Norwegian 4x4 Heart Rate

Norwegian 4×4 is the Helgerud VO2max interval protocol, built for athletes who want to push their aerobic ceiling. Connect any Bluetooth heart-rate monitor — Whoop broadcast, Polar H10, Wahoo TICKR, Garmin HRM, or any device exposing the standard heart-rate service — and the app guides you through a 5 min warm-up, 4 × (4 min hard / 3 min easy), and 5 min cool-down. A live heart-rate gauge shows when you're in the target zone (85–95% HRmax during work intervals, 60–70% during rest). The app speaks "Speed up" or "Slow down" until you hit the zone. Audio cues mark each phase transition; a 9-second countdown gives you time to start working out. All data stays on your phone. No accounts, no analytics, no servers.