N4x4 vs VO2 Flow

Supports iPhone Duo, with expanded workout views and hands-free training when propped open. The best Norwegian 4x4 App on the App Store. 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.

VO2 Flow is built for the Norwegian 4x4 protocol, the research-backed interval method athletes use to raise VO2max. Four 4-minute intervals at 90 to 95% max heart rate, with 3-minute recoveries. Studies show a 10% VO2max increase in 8 weeks. Run Norwegian 4x4 or build your own protocol. Tabata. 10x1. 8x2. Any work and rest combination, any number of rounds. Free in the app: - Norwegian 4x4 protocol built-in and ready to start - Workout history, every completed session logged and grouped by month - One custom interval profile - Apple Music integration, album art and controls during your session - Audio cues and haptic feedback on every phase transition - Keep screen awake during workouts - Landscape and portrait support Pro upgrade, $4.99 one-time: - Remove all ads forever - Unlimited custom interval profiles - Achievement badges, 7 milestones you can share - Every future Pro feature, no subscription This is a timer, not a fitness platform. No social feed. No subscription. No complexity. Launch, pick a profile, start.