Most Norwegian 4x4 timers count the clock. Stellar Intervals counts what matters: the time your heart is truly in the zone — and how fast it recovers afterward.

THE PROBLEM WITH CLOCK-BASED 4x4
The Norwegian 4x4 protocol calls for four 4-minute intervals above ~85% of your max heart rate. But it takes time to climb into that zone — often 60 to 90 seconds. A standard timer counts that ramp-up as "work," so you get an uneven, watered-down dose every session. Two workouts that look identical on the clock can be completely different efforts.

TRUE TIME IN ZONE
Stellar Intervals fixes this with a heart-rate-gated timer. The interval clock starts only when you cross your 85% threshold — and pauses the instant you drop below it. No more junk time. Every session delivers a true 16 minutes of high-intensity stimulus. Repeatable, predictable, honest.

A CLEAN SIGNAL: HEART-RATE RECOVERY
Here's what true time in zone unlocks. Heart-rate recovery (HRR) — how many beats per minute your heart drops in the first minute after an interval — is one of the most studied markers of cardiovascular fitness. But it's only meaningful if the effort before it was consistent. Because Stellar Intervals standardizes every interval to the same load, your HRR becomes a clean, comparable number you can trust. Track it interval to interval and week to week, and watch your fitness improve in a metric measured from your own training.

THE SCIENCE
The Norwegian 4x4 protocol is the gold-standard interval method for improving VO2 max — and VO2 max is one of the strongest predictors of long-term health and longevity. Stellar Intervals helps you execute the protocol precisely and follow your progress through HRR, a marker researchers have long tied to heart health and aerobic fitness.

FEATURES
Heart-rate-gated timer — counts only true time in zone
Auto-pause when you fall below 85% HRmax
Per-interval heart-rate recovery (HRR) tracking
Long-term HRR and time-in-zone trends
Works with Apple Watch and Bluetooth chest straps
Sport-agnostic: run, bike, row, ski, or elliptical
Apple Health integration
Clean, distraction-free workout screen

HONEST BY DESIGN
Stellar Intervals does not estimate or claim to measure your VO2 max — no app can do that accurately from your wrist. What we do measure, we measure for real: your true time in zone and your heart-rate recovery. Two numbers you can trust, both improving as you train.

WHO IT'S FOR
Runners, cyclists, and rowers chasing VO2 max gains. Longevity-minded people who already track HRV, sleep, and biomarkers. Anyone who wants their 4x4 done with precision instead of guesswork.
Requires a heart rate source (Apple Watch or Bluetooth HR monitor).
Start your first true interval today.