REF-304 · Seiler KS & Kjerland GØ, 2006 · Scandinavian Journal of Medicine & Science in Sports, · Paper · PubMed
Quantifying Training Intensity Distribution in Elite Endurance Athletes: Is There Evidence for an "Optimal" Distribution?
11 nationally competitive junior XC skiers over 32 days: ~75% of endurance sessions in zone 1, 6-8% in zone 2, 17-18% in zone 3 — a polarized pattern with little training at threshold
Participants
n=11 male well-trained junior cross-country skiers, nationally competitive
Protocol
Treadmill tests to exhaustion to determine heart rate and VO2 at the ventilatory thresholds (VT1, VT2), VO2max and maximal heart rate; VT1 and VT2 delineated three intensity zones
All training sessions over 32 consecutive days (N=384: 37 strength, 347 endurance) quantified by continuous heart-rate registration and session RPE
A subset of 60 consecutive sessions additionally quantified with blood lactate
Key Results
Endurance sessions (n=318) by heart rate: 75 ± 3% zone 1, 8 ± 3% zone 2, 17 ± 4% zone 3; by session RPE: 76 ± 4% zone 1, 6 ± 5% zone 2, 18 ± 7% zone 3 — the two methods agreed
Of the 60 lactate-measured sessions: 71% at ≤ 2.0 mM, 7% between 2 and 4 mM, 22% above 4 mM (mean 9.5 ± 2.8 mM in those sessions)
Training was organised after a POLARIZED pattern: most sessions clearly below (about 75%) or with substantial periods above (15-20%) the "lactate accommodation zone" bounded by VT1 and VT2 — zone 3 exceeded zone 2
The authors note the pattern matches observational studies of elite endurance athletes across several sports: elite endurance athletes train surprisingly little at lactate-threshold intensity
Mechanism
Descriptive study — no mechanism tested. The authors' reading: a high share of clearly easy sessions plus a meaningful share of clearly hard ones, with the zone between VT1 and VT2 (threshold) used sparingly. Inference, not tested: the easy majority allows the hard minority to be done well; heavy use of the middle zone would add fatigue without adding a distinct stimulus.
Practical Application
In 11 nationally competitive junior XC skiers, ~75% of endurance sessions were in zone 1 (below VT1), only 6-8% in zone 2 (between VT1 and VT2) and 17-18% in zone 3 (above VT2) — session distribution counted by sessions, with heart rate and session RPE agreeing
This is the descriptive origin of the three-zone (VT1/VT2) framework and of the "polarized" label: zone 3 was larger than zone 2, and the threshold zone was the least used
Session RPE reproduced the heart-rate-based distribution (76/6/18 vs 75/8/17%), so session RPE alone is an adequate way to audit one's own intensity distribution
Not from this paper; the base's own practice: count sessions, not minutes, when checking your own distribution against this pattern (the paper counted sessions); if most weeks show a large share in zone 2, that is the "drift toward the middle" this pattern argues against
Whether polarized beats other distributions was tested later — see Stöggl & Sperlich 2014 REF-303
Limitations
Small sample (n=11), single sport (XC skiing), 32-day window
Descriptive — no intervention, no performance outcome
Junior athletes — may differ from senior elite
Distribution reported by session count, not by time in zone
How this entry was checked and changed
Why this level
regraded 2026-09-27 from B+: descriptive observational study, n=11, no intervention
Content correction 2026-09-27
"n=9" -> n=11 (the abstract: eleven male athletes).
"~75% zone 1, ~15% zone 2, ~10% zone 3 — pyramidal" -> the abstract: 75-76% zone 1, 6-8% zone 2, 17-18% zone 3 (sessions), i.e. a POLARIZED pattern (zone 3 > zone 2), which is what the authors call it; lactate subset 71% / 7% / 22%.
Added the protocol facts from the abstract (32 days, 384 sessions, 60 lactate-measured sessions, VT1/VT2 zones).
Tier 4-5 -> Tier 3 (nationally competitive juniors); Level B+ -> B (descriptive observational, n=11).