REF-1222 · Solli, Tønnessen & Sandbakk, 2019 · Frontiers in Physiology · Paper · PubMed · Free full text

Must my hard intervals come late in the build, near the race?

Not necessarily: one world-class skier won with HIT heaviest early in concentrated blocks and, years later, with HIT rising toward racing. One athlete, no tests.

C+thin evidenceMeasured on world-class athletes

Solli and colleagues compared two seasons of the world's most successful cross-country skier, both of which brought World Cup victories in sprint and distance races and the overall and sprint World Cup titles: one using block periodization of high-intensity training (BP, 2005-2006) and one using a traditional model (TRAD, 2014-2015).

The seasons were picked for success, equal endurance training load (TRIMP: minutes at low, moderate and high intensity weighted 1, 2 and 3) and detailed records.

Weekly load showed no evidence of a difference (1058 ± 368 vs 1084 ± 339 TRIMP; the seasons were chosen for equal load), but BP was 15% lower in volume (15 vs 18 h a week; 795 vs 938 h a year), with less low-intensity (LIT) time, far fewer moderate sessions (13 vs 38 a year) and twice as many HIT sessions (157 vs 77).

In BP, HIT was high from the first preparation period and fell toward competition, and 73 of the HIT sessions came in seven HIT blocks of 7-11 days with 8-13 HIT sessions each, about four weeks apart; in TRAD, HIT rose toward the competition period.

Average international race rank was 4.2 in BP and 2.8 in TRAD, and the authors found no difference in how performance developed through each competition season. Looking back, athlete and coaches judged the block model effective for her first seasons with it (2004-2006) but said she should then have switched focus; her performance stagnated in the following years.

Our reading: one exceptional athlete succeeded with HIT front-loaded in concentrated blocks and with HIT rising toward racing at the same load, which argues against any rule that the hard work must come late — and says nothing about which is better.

Protocol

Participant: n = 1, the most-decorated winter Olympian; ethics review by the regional committee of mid-Norway, approval by the Norwegian Social Science Data Services, written consent to publication.
Seasons and selection: BP 2005-2006 and TRAD 2014-2015, chosen on three criteria: a successful season (World Cup wins in sprint, 0.8-1.3 km, and distance, 10-30 km, races, and the sprint and overall World Cup), equal endurance training load by TRIMP, and detailed session information.
Data: daily training diaries designed by the Norwegian Ski Association and the Norwegian Olympic Federation, systematized by phase (general preparation 1 and 2, specific preparation, competition), form (endurance, strength, speed), intensity and mode (specific: skiing and roller skiing; non-specific: running and cycling); illness days from the diaries; semi-structured interviews with the athlete and her coaches.
Intensity zones (heart rate): LIT below 87%, MIT 87-92%, HIT above 92% of HRmax.
Load: TRIMP = minutes in each zone × 1, 2 or 3 (LIT, MIT, HIT), summed.
HIT block definition: more than 4 HIT sessions (competitions not counted) within 7 days, since her usual MIT plus HIT was 2-3 sessions a week.
Performance: average rank in World Cup races and World or Olympic championships, first part of the competition season vs the second; no physiological test data were available for BP.
Statistics: paired t-test or Wilcoxon signed-rank test on weekly values, BP vs TRAD.
Funding and interests: no commercial or financial conflicts declared; funding not stated.

Key Results

Whole year (weekly values from Table 1, mean ± SD, * significantly different, P < 0.05; annual counts, race rank and illness, no-training and altitude days from the Results text):

MeasureBP (2005-06)TRAD (2014-15)
Annual volume795 h, 478 sessions938 h, 538 sessions
Hours a week15.3 ± 5.918.0 ± 6.7 *
Endurance load (TRIMP a week)1058 ± 3681084 ± 339
LIT hours a week12.6 ± 5.214.8 ± 5.3 *
MIT sessions a year1338
HIT sessions a year15777
HIT hours a week1.6 ± 1.20.7 ± 0.6 *
Time in LIT / MIT / HIT88 / 1 / 11%92 / 4 / 4%
Sessions LIT / MIT / HIT61 / 3 / 36%76 / 8 / 16%
HIT hours a week: GP1 / GP2 / SP / CP1.9 / 1.7 / 1.7 / 1.20.4 / 0.8 / 0.7 / 1.0
HIT sessions a week: GP1 / GP2 / SP / CP3.8 / 3.3 / 3.6 / 2.30.8 / 1.5 / 1.9 / 2.1
Strength hours a week0.8 ± 0.61.7 ± 1.2 *
Average international race rank4.2 ± 4.42.8 ± 5.8
Illness days / days without training / days at altitude7 / 36 / 589 / 36 / 54
The HIT blocks (BP): seven blocks of 7-11 days with 8-13 HIT sessions each, from the first preparation period to the specific preparation period and none in the competition period; 73 of the 157 HIT sessions and 35.6 h of HIT, 45% of the year's HIT; 27 ± 12 days (15-48) between blocks; 16.2 ± 1.3 h a week over 10.8 sessions; time in LIT/MIT/HIT 74/1/26%; weekly load 1366 ± 68 TRIMP; about 8 HIT sessions per 7 days, with 1-3 days holding two HIT sessions; 87% of HIT sessions were 4-7-min intervals, most typically 5 × 4 min with 2-3 min recovery.
Between blocks: about 1 h and 2 HIT sessions a week; 20 h a week in general preparation and 16 h in specific preparation. In the 7 days after each block: 1198 TRIMP, 19.7 h, about 7 LIT, 1-2 MIT/HIT and 1-2 strength sessions.
Intensity steering: her coaches' rule was that after the last interval she "should potentially be able to perform one additional interval at approximately the same pace" — HIT sessions were held below maximal effort.
Numbers that do not add up: the Discussion gives competition-period HIT as 1.3 h a week in BP and 1.2 h in TRAD; Table 1 gives 1.2 and 1.0. "A progressive increase in HIT toward the competition period" in TRAD holds for sessions (0.8 to 2.1 a week) but not quite for hours (0.8 in GP2, then 0.7 in SP).The week after each block (19.7 h) had more hours than a block week (16.2 h a week): the "reduction in training load after each HIT block" was in TRIMP and HIT, not in hours (our reading of the paper's numbers).

Mechanisms

Measured: no physiological variable in either season; only training content, illness days and race ranks.
Authors' interpretation (from interviews): the early HIT blocks aimed to raise VO2max early so she could train at higher speeds through the preparation periods; five exercise modes, loading upper and lower body differently, and the steady, submaximal steering of each session let her repeat about 8 HIT sessions a week at high quality; the drop in load and HIT after each block, they "speculate", kept illness days low.
Authors' interpretation: the more frequent long LIT sessions in TRAD (about 5 vs 3 a week over 90 min) and its long MIT sessions near 90% HRmax may have compensated for its fewer HIT sessions.
Inference: "equal load" depends on counting a HIT minute as three LIT minutes; under another weighting the two seasons would not be equal, so the comparison is between two plans that fit one crude load scale, not two equal doses.

Practical Application

Must my hard intervals come late in the build, near the race? Not necessarily: one world-class skier won with HIT heaviest early in concentrated blocks and, years later, with HIT rising toward racing. One athlete, no tests.
What a world-class HIT block looked like (C+, n = 1, Tier 5): 7-11 days, 8-13 HIT sessions (about 8 in 7 days, some days with two), typically 5 × 4 min above 92% HRmax, held so one more rep was possible, about 16 h a week, then a normal-volume week with 1-2 hard sessions; blocks about every four weeks through preparation, none in the racing season.
Do not scale this down to an amateur as written. She trained about 800 h a year and had raised her load progressively since junior level before using these blocks. The base's tested loading microcycle for well-trained and elite athletes is about 5 HIT sessions in 6-7 days (SYNTH-33, REF-136, REF-048); the size of her blocks (8-13 HIT sessions in 7-11 days) is beyond anything tested in the base's trials (our reading).
Blocks may have a shelf life (retrospective opinion, one athlete): her performance stagnated after the first seasons of the block model; she and her coaches judged in hindsight that she should then have shifted to maintaining capacity and other factors, and her next major improvement, in 2010, coincided with a change to a traditional model (this paper, citing REF-646).
This is not evidence that block beats traditional, or the reverse. Race ranks were 4.2 vs 2.8, the seasons nine years apart, and the authors found no difference in performance development; the comparison trials are in REF-946, REF-1215 and REF-1216.

Limitations

One athlete. An exceptional individual's two seasons cannot show which model is better or what others should do.
Nine years apart. BP came earlier in her career and TRAD later (2014-2015), with different training history, experience and race programme; the authors name this as the main limitation.
Seasons chosen for the comparison. Both were picked because they were successful and had equal TRIMP; seasons that went badly under either model are not shown.
Crude load and no tests. TRIMP weights zones 1-2-3 by heart rate; no physiological test data exist for BP; performance is average race rank, which depends on the field and the race programme.
Self-reported diaries and retrospective interviews, including the coaches' and athlete's hindsight judgement on stagnation.
Statistics on weekly values of one person treat weeks as independent observations; the P values describe the two years' weeks, not a population.
Funding: not stated; the authors declare no commercial or financial conflict.
Conflicts with the base: REF-646's practical line presents "a small, deliberate high-intensity share that grows as competition nears" as the template from this same athlete; this paper shows she also succeeded with the opposite HIT progression.
How this entry was checked and changed
Why this level

single case study (rubric C+), descriptive; read from the full text (open access, CC BY 4.0). The two seasons were chosen by the authors for success and equal load, and performance was compared only as average race rank

Full text (checked 2026-10-26)
Weekly endurance training load did not differ between BP and TRAD (1058 ± 368 vs. 1084 ± 339 TRIMP); training volume was lower in BP (15 ± 6 vs. 18 ± 7 h/wk, P = 0.001) (full text)
Total annual training volume was 795 h across 478 sessions in BP and 938 h over 538 sessions in TRAD; MIT 13 vs. 38 sessions, HIT 157 vs. 77 sessions a year (full text)
Seven HIT blocks of 7–11 days with 8–13 HIT sessions; 35.6 h of HIT in blocks, 45% of annual HIT volume; 27 ± 12 days (15–48 days) between blocks (full text)
During HIT blocks: 16.2 ± 1.3 h/wk across 10.8 ± 0.4 sessions/wk, 74/1/26% LIT/MIT/HIT by time; weekly load 1366 ± 68 TRIMP (full text)
In the 7 days after each block: 1198 TRIMP and 19.7 h/wk (full text)
Average rank in international competitions 4.2 ± 4.4 in BP vs. 2.8 ± 5.8 in TRAD (full text)
Intensity zones: LIT below 87%, MIT 87–92%, HIT above 92% of HRmax (full text)
Discussion: HIT 1.9 h/wk in GP1, 1.7 h/wk in SP and 1.3 h/wk in CP in BP; TRAD from 0.4 h/wk in GP1 to 1.2 h/wk toward CP (Table 1: CP 1.2 and 1.0) (full text)
The authors declare no commercial or financial relationships that could be construed as a potential conflict of interest (full text)