Questions, answered.

The Norwegian 4x4 is an interval workout: 4 minutes at 85-95% of your max heart rate, then 3 minutes easy, repeated 4 times, with a warm-up and cooldown. Developed at NTNU in Norway, it's one of the most studied methods for raising VO2 max.

No watch, no problem: Viking 4x4 works as a fully guided timer. With an Apple Watch you get live heart rate, zones, interval reports, and recovery tracking.

Other apps time the protocol. Viking 4x4 measures whether it's working: per-interval heart rate reports and 60-second heart rate recovery trends over time.

1-3 times per week. Once weekly was the dose used in the two-year study that improved VO2 max 18%. More isn't better for this protocol.

It's demanding. Start with shorter work intervals and build up, and talk to your doctor before starting any high-intensity program, especially with existing heart conditions.

85-95% of your max heart rate, sustained for the full 4 minutes. The app shows a live target and tells you when you're in the zone.

Yes. Workouts save with your chosen activity type, heart rate samples, and calories. Your data stays on your device and in your Health app. No account required.

The drop in your heart rate 60 seconds after peak effort. It reflects cardiac efficiency, and in a major NEJM study, slower recovery predicted mortality. Watching it improve is the clearest sign the 4x4 is working.

Free at launch on iPhone.

Yes. Any cardio you can push to 85-95% max HR works. The app supports running, cycling, rowing, elliptical, stair stepper, and more.