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NTRVL requires AirPods Pro 3 or Powerbeats Pro 2 with iOS 26 or later. Make sure your AirPods are connected and the in-ear detection sensor is clean. Heart rate data flows through Apple Health — check that NTRVL has read/write permissions in Settings → Health → NTRVL.

Apple Watch is an optional companion screen — it shows your workout in real-time (HR arc, zone, countdown). However, heart rate data must come from AirPods Pro 3 or Powerbeats Pro 2. Apple Watch alone is not sufficient for HR monitoring in NTRVL.

It's a scientifically validated interval training method from Helgerud et al. (2007): 4 intervals of 4 minutes at Zone 4 (85–95% max HR), separated by 3 minutes of active recovery at Zone 2. Total workout time is about 40 minutes including warm-up and cool-down.

NTRVL uses the Karvonen Heart Rate Reserve (HRR) method with your max HR estimated via the Tanaka formula (208 − 0.7 × age). You can override your max HR in Settings if you know your actual measured max.

Everything stays on your device. NTRVL has no server, no cloud, no accounts. Workout data is stored in Apple Health and locally on your iPhone. See our Privacy Policy for details.

Go to iPhone Settings → your name → Subscriptions → NTRVL → Cancel. Subscriptions are managed entirely by Apple. You'll keep Pro features until the end of your current billing period.

NTRVL uses your resting heart rate from Apple Health to calculate zones. If Health hasn't received a recent resting HR reading, your zones might be slightly off. Wearing your Apple Watch to sleep or checking your resting HR periodically will keep the data fresh.