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.
4 minutes hard.
Push toward 85–95% of max heart rate.
3 minutes easy.
Let the heart rate fall — and keep moving.
See every interval.
Target time, peak HR, and 60-second recovery.
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.
Real app screen · demo heart-rate data
See every interval.
Prove every adaptation.
- Peak heart rate — at the end of each of the four work bouts.
- Heart rate 60 seconds later — captured automatically during every recovery.
- HRR-60 per interval — the drop between the two — the number that shows adaptation.
- A session rating — your average recovery scored Low, Fair, Good, Strong, or Elite.
- Time in every zone — exactly how long you held each one, plus the full heart rate chart.
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
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 →
Your Recovery Score
Every session's average HRR-60 gets a rating, and your history charts it across your last 12 sessions — so you can see the trend, not just today's number.
- Low<12 BPM
- Fair12–17 BPM
- Good18–24 BPM
- Strong25–34 BPM
- Elite35+ BPM
Example: last 6 sessions · HRR-60 in BPM
A coach in your ears.
Not a drill sergeant.
Voice cues + haptics
Spoken coaching and taps for every phase — push, hold, ease off — no need to look at a screen.
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.
WORK 2 OF 4
03:17
163 BPM
42 CAL
Apple Watch companion app
Made for your
Apple Watch.
Press start on your iPhone and the Viking 4x4 Watch app launches itself. No fumbling, no separate setup — the Watch runs a real workout session on your wrist while the iPhone runs the show.
- Auto-launch — starts the moment you press Start Workout on the phone.
- Live HR streaming — heart rate and calories flow to your iPhone about every second.
- Wrist display — phase, countdown, live BPM, and calories at a glance mid-interval.
- Saves to Apple Health — the workout is recorded from the Watch, exactly like a native session.
No Watch? Viking 4x4 still runs the full guided protocol as a voice-coached timer.
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 · ~45 minutes
References
What the experts say about the protocol
Has called the Norwegian 4x4 one of the best protocols for improving VO2 max, and points to it repeatedly across FoundMyFitness.
Sources + full context →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 →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.
Finish and raise the horn: SKÅL. Every session ends with your report — duration, calories, peak and average heart rate, and exactly how long you held every zone — saved to your history so you can watch the trend build.
Consistency builds
the engine.
The best protocol is the one you keep doing. Every finished 4x4 lands in your history, and your Recovery Score tracks your average HRR-60 across your last 12 sessions — with the change since last time, so you can see whether your heart is recovering faster.
One to three sessions a week is the studied range. Show up, finish strong, and let the trend line do the convincing.
Real app screen · demo heart-rate data
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, the companion app launches automatically when you press start, streams live heart rate to your iPhone every second, mirrors your phase and countdown on your wrist, and saves the workout to Apple Health.
How is Viking 4x4 different from other 4x4 apps?
Most 4x4 apps time the protocol and show heart rate zones. Viking 4x4 measures whether it's working: after every interval it records how far your heart rate drops in 60 seconds (HRR-60), rates the session from Low to Elite, and trends your Recovery Score across sessions.
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.
Your first 4x4 is free · iPhone · Apple Watch companion