The I1–I5 federation scale
The Norwegian Olympic federation's I-scale pairs heart-rate bands with lactate guideposts: I1 (60–72% HRmax) below 2 mmol/L, I2 (72–82%) around 2, I3 (82–87%) at 2.5–4, I4 (87–92%) at 4–6 and I5 (92–100%) above 6 mmol/L. I3 marks the threshold transition.
Why Nordic training is polarized
World-class Nordic programs log large volumes of easy I1–I2 work and a small dose of hard I3–I5 intervals. Seiler's analyses put roughly 80% of sessions at low intensity and 20% hard. Easy volume builds the base with low fatigue.
The biathlon shooting demand
Biathletes arrive at the range in I4–I5 and must drop heart rate within seconds to steady the rifle. Many train approach-and-shoot sequences to learn how fast it falls.
Worked example
For a skier or biathlete with a maximum heart rate of 190 bpm:
| I1, Easy (60–72%) | 114–137 bpm |
| I2, Steady (72–82%) | 137–156 bpm |
| I3, Threshold (82–87%) | 156–165 bpm |
| I4, VO₂max (87–92%) | 165–175 bpm |
| I5, Anaerobic (92–100%) | 175–190 bpm |
Most weekly time sits in I1–I2. I3–I5 is for a handful of hard sessions.
Frequently asked questions
What is the I1–I5 training scale?
The I1–I5 scale is the Norwegian Olympic federation's five-zone intensity model for cross-country skiing, biathlon and nordic combined. I1 (60–72% HRmax) is easy, I5 (92–100%) anaerobic, each with a lactate guidepost from under 2 mmol/L to over 6.
Why do Nordic skiers train so much easy volume?
Nordic skiing has a very high aerobic demand, and that base is built most efficiently with large volumes of I1–I2 work. Polarized programs put roughly 80% of sessions at low intensity and 20% hard.
How do I1–I5 zones map to lactate?
I1 sits below 2 mmol/L, I2 around 2, I3 at 2.5–4 mmol/L (the threshold transition), I4 at 4–6, and I5 above 6 mmol/L. Lactate anchoring keeps the zones comparable across athletes.
How do biathletes control heart rate for shooting?
Biathletes arrive at the range in I4–I5 and must drop heart rate within seconds to steady the rifle. They train approach-and-shoot sequences to learn how quickly it falls.
Is I3 the same as my lactate threshold?
Roughly, yes. I3 spans 82–87% of maximum heart rate and 2.5–4 mmol/L lactate, where lactate begins to climb steeply. At a 190 bpm maximum that is 156–165 bpm.
Sources
- Norwegian Olympic Federation (Olympiatoppen). The I1–I5 intensity-scale used across cross-country skiing, biathlon and nordic combined.
- Seiler & Tønnessen (2009). “Intervals, thresholds, and long slow distance: the role of intensity and duration in endurance training.” Sportscience 13:32–53.