The Norwegian 4x4, explained

What it is

The Norwegian 4x4 is an interval workout. You do four rounds of 4 min at 90 to 95 % of your max heart rate. Between rounds you keep moving for 3 min at an easy effort, at about 70 % of max heart rate. With a warm-up before and a cool-down after, the whole session takes about 40 min.

The name describes the format: four rounds of 4 min. It is the protocol Helgerud and colleagues tested in a 2007 trial, where it raised VO₂max more than the same amount of work done at a lower intensity. VO₂max is the highest rate at which your body can take up and use oxygen during exercise, a common measure of aerobic fitness.

The session, minute by minute

This is the standard session, as Fjell runs it by default. Start times are in minutes and seconds from the beginning.

• There are three recoveries, not four: round 4 runs straight into the cool-down.
• Recovery is active. Walk, jog or pedal easily rather than stopping. The trialled protocol used about 70 % of max heart rate; Fjell's default recovery band is 50 to 70 %.
• The warm-up and cool-down targets are Fjell's defaults. You can change any of it in Fjell's Tune screen.

Your max heart rate and zones

Every target in the 4x4 is a share of your max heart rate, so the number you start from matters.

The age formula

The age formula Fjell uses is 208 − 0.7 × age. It comes from an analysis of 351 studies covering 18,712 people, confirmed in a lab test of 514 more. At age 35 it gives 184 bpm; at age 41, 179 bpm.

It describes the average. Individuals differ, and a measured maximum beats a formula.

Tanaka, Monahan, Seals, J Am Coll Cardiol 2001;37:153–156. doi.org/10.1016/s0735-1097(00)01054-8

A measured max

If you have a reliable reading from a recent all-out effort, use it. A wrist sensor can show brief spikes that are not real, so trust a value you held for a while over a single peak. That is why Fjell looks at your best 30-second heart rate rather than the highest single reading.

How Fjell sets it

• Automatic: your best 30-second heart rate in Apple Health over the last 6 months, unless it is under 92 % of the age formula; then the formula.
• Manual: any number from 140 to 220 bpm, set in Tune.

Zones for common max heart rates

Each value is max heart rate times the percentage, rounded to the nearest whole beat, the way Fjell calculates it.

With a max of 186 bpm, for example, the work zone is 167 to 177 bpm, and Fjell starts the first round with "Round 1 of 4. Go. Target 167 to 177."

How it should feel

Hard, but steady. You should be able to finish all four rounds at the same effort. If round 4 falls apart, round 1 was too hard.

Heart rate needs time to climb. When a hard interval starts, oxygen uptake takes time to catch up, so the first part of each round is catch-up. Aim to reach the zone within the first half of each round, then hold it there.

Fjell helps with a halfway check. During each round it tells you whether you are on pace to be in the zone by 2:00, with lines like "On pace for the 2:00 check" or "About 6 bpm short of the zone at 2:00". Once you are in, it shows how long that took, for example "In zone after 0:52".

The halfway check is Fjell's own rule of thumb built on this. It has not been tested as a standard.

In recovery, slow right down but keep moving, so your heart rate can come down before the next round.

How often

In the trial behind the protocol, healthy, moderately trained men did 3 sessions a week for 8 weeks.

A common pattern is one or two sessions a week, alongside easier training. Give it time: the trial ran for 8 weeks.

This is general guidance, not a prescription. If you are not sure what suits you, ask a doctor or a qualified coach.

Fjell's weekly goal starts at one session a week, and you can change it in Tune. Its Recovery tab shows how your overnight heart rate variability responds, as a trend against your own baseline. A dip for a night or two after a hard session is normal.

Where to do it

Anything that lets you work hard for 4 min, then keep moving easily for 3 min:

• a treadmill with an incline,
• a hill outdoors,
• a bike or an indoor bike,
• a rowing machine,
• cross-country skis,
• stairs or a stair machine.

In Tune, under Workout type, Fjell has Running, Incline walking, Cycling, Rowing, Elliptical, Cross-country skiing, Stair climbing and Other (HIIT), with an Indoor switch. Your choice sets the workout type saved to Apple Health.

Common mistakes

• Starting too hard. Round 1 feels easy, so you push, and rounds 3 and 4 fall apart. Build into the zone instead.
• Staying too easy. If you never reach 90 % of max, you are doing a different workout.
• Standing still in recovery. The trialled protocol used active recovery at about 70 %. Keep walking, jogging or pedalling easily.
• Skipping the warm-up. Give yourself the 10 min to build up before round 1.
• Chasing pace instead of heart rate. The 4x4 is set by heart rate. Hills, heat, wind and tiredness change the pace it takes, so let heart rate set the effort.
• Trusting a formula over a measured max. The formula describes the average. If your own hard efforts say otherwise, go with them.
• Forgetting the lag. Heart rate trails effort, and a wrist sensor adds a few seconds more. Build steadily and give the number a moment before you push harder.

What the research shows, and what it does not

Every figure below comes from the abstract of the paper cited.

What is not proven

• That the 4x4 makes you live longer. In the five-year Generation 100 trial in older adults, the group assigned to interval training had a lower death rate than the control group, but the difference was not statistically significant. Stensvold et al., BMJ 2020;371:m3485. doi.org/10.1136/bmj.m3485
• That fitness itself causes a longer life. Fitness going with longer life is an association, not proof of cause.
• That the halfway check is the right target. It is Fjell's own rule of thumb and is untested as a standard.
• That your results will match the averages. These are group averages from specific populations, and your response will differ.

Questions