N4x4

Train the Norwegian 4x4 without watching the clock or guessing your effort.

N4x4 reads your live heart rate — from Apple Watch, AirPods Pro 3, Garmin, WHOOP, Polar, or any Bluetooth monitor — and turns it into clear push, hold, and ease-off cues through every interval. When you finish, you see exactly how long you held the target zone, and how today compared with last time.

Four 4-minute efforts, three easy recoveries. 33 minutes, warm-up and cooldown included. It trains VO2 max, the fitness marker most closely tied to how well you age, and it's one of the most studied ways to raise it. If you're going to invest time in your health, this is a strong place to start.

HIT THE RIGHT ZONE • Target zones calculated from your own max heart rate • The number turns orange when you're below the zone and red when you're above, on both iPhone and Apple Watch • Spoken and haptic nudges when you drift out and stay out • A haptic countdown into every interval change: two taps, then the buzz

USE THE SENSOR YOU ALREADY OWN • Apple Watch: install N4x4 on your wrist and heart rate streams automatically • AirPods Pro 3: the earbuds you're already wearing double as your heart rate monitor (iPhone on iOS 26) • Garmin (Broadcast Heart Rate), WHOOP (HR Broadcast), Polar, Wahoo, COROS, Powerbeats Pro 2, and any standard Bluetooth chest strap or armband • Wearing more than one? You choose which source wins • A built-in guide with exact, per-model setup steps

EASY TO KEEP THE HABIT • Voice coaching through every interval, with the phone locked and your music ducked • Night-before, morning-of, and comeback reminders — each on its own switch • Weekly streaks, and a Live Activity on the Lock Screen and Dynamic Island

FINISH WITH PROOF • Every interval charted against its target zone • Time in zone, time to zone, and a note per interval for your settings • Last session laid over today's, so progress is easy to see • Your usual workout — Cycle, Run, Kettlebells, and more — pre-selected when you save

SEE THE PAYOFF • Your VO2 max trend from Apple Health • Training calendar, milestones, and full session history

Works on the treadmill, rower, bike, stair climber, with kettlebells, or on the open road — any cardio that gets your heart into the zone.

No account. No subscription. No ads. Your data stays on your device and in Apple Health.

Start with one hard session a week. Add a second when you're ready.