Fjell: Norwegian 4x4 Intervals
Heart rate coach for VO2 max
Free · In‑App Purchases
Climb four peaks. Fjell coaches the Norwegian 4x4 by your heart rate and tells you during every interval whether you’ll reach your zone by halfway. 7 days free.
Climb four peaks. Fjell coaches the Norwegian 4x4 by your heart rate: four rounds of 4 minutes at 90 to 95 % of your max heart rate, with 3 minutes of easy recovery between them. Afterwards your heart rate draws every round as a peak. Fjell is Norwegian for mountain.
KNOW BY MINUTE TWO
During every work interval, Fjell’s halfway check watches how fast your heart rate climbs and tells you whether you will reach your zone by the 2:00 mark: “On pace · 5 bpm to go by 2:00” or “Behind · 9 bpm to go by 2:00”. Once you are in, it tells you how long that took: “In zone after 0:52”. The halfway check is Fjell’s own rule of thumb, built on how oxygen uptake catches up at the start of a hard interval. It expects your heart rate to level off, not climb in a straight line.
COACHED BY YOUR HEART RATE
• Live heart rate from Apple Watch against your zone: green in the zone, amber above it, with arrows that tell you to push or ease off.
• Hear when you reach the zone: a short chime, then “In zone”, or “Back in zone” when you return to it.
• Voice cues and beeps, even with the screen locked: a 10-second heads-up, a 3-2-1 countdown and the round. Your music dips while Fjell speaks and podcasts pause, then both come back; you can turn that off.
• A Live Activity on the lock screen and in the Dynamic Island with the time left, your heart rate and a pause button.
• Taps on Apple Watch: a soft heads-up 10 seconds before each change, a distinct tap as it happens, and in work rounds a tap when you reach the zone, go above it or drop below it. Apple Watch keeps to your plan even while your iPhone is asleep.
• Zones from your own max heart rate: your best 30-second heart rate in Apple Health, the age formula 208 − 0.7 × age when there is none, or a number you set. If your hardest 30 seconds beat it, the recap offers the new number in one tap.
SEE EVERY ROUND
• A recap of the whole session: every round as a peak, all rounds on one chart, and your minutes in zone, above and below. Each round shows its time to zone, ramp and time back into recovery, and your quickest round is marked, or “Best yet” when it beats every earlier session like it.
• Over time: minutes in zone, time to zone and time back into recovery across your last 8 sessions like this one.
• History and streaks that count only finished 4x4 sessions.
• See how you recover from each session: your overnight heart rate variability against your own baseline.
• Make it yours: rounds, interval lengths, zones and the halfway point are all adjustable. Run, walk uphill, cycle, row, ski or climb stairs.
WORKS WITH APPLE HEALTH
With your permission, Fjell reads your heart rate, heart rate variability, sleep and date of birth from Apple Health to set your zones and show your recovery. It saves your interval workouts to Apple Health.
PRIVATE BY DESIGN
No account. No ads. No analytics. No tracking. Your data stays on your iPhone and in Apple Health.
NO APPLE WATCH?
Choose Timer only for voice cues, beeps and the countdown without heart rate. Apple Watch adds live heart rate, the halfway check, the zone chime and taps on your wrist.
The 4x4 is hard. If you have a heart condition or other health concerns, talk to a doctor first. Fjell is not medical advice.
SUBSCRIPTION
New subscribers get a 7-day free trial. After that, Fjell costs 2.99 USD a month or 19.99 USD a year. Prices vary by country. Payment is charged to your Apple Account when you confirm the purchase, or when the free trial ends. Your subscription renews automatically unless you cancel it at least 24 hours before the end of the current period. Manage or cancel it any time in Settings.
Terms of Use: https://www.apple.com/legal/internet-services/itunes/dev/stdeula/
Privacy Policy: https://fjell4x4.com/privacy
Ratings & Reviews
- This app hasn’t received enough ratings or reviews to display an overview.
• Hear when you reach the zone: a short chime, then “In zone” (or “Back in zone” if you dropped out), from your iPhone over your music.
• Music lowers while Fjell speaks, from the 10-second heads-up through Go, then comes back, and podcasts pause and pick up again. It is on by default; turn it off in Tune › Sound › Lower music during cues.
• Apple Watch shows the halfway check too, under the countdown in each work round. It taps when you enter the work zone, go above it or drop below it, gives each stage change its own tap and a softer 10-second heads-up, and keeps to your plan on its own while your iPhone is asleep.
• A fuller recap: all rounds on one chart, minutes in zone, above and below, and time into the zone and back into recovery. Your quickest round is marked (“Best yet” when it beats every earlier session like it), and Over time shows your last 8 sessions like this one.
Also: a steadier halfway check that says “Push harder” only when you are clearly behind, swipe to delete a workout in History, a bolder work block and a new app icon.
Fixes: “bpm” no longer breaks onto a second line beside the target zone on the workout screen, the trial reminder switch in Tune now says when notifications are off for Fjell and how to turn them on, more room at the screen edges on Apple Watch, and Fjell keeps timing on your iPhone if another workout app takes over your Apple Watch.
The developer, David van Rooij, indicated that the app’s privacy practices may include handling of data as described below. For more information, see the developer’s privacy policy .
Data Not Collected
The developer does not collect any data from this app.
Accessibility
The developer has not yet indicated which accessibility features this app supports. Learn More
Information
- Seller
- David van Rooij
- Size
- 9.2 MB
- Category
- Health & Fitness
- Compatibility
Requires iOS 18.0 or later.
- iPhone
Requires iOS 18.0 or later. - Apple Watch
Requires watchOS 11.0 or later.
- iPhone
- Languages
- English
- Age Rating
9+
- 9+
- Contains
Health or Wellness Topics
- In-App Purchases
Yes
- Fjell Yearly $19.99
- Fjell Monthly $2.99
- Copyright
- © 2026 David van Rooij
