Norwegian 4x4. The Protocol.

The most studied HIIT protocol for improving VO2 max. Four hard intervals. Thirty-five minutes total. Twice a week.

The Protocol
  • Warm up for 10 minutes at easy effort.
  • Run at 85–95% of maximum heart rate for 4 minutes.
  • Recover at easy effort for 3 minutes.
  • Repeat 4 times.
  • Cool down for 5 minutes.
Effort
  • Work intervals — hard enough that you can't hold a conversation. 85–95% of maximum heart rate.
  • Recovery intervals — easy enough to bring your heart rate down. 70% of maximum heart rate or below.
  • No heart rate monitor? Use perceived effort — 8–9 out of 10 during work, 5–6 during recovery.
What It Does
  • Raises VO2 max by approximately 7–10% in 8 weeks.
  • Improves heart efficiency and cardiovascular endurance.
  • Produces the strongest acute nitric oxide effect of any training method.
Caveats
  • Do not attempt without a base of aerobic fitness. Build Zone 2 first.
  • Twice a week is enough. More is not better.
  • Works on any cardio modality — running, cycling, rowing.

PS. It is not complicated. It is just hard.