NTRVL: 4×4 Trainer

Your watch can now run the whole workout — the phone can stay home. Plus an 8-week program and heart rate recovery after every session, free.

ADD YEARS TO YOUR LIFE IN 40 MINUTES A WEEK.

NTRVL is the cleanest way to do the Norwegian 4×4 — the interval protocol that 30+ years of research has shown to raise VO₂ Max faster than any other workout. And VO₂ Max isn't just a fitness number. It's the single strongest predictor of how long you live.

You don't need a gym. You don't need a trainer. You don't even need a heart-rate sensor — without one, NTRVL runs the protocol by feel. With AirPods Pro 3, Powerbeats Pro 2 or an Apple Watch, it turns them into a coach.

THE PROTOCOL

• 10-minute warm-up • 4 × 4 minutes in Zone 4 (90% max HR), with 3 minutes of active recovery between • 5-minute cool-down

That's it. Forty minutes, three times a week. Backed by NTNU in Trondheim, where the protocol was developed.

START ON EITHER DEVICE

Start on the watch and the phone can stay home — the watch measures your heart rate, counts the intervals, buzzes at every change and writes the session to Apple Health. Start on the phone and the watch does the measuring. Whichever device you start on runs the session; the other becomes a live screen you can control from either side.

Pull your AirPods out mid-interval and the watch takes over the heart rate without stopping the workout.

AN 8-WEEK PROGRAM, NOT A STOPWATCH

Three sessions a week, 24 in total. NTRVL tells you where you are and when the next one is due.

TWO NUMBERS AFTER EVERY SESSION

• Zone accuracy — how much of each interval you actually spent in Zone 4. • Heart rate recovery — how far your pulse falls in the first minute after an interval. It improves in two to three weeks, long before VO₂ Max moves, so it is the earliest honest sign that the training is working. Free, every session.

WHAT NTRVL DOES

• Reads your live heart rate from AirPods, Powerbeats or Apple Watch • Calculates your personal training zones using Karvonen HRR + Tanaka max HR — adjusted for your age, resting HR and biological sex from Apple Health • Adaptive warm-up — intervals don't start until your heart is actually ready, or skip it when you're already warm • Voice cues at every phase change, with your music ducked • Live Activity on the Lock Screen and in the Dynamic Island • Run, bike, row or anything else — the session is written to Apple Health as that activity, with energy burned and heart-rate samples

WHY VO₂ MAX MATTERS

Higher VO₂ Max is linked to lower risk of cardiovascular disease, dementia, type-2 diabetes, and all-cause mortality. A 5-point increase in VO₂ Max can add years of healthy life. The Norwegian 4×4 raises VO₂ Max more than any other protocol of equal duration. NTRVL gives you a structured way to do it for the rest of your life.

PRIVACY

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

REQUIREMENTS

• iPhone running iOS 17 or later • Optional for heart rate: AirPods Pro 3, Powerbeats Pro 2, or Apple Watch (Series 4+) • The Apple Watch app can run a session on its own, with or without the phone nearby

NTRVL Pro (free 7-day trial)

• Recovery Trend — your heart rate recovery, week over week • Training Calendar — every session, colour-coded • Unlimited history and PDF export for your coach • Custom interval lengths and counts

ntrvl.app

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