For iPhone and Apple Watch
The Norwegian 4x4, coached by your heart rate.
Four rounds of 4 min at 90 to 95 % of your max heart rate. During each round, Fjell tells you whether you will reach the zone by 2:00, while there is still time to push. The recap draws every round as a peak.
Coming soon to the App Store, for iPhone and Apple Watch.
See the halfway check7 days free, then 2.99 a month or 19.99 a year.
The free trial is for new subscribers. Same prices in USD, CAD and EUR, local prices elsewhere.
The halfway check
When a hard round starts, your heart rate takes a while to climb. Fjell watches the climb and tells you, while there is still time to push, whether you will be in the zone by 2:00.
Work
Time leftRound 1 of 4
0:00
In zone after 0:52
Heart rate
176 bpm
In zone Last 30 seconds. Give it everything.
Good start: heart rate climbs from 126 bpm and reaches the zone after 0:52, before the halfway check at 2:00. The round ends at 176 bpm, in the zone.
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 has not been tested as a standard. Read the science.
A simulated round for a max heart rate of 186 bpm, so the zone is 167 to 177 bpm (90 to 95 %). The 4 min round plays in about 12 s.
Your zones
Every zone is a share of your max heart rate. Enter your age, or your max heart rate if you know it.
15 to 90 years
140 to 220 bpm, the range Fjell accepts. When you enter it, it replaces the age estimate.
Max heart rate 186 bpm
An example. Enter your age or your max heart rate.
The formula, 208 − 0.7 × age, comes from 351 studies covering 18,712 people and was confirmed in a lab test of 514 more (Tanaka, Monahan, Seals, J Am Coll Cardiol 2001;37:153–156). It describes the average: individuals differ, and a measured maximum beats a formula. Fjell uses your hardest sustained effort from Apple Health when there is one, or a number you set. Nothing you type here leaves this page.
A session, step by step
A 10 min warm-up, four rounds of 4 min with 3 min of easy recovery between them, and a 5 min cool-down. Fjell talks you through every change, over your music.
0:00
Warm up. 10 minutes.
10 seconds. Round 1 next.Then
3,
2,
1, with beeps.
10:00
Round 1 of 4. Go. Target 167 to 177.
Halfway. In zone.or
Halfway. Not in zone yet. Push a little harder.
14:00
Recovery. 3 minutes. Keep moving, easy.
17:00
Round 2 of 4. Go.
Round 3 of 4. Go.
Last round. Go.
35:00
Cool down. 5 minutes.
Workout complete. Nice work.
Every change gets the same heads-up 10 s ahead and a 3, 2, 1 countdown with beeps. With Apple Watch, you also feel a tap on your wrist. Targets shown for a max heart rate of 186 bpm. Rounds, lengths and zones are all adjustable in Tune.
The app
The running screen is two instruments: time left on a block in the colour of the stage, and your heart rate under it, against the zone. The status line under the clock carries the halfway check.
A Live Activity and the Dynamic Island show the stage, the time left and your heart rate, with a pause button. Apple Watch gives a haptic before every change, so you feel it coming.
The recap draws your heart rate over the plan, with the time in zone above every round. Each round gets its time to zone and its halfway verdict.
The science
Every figure here is from the abstract of the paper it cites. Where an idea is Fjell's own, it says so.
In a trial of 40 healthy men, 4 x 4 min at 90 to 95 % of max heart rate, 3 times a week for 8 weeks, raised VO₂max by 7.2 %. The same total work at 70 % or 85 % of max heart rate raised it significantly less (P < 0.01). Stroke volume rose about 10 % and tracked the VO₂max gain.
Caveat: healthy, moderately trained men, over 8 weeks.
A meta-analysis of 28 controlled trials in 723 healthy adults aged 18 to 45 found that high-intensity intervals improved VO₂max by about 1.2 mL/kg/min more than continuous endurance training. Both beat no exercise: by 5.5 and 4.9 mL/kg/min.
Caveat: the edge is small. Both work.
Across 33 cohort studies, with about 103,000 people in the death analysis, each 1-MET higher aerobic capacity (about 3.5 mL/kg/min) went with a 13 % lower risk of death from any cause. Low fitness carried a 70 % higher risk than high fitness.
Caveat: an association from observational studies, not proof that training causes it.
Training raises the part of heart rate variability linked to vagal tone (13 studies, effect size 0.48). After one hard session, the heart's autonomic recovery takes at least 48 hours, so a dip the next day is expected. That is why Fjell shows a 7-night trend.
Caveat: HRV is personal, and in very fit athletes it can fall as fitness rises.
Oxygen uptake takes time to catch up when a hard interval starts. Intervals are thought to work best with several minutes a session above about 90 % of VO₂max. Being in the zone by halfway leaves the second half of the round at target.
Caveat: The halfway check is Fjell's own rule of thumb built on this. It has not been tested as a standard.
Every zone is a share of your max heart rate. Fjell takes your best 30-second heart rate in Apple Health over the last 6 months. If that is under 92 % of the age formula, 208 − 0.7 × age, it uses the formula instead. Or you set the number yourself.
Caveat: the 30-second window and the 92 % bar are Fjell's own choices. The formula describes the average; a measured maximum beats it.
Why pay for it
A timer counts the minutes. Fjell watches your heart rate through them and tells you, mid-round, whether you'll make the zone.
| Interval timer | Workout app | Fjell | |
|---|---|---|---|
| Times 4 rounds of 4 min with 3 min recovery | Yes | Yescustom workout | Yes |
| Heart rate against your 4x4 zone | No | Yesrange alert | Yes |
| Tells you mid-round whether you will be in the zone by halfway | No | No | Yes |
| Time to zone for every round | No | No | Yes |
| Streaks that count only finished 4x4s | No | No | Yes |
| Your HRV the night after each session, against your own baseline | No | No | Yes |
| Price | Often free | Built in | 2.99 a month |
Workout app: Apple's Workout app on Apple Watch, set up as a custom workout with a heart-rate range alert. Fjell's yearly plan is 19.99 a year.
Price and privacy
7 days free, then 2.99 a month or 19.99 a year. Cancel anytime in Settings.
What you pay for: the live halfway forecast, heart rate against your own zones, coaching on the lock screen and Apple Watch, recaps, streaks and recovery. No ads, no account, no tracking.
Your data stays on your iPhone and in Apple Health.
FAQ
No. Timer only gives you the voice cues, the beeps and the countdown, without heart rate. Apple Watch adds live heart rate, the halfway check, haptics on your wrist, and the overnight HRV that Recovery reads.
An iPhone with iOS 18 or later. Apple Watch is optional and needs watchOS 11 or later.
Yes. Cues and beeps play over your music and podcasts, also with the screen locked. To have the music dip while Fjell speaks, turn on Lower music during cues in Tune.
Open the Settings app, tap your name, then Subscriptions, Fjell, Cancel Subscription. Cancel at least 24 hours before the current period ends, or the subscription renews.
The 4x4 is hard. If you have a heart condition or other health concerns, talk to a doctor first. Fjell is not medical advice.
If you are new to hard training, start easier, with fewer rounds or a lower zone (both are in Tune), and stop if you feel unwell.
Anywhere you can work hard for 4 min at a time: on a treadmill, up a hill outdoors, on a bike, a rower or cross-country skis, or on stairs.
More answers on the support page.
