Lower all-cause mortality risk
associated with each 1-MET higher level of cardiorespiratory fitness in a meta-analysis.
JAMA meta-analysis ↗ The Norwegian interval, guided
Four hard intervals.
One clearer way to train.
NORx4 turns the proven Norwegian 4×4 structure into a guided workout for iPhone and Apple Watch—with personal heart-rate zones and progress you can see.
Coming soon to the App Store
4 × 4Structured work intervals
90–95%Personalized max-HR target
iPhone + WatchGuidance wherever you train
The session
A demanding workout.
A simple rhythm.
NORx4 keeps the structure visible and the next step obvious, so you can focus on effort instead of watching the clock.
01
5MIN
Warm up
02
4MIN
Work
03
3MIN
Recover
04
×4ROUNDS
Repeat
05
5MIN
Cool down
Why work on VO₂ max?
Your aerobic reserve is a longevity signal.
VO₂ max reflects how effectively your body can take in and use oxygen during hard effort. It is one of the clearest measures of cardiorespiratory fitness—the capacity that helps you climb, carry, travel, and keep doing more as the years add up.
Training it is not about chasing a single score. It is about building more capacity between everyday effort and your upper limit.
Lower cardiovascular event risk
associated with each 1-MET higher level of cardiorespiratory fitness in the same analysis.
33-study analysis ↗Adults in a large fitness cohort
where higher treadmill-measured fitness was associated with lower long-term mortality.
JAMA Network Open study ↗These population studies show associations, not a guarantee of individual outcomes or proof that fitness alone caused the difference. NORx4 is a fitness tool, not medical advice.
Inside NORx4
Know where you are.
Know what comes next.
Live interval timing, target-zone guidance, clear recovery cues, and a useful record of every completed session.
Your protocol
Review the warmup, hard intervals, recovery targets, and cooldown before you begin.
Guided intervals
See the clock, target zone, heart rate, and remaining work at a glance.
Progress over time
Follow VO₂ max trends, completed sessions, streaks, and best efforts.
What NORx4 does
Your zones, not averages
Use Apple Health metrics or enter your profile manually to set training zones around you.
Built for Apple Watch
Follow interval cues and live heart rate from your wrist while your phone stays out of the way.
Progress with context
Review sessions, consistency, time in zone, and VO₂ max trends without drowning in charts.
Designed with restraint
Your health data stays yours.
NORx4 reads only the Apple Health metrics you allow. Health information is processed on your device and is never used for advertising or tracking.
Train with intention
Build the engine.
Keep the signal clear.
NORx4 is coming soon to iPhone and Apple Watch.