Norwegian 4x4 Protocol

The 4x4 app that shows your heart getting stronger.

Guided Norwegian 4x4 intervals with live Apple Watch heart rate, per-interval reports, and recovery trends. The protocol is proven. Now you can prove it's working on you.

Launching soon on iPhoneSee how it works
4×4 PROTOCOLLIVE WATCH HRINTERVAL REPORTSHRR TRENDS
01 TRAIN

4 minutes hard.

Push toward 85–95% of max heart rate.

02 RECOVER

3 minutes easy.

Let the heart rate fall — and keep moving.

03 MEASURE

See every interval.

Target time, peak HR, and 60-second recovery.

04 IMPROVE

Watch adaptation happen.

Compare recovery session over session.

Built on the protocol validated by 20+ years of research at the Norwegian University of Science and Technology, and discussed by Dr. Peter Attia, Dr. Rhonda Patrick, and researchers behind the landmark studies.

Experts cited are not affiliated with Viking 4x4.

Timers are easy.
Proving it worked is harder.

Every 4x4 app can count to four minutes. What no timer tells you is whether your heart is actually adapting. VO2 max is the single strongest fitness predictor of how long you live — and if your app can't show you progress, you're training on faith.

Stronger heart earned

See every interval.
Prove every adaptation.

  • Peak heart rate — for each of the four work bouts.
  • Time in 85–95% — the minutes that actually drive adaptation.
  • 60-second recovery — measured automatically after every interval.
  • Best interval — flagged in gold when you earn it.
  • Baseline comparison — today's recovery against your 4-week average.

Heart rate recovery

The number that matters
after the interval.

Heart rate recovery is how far your heart rate falls in the 60 seconds after peak effort — a direct read on cardiac efficiency and vagal tone. It's the fitness metric your cardiologist actually cares about, and Viking 4x4 measures it after every single interval.

↓ 33BPM

60-second heart rate recovery

180160140120WORKRECOVERY · 60 SEC171 BPM138 BPM−33BPM

Faster recovery = a stronger engine

As your heart gets more efficient, it slows down faster after hard efforts. In a landmark NEJM study, recovery of 12 BPM or less at one minute predicted mortality. Watching your own number climb week over week is the clearest evidence the protocol is working. Read the HRR science →

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Last 6 sessions · HRR-60 in BPM

A coach in your ears.
Not a drill sergeant.

Voice guidance

Clear audio cues for every phase — push, hold, ease off.

Live zone coaching

Told to pick it up or ease back against your live heart rate.

Minute alerts + 20-second finish cue

Countdowns that keep you honest to the final seconds.

Works with the screen locked

Phone in your pocket, coach in your ears.

Any cardio type

Run, cycle, row, climb — anything you can push to 85–95%.

Stay in the zone that
builds your engine.

Your live Apple Watch heart rate rides over color-coded zone bands. The 85–95% target is marked in gold — the intensity the research says drives the adaptation. Hover or tap a zone to see its job.

Z590–100%

Red line. Only the last minute of an interval should touch it.

Z480–90%

Hard. The lower half of the 4x4 work target lives here.

Z370–80%

Transition — you pass through it, you don't camp in it.

Z260–70%

Easy aerobic. Recovery intervals and base volume.

Z150–60%

Warm-up and cooldown.

Your heart data is yours.

  • No account
  • No cloud
  • No tracking
  • Apple Health only
  • Data stays on device

The science

Simple protocol.
Serious stimulus.

What is the Norwegian 4x4 protocol?

The Norwegian 4x4 is an interval workout: 4 minutes at 85–95% of your max heart rate, then 3 minutes easy, repeated 4 times, with a warm-up and cooldown. Developed at NTNU in Norway, it's one of the most studied methods for raising VO2 max.

4 hard · 3 easy · repeat 4 times · ~38 minutes

+7.2%VO2 max in 8 weeks of 4x4 trainingHelgerud et al., 2007~20 yrsyounger cardiac remodeling after 2 years of training including weekly 4x4Howden/Levine et al., Circulation 2018~4xthe mortality risk carried by low fitness vs high fitness across 122,007 patientsMandsager et al., JAMA Network Open 2018

References

  1. Helgerud J, et al. Med Sci Sports Exerc. 2007;39(4):665-671.
  2. Howden EJ, et al. Circulation. 2018;137(15):1549-1560.
  3. Mandsager K, et al. JAMA Netw Open. 2018;1(6):e183605.
  4. Cole CR, et al. N Engl J Med. 1999;341(18):1351-1357.

What the experts say about the protocol

RPDr. Rhonda PatrickFoundMyFitness

Has called the Norwegian 4x4 one of the best protocols for improving VO2 max, and points to it repeatedly across FoundMyFitness.

Sources + full context →
PAPeter Attia, MDThe Drive

Writes that low VO2 max predicts mortality more strongly than smoking or diabetes — and prescribes 4-minute max-aerobic intervals to raise it.

Sources + full context →
BLDr. Benjamin LevineUT Southwestern

His two-year trial used a weekly 4x4 as its intensity anchor — and remodeled sedentary middle-aged hearts to look ~20 years younger.

Sources + full context →

Statements describe the Norwegian 4x4 protocol and VO2 max generally, not Viking 4x4. Experts cited are not affiliated with Viking 4x4.

How it works

01

Set your target.

Enter your max heart rate — from a field test, a formula, or let the app recalibrate from observed peaks. Pick your cardio type and your interval lengths. The default is the studied protocol.

02

Train with a voice in your ear.

Four minutes hard, three easy, four times. The coach calls every phase, the zones glow live on screen, and the gold chip tells you the moment you're in target.

03

Prove it.

Peak heart rate, time in zone, and 60-second recovery for every interval — then watch your recovery trend improve across weeks. SKÅL.

Consistency builds
the engine.

The best protocol is the one you keep doing. Weekly reminders on your schedule, streak tracking, and encouragement built from your own numbers — “Your recovery is 8 BPM faster than a month ago.” One or two sessions a week is the studied dose. Viking 4x4 keeps you on it.

Questions, answered.

What is the Norwegian 4x4 protocol?

The Norwegian 4x4 is an interval workout: 4 minutes at 85-95% of your max heart rate, then 3 minutes easy, repeated 4 times, with a warm-up and cooldown. Developed at NTNU in Norway, it's one of the most studied methods for raising VO2 max.

Do I need an Apple Watch?

No watch, no problem: Viking 4x4 works as a fully guided timer. With an Apple Watch you get live heart rate, zones, interval reports, and recovery tracking.

How is Viking 4x4 different from other 4x4 apps?

Other apps time the protocol. Viking 4x4 measures whether it's working: per-interval heart rate reports and 60-second heart rate recovery trends over time.

How often should I do the 4x4?

1-3 times per week. Once weekly was the dose used in the two-year study that improved VO2 max 18%. More isn't better for this protocol.

Is the Norwegian 4x4 safe for beginners?

It's demanding. Start with shorter work intervals and build up, and talk to your doctor before starting any high-intensity program, especially with existing heart conditions.

What heart rate should I hit during work intervals?

85-95% of your max heart rate, sustained for the full 4 minutes. The app shows a live target and tells you when you're in the zone.

Does Viking 4x4 sync with Apple Health?

Yes. Workouts save with your chosen activity type, heart rate samples, and calories. Your data stays on your device and in your Health app. No account required.

What's heart rate recovery and why does the app track it?

The drop in your heart rate 60 seconds after peak effort. It reflects cardiac efficiency, and in a major NEJM study, slower recovery predicted mortality. Watching it improve is the clearest sign the 4x4 is working.

More questions? Read the full FAQ →

SKÅL!

Your heart. Stronger. Provably.

Launching soon on iPhone

Free at launch · iPhone · Apple Watch heart-rate support