Seiler & Kjerland 2006, Intensity distribution in elite endurance athletes

Sample was 11 nationally competitive junior male cross-country skiers, not a broad elite cohort. They clustered ~75-80% of sessions easy, 15-20% hard. The polarised pattern was later supported by wider observational work; this paper alone is small and sport-specific. The 80/20 split is sensitive to whether it is counted by session or by time-in-zone. Moderate (small, descriptive).