The protocol
A full session is 35 minutes from the first easy pedal stroke to the last. The interval and recovery durations are fixed, and that is what makes it the 4x4 rather than just some intervals. The warm-up and cool-down are yours to adjust.
| Phase | Duration | Target effort | % of HRmax |
|---|---|---|---|
| Warm-up | 5:00 | Easy, conversational | 50 to 70% |
| Interval 1 | 4:00 | Hard, a few words at a time | 85 to 95% |
| Recovery 1 | 3:00 | Easy, keep moving | 60 to 70% |
| Interval 2 | 4:00 | Hard | 85 to 95% |
| Recovery 2 | 3:00 | Easy, keep moving | 60 to 70% |
| Interval 3 | 4:00 | Hard | 85 to 95% |
| Recovery 3 | 3:00 | Easy, keep moving | 60 to 70% |
| Interval 4 | 4:00 | Hard, same as the first | 85 to 95% |
| Cool-down | 5:00 | Very easy | 50 to 60% |
How to run a session
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Know your maximum heart rate
If you have never tested it, estimate: 208 - 0.7 x your age. A 40-year-old lands around 180 bpm, which puts the interval range at roughly 153 to 171 bpm.
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Pick something you can hold steady
A stationary bike, a treadmill incline, a rowing machine, a hill. Anything where you can raise effort smoothly and will not have to stop for traffic.
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Warm up for five minutes
Not a formality. Going into the first interval cold makes it feel far worse than it should and skews the whole session.
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Build into each interval
Take the first 60 to 90 seconds to climb into the target range, then hold it for the rest. Starting at a sprint means fading before four minutes is up.
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Recover actively
Keep the legs turning through the three minutes. You are not meant to fully recover. Arriving at the next interval slightly elevated is part of the design.
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Finish all four the same
The best-run session has four near-identical intervals. If number four falls apart, number one was too hard.
How hard is hard?
85 to 95 percent of maximum heart rate is specific enough to aim at and vague enough to get wrong, so use effort as the cross-check. During an interval you should be able to get out three or four words, not a sentence. You should want it to end. You should also be confident you could do it again after three minutes off, because you will, three more times.
Heart rate lags effort by 30 to 60 seconds, so judge an interval by where your heart rate sits in the last two minutes, not the first.
A note on measurement
Wrist-based heart rate is good at steady effort and less good at sudden changes. If a reading looks impossible, like 40 bpm mid-interval or 190 during a recovery, it is usually the sensor, not you. Tighten the strap a notch above the wrist bone and give it a minute. A Bluetooth chest strap reads more accurately during hard efforts if you want the most reliable numbers.
How often should you do it?
Once or twice a week, with at least 48 hours between sessions. It is a genuinely hard workout and it does not stack well. Three or four a week produces worse results than two, because you stop being able to hit the target range.
Fill the rest of the week with easy aerobic work. The 4x4 is the sharp end of a training week, not the whole of it.
Common mistakes
- Starting too hard. The most common one by far. Interval one at 98 percent guarantees interval four is a survival exercise.
- Recovering too completely. Stopping dead for three minutes turns the session into four separate efforts instead of one continuous stimulus.
- Shortening the interval. Three-minute intervals are a different workout. The final minute is where most of the benefit is.
- Skipping the warm-up. Costs you the first interval and makes the session feel harder than it is.
- Judging by pace instead of heart rate. On a bad day the same effort is slower. Chase the heart rate range, not the numbers on the display.
What to track
Two numbers tell you most of what you need to know over a few months:
- Time in zone. How much of each interval you actually spent in the target range. Above 50 percent is a well-executed session.
- One-minute heart rate recovery. How far your heart rate drops in the 60 seconds after an interval ends. As aerobic fitness improves, that drop gets bigger.
Neither is a medical measurement. Both are useful as trends rather than as single readings.
Running it with the app
Norwegian 4x4 does the timing and the zone math so you can keep your eyes off the screen. It announces each phase, shows a live target range using your Apple Watch or any Bluetooth heart rate monitor, and tells you in plain words whether to push harder, hold, or ease up. Afterwards it saves the session to Apple Health and charts your time in zone and recovery across sessions.
This guide is general training information, not medical advice. High-intensity interval training is demanding. If you are new to it, pregnant, or managing a heart condition, speak to a doctor before starting.