NTRVL: Norwegian 4x4 Trainer
iOS app by Andrii Gryshutkin. Health & Fitness · Andrii Gryshutkin
- Store rating
- 0 / 5
- Store rating count
- 0
- Download price
- Free to download
- In-app purchases
- Unknown
- Version
- 1.4
- Listing last refreshed
- 2026-09-16
View the original store listing
Store description excerpt
Four hard four-minute intervals. Three minutes easy in between. That is the Norwegian 4x4, and NTRVL runs the whole thing for you. 10-minute warm-up. Four intervals of 4 minutes in zone 4, with 3 minutes of active recovery. 5-minute cool-down. 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 - AirPods Pro 3 or Powerbeats Pro 2: train with just your phone and your earbuds - Apple Watch: start on your wrist and leave the phone at home - 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. 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 every zone change - Live Activity on the Lock Screen and in the Dynamic Island - Skip the warm-up when you are already warm, or let it run - Run, bike, row, ski-erg, stairs. The session is written to Apple Health as that activity, with heart-rate samples and energy burned ABOUT VO2 MAX VO2 max is a measure of aerobic fitness: how much oxygen your body can use at full effort. It is trainable, and interval work is one of the most studied ways to improve it. The 4x4 protocol was developed at NTNU in Trondheim, Norway, 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 One full short session, a single interval of about 12 minutes, free. No card, no account. NTRVL Pro unlocks the full four-interval protocol, the 8-week programme, recovery trend, training calendar, unlimited history, PDF export for your coach, and custom interval lengths and counts. Pro is 4.99 USD per month, or 29.99 USD per year with the first week free. REQUIREMENTS - iPhone running iOS 17 or later - Optional for heart rate: AirPods Pro 3,
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