Train your VO₂ max

A dead-simple iPhone interval timer for the Norwegian 4x4 protocol. Or any custom interval workout you want to build.

Home screen showing warm-up, work, rest, interval, and cool-down settings above a workout timeline chart — Set it up once

Live workout screen with a countdown ring reading 3:58 remaining during work interval 1 of 4 — Run the workout

Settings screen with sound and vibration toggles and separate transition sounds for work and rest — Pick your alerts

Customizable intervals

Configure the duration of warm-up, work, rest and cool-down, as well as the number of sets.

Sound and vibration alerts

Never miss a phase change! Tones play at every transition, even when the app is in the background, and when you're listening to music or a podcast.

Local-first privacy

Get started immediately with no sign-up or account required. Your settings and workout history stay completely private and local to your iPhone.

What is the Norwegian 4x4 protocol?

It's a high-intensity interval training (HIIT) protocol designed to maximize VO₂ max and cardiovascular health and performance. It consists of four intervals of four minutes each, performed at 85-95% of your max heart rate, with three minute rest/active recovery periods in between.

What is VO₂ max?

Also known as the maximal oxygen consumption, or maximal aerobic capacity, it's the maximum rate of oxygen consumption during intense exercise. It's a measure of your cardiovascular fitness and endurance.