VO2 Flow: Norwegian 4x4 Timer

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.