never again.

Your best investment in the
next decade of your life.

Warm up

Go hard

Recover

Repeat ×4

Your wrist does the counting.

Works with what's alreadyon your wrist. Or in your ears.

Take their word for it,not ours.

Built to be looked at once,then never again.

Your best investment in thenext decade of your life.

Works with what's already
on your wrist. Or in your ears.

Take their word for it,
not ours.

Built to be looked at once,
then never again.

Your best investment in the
next decade of your life.