NTRVL: Norwegian 4x4 Trainer

Your first session is eight minutes. Two minutes of warm-up, one four-minute interval, two minutes to cool down. Long enough to feel what zone 4 is, short enough that you actually finish it.

The full Norwegian 4x4 is there when you want it: four hard four-minute intervals with three easy minutes between them, the protocol from Helgerud et al., 2007. NTRVL counts every phase, tells you when to push and when to ease off, and saves the session to Apple Health.

HEART RATE, HOWEVER YOU HAVE IT
- Apple Watch: start on your wrist and leave the phone at home
- AirPods Pro 3 or Powerbeats Pro 2: train with just your phone and your earbuds
- No sensor at all: NTRVL runs the protocol by feel, on effort alone
Pull your AirPods out mid-interval and the watch takes over the heart rate without stopping the workout.

YOUR NUMBER WITHOUT A SENSOR
Heart rate recovery does not need a monitor. It needs a finger and twenty seconds. When the interval ends, NTRVL walks you through counting your own pulse: find it, tap along with it, catch your breath, tap again a minute later. The difference is your recovery. It is counted by hand, and the app says so.

LIVE, NOT CATCHING UP
On the watch, heart rate comes from the workout session as it is measured, not from a later read of your health data. In a four-minute interval the number has to move when you do.

START ON EITHER DEVICE
Whichever device you start on runs the session. The other becomes a live screen you can control from either side. Phone-first or watch-first, both work.

TWO NUMBERS AFTER EVERY SESSION
- Zone accuracy: the share of each interval you actually spent in zone 4, interval by interval.
- Heart rate recovery: how far your pulse falls in the first 60 seconds after a hard interval. It moves within a few weeks, so it is the earliest honest sign the training is landing.

ZONES CALCULATED FOR YOU
NTRVL works out your zones with the Karvonen heart-rate-reserve method over a Tanaka maximum, using your age, resting heart rate and biological sex from Apple Health. Tested your max HR in a lab? Enter it by hand and NTRVL uses that instead.

AN 8-WEEK PROGRAMME, NOT A STOPWATCH
Three sessions a week, 24 in all. NTRVL tells you where you are in the block and when the next one is due.

BUILT FOR TRAINING, NOT FOR STARING AT A SCREEN
- Voice cues at every phase change, with your music ducked
- Haptics on the watch for every phase and zone change
- Live Activity on the Lock Screen and in the Dynamic Island
- Skip the warm-up when you are already warm
- Run, bike, row, ski-erg, stairs. Written to Apple Health as that activity, with heart-rate samples and energy burned

ABOUT VO2 MAX
VO2 max is how much oxygen your body can use at full effort. Interval work is one of the most studied ways to improve it. The 4x4 protocol was developed at NTNU in Trondheim and is among the most researched interval sessions there is.

PRIVACY
Your heart-rate data never leaves your iPhone. No account, no server, no analytics on your health. Everything is processed on device and stored in Apple Health, where you own it.

TRY THE PROTOCOL FREE
Your first session is free and takes eight minutes. No card, no account. NTRVL Pro unlocks the full four-interval protocol, the 8-week programme, the recovery trend across weeks, the training calendar, unlimited history, export for your coach, and custom interval lengths and counts. Pro is 1.99 USD per month, or 9.99 USD per year with the first week free.

REQUIREMENTS
iPhone on iOS 17 or later. For heart rate, optionally an Apple Watch Series 4 or later, AirPods Pro 3, or Powerbeats Pro 2. The watch app can run a session on its own.

NTRVL is a training tool, not a medical device, and does not provide medical advice. High-intensity training is not right for everyone. Talk to a doctor before starting a new training programme.

ntrvl.app

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