Norwegian 4×4Heart-rate ERG training for your smart trainer

The Norwegian 4×4 — four hard four-minute efforts — on your Bluetooth smart trainer, with the resistance adjusting itself every block to hold your heart rate in the 90–95% zone.

Download on the App Store See how it works

One-time purchase · iPhone · iOS 17 or later

01

The App

A look

Made to read at arm's length.

  1. Norwegian 4x4 session screen: heart rate, target watts, cadence and a circular countdown during a work block
  2. Norwegian 4x4 home screen with a trend chart of power at target heart rate over recent sessions
  3. Norwegian 4x4 session history list, each row exportable as a .fit file
  4. Norwegian 4x4 in-app screen explaining the 4x4 interval protocol and the supporting research
  5. Norwegian 4x4 pairing screen listing discovered Bluetooth FTMS trainers sorted by signal strength

Heart rate, target watts, time left — one glance, at effort.

What it does

It sets your heart rate, not your watts.

A fixed wattage ignores how you show up on the day — the same 300 W can be 88% of your max after good sleep and 97% after a rough night. You pick the zone; the app finds the watts, and every session it shows the one number that matters: how many it took to hold 90–95%.

Norwegian 4×4

Just ride. Resistance shifts each block to keep you at 90–95% of max.

Trainer support

Works with your smart trainer.

If it speaks FTMS — the Bluetooth standard for trainer control — Norwegian 4×4 can drive it. Tested on JetBlack and Wahoo; the standard covers most modern trainers.

Wahoo Kickr Tacx Elite Saris JetBlack Zwift Hub Van Rysel Any FTMS trainer

Heart rate comes from any Bluetooth strap or sensor, or the trainer's own bridge.

Try it on your next interval day.

Download on the App Store

One-time purchase · iPhone · iOS 17 or later

02

The Protocol

Structure

The real 4×4.

4 × 4 minWork @ 90–95% HRmax
3 minActive recovery
4Intervals
16 minTotal work

The interval session developed at NTNU Trondheim. In the trial that made its name, the 4×4 group gained roughly twice the VO₂max of steady training over eight weeks — for the same time on the bike.

Helgerud J, et al. Aerobic high-intensity intervals improve VO₂max more than moderate training. Med Sci Sports Exerc. 2007;39(4):665–71. doi:10.1249/mss.0b013e3180304570

03

FAQ

Questions

Before you buy.

Which smart trainers does it work with?

Any Bluetooth trainer that supports FTMS, the standard trainer-control profile — Wahoo Kickr, Tacx, Elite, Saris, JetBlack, Zwift Hub, Van Rysel and others. Tested on JetBlack and Wahoo hardware.

Do I need a heart-rate strap?

You need some heart-rate source. A Bluetooth chest strap is most accurate, but any Bluetooth heart-rate sensor works, and some trainers bridge heart rate themselves.

Does it need Apple Health?

No. Health is optional and every metric works with no permission granted. If you allow it, sessions save as indoor cycling workouts and your VO₂max and resting heart rate help set targets.

Is there a subscription?

No. One payment, no account, no cloud, no ads.

Can I get sessions into Strava, Garmin or TrainingPeaks?

Every session writes a standard .fit file to the Files app. Upload it anywhere that accepts .fit.

What is the Norwegian 4×4 (4x4)?

Four four-minute efforts at 90–95% of maximum heart rate, each followed by three minutes easy. An interval session developed at NTNU Trondheim, known for large VO₂max gains for the time invested. It's been tested across running, skiing and cycling — this app is built specifically for the cycling version, on a smart trainer.

Norwegian 4×4 App Store