Kelemen et al. 2024, The Norwegian double-threshold method

Systematic review of 7 studies tracking 13 elite Norwegian middle- and long-distance runners (Scientific Journal of Sport and Performance 2024). They run ~120–180 km/week, with 75–80% at low intensity (Zone 1, under 2 mmol/L lactate, 62–82% HRmax) and 15–20% at the anaerobic threshold (Zone 2, defined as 2.0–4.0 mmol/L, 82–92% HRmax). The signature is the “double-threshold day” — two controlled sub-threshold interval sessions in one day (morning kept below ~2.5 mmol/L, afternoon below ~3.5 mmol/L) — performed two to four times a week, with intensity policed by lactate meter and heart rate. Intense Zone 3 aerobic-capacity intervals (4.0–8.0 mmol/L) are largely avoided in the base period. Description strong; causal superiority over other intensity distributions not established.