REF-1215 · Rønnestad et al., 2016 · Scandinavian Journal of Medicine & Science in Sports · Paper · PubMed

Can a block of intervals close to the season still pay?

In 19 elite skiers, 5 weeks approaching competition, front-loading the same 11 HIT sessions raised peak power 4% vs −3% when spread. One small trial, one lab.

Blimited evidenceMeasured on highly trained to elite athletes

Rønnestad and colleagues compared two ways of organizing five weeks of endurance training in elite cross-country skiers approaching the competition period. The block group (BP, 10 skiers) did five high-intensity interval sessions (HIT) in week 1 and three in week 3, with one HIT session in each of the other weeks.

The traditional group (TRAD, 9 skiers) did two HIT sessions a week, three in week 3. Both did similar totals of HIT and of low-intensity training (LIT).

BP gained more peak power output (+4 ± 4% vs −3 ± 6%) and power output at 4 mmol/L blood lactate (+11 ± 10% vs +2 ± 4%; both P < 0.01), and VO2max rose 2 ± 2% in BP (P < 0.05) with no change in TRAD; the effect sizes for BP vs TRAD were moderate. The authors conclude that block periodization had superior effects on several endurance and performance indices.

Our reading: a positive result in high-level athletes close to the season, which argues against the idea that a block needs a long runway to pay; part of the peak-power difference is TRAD losing 3%, the VO2max comparison between groups is not reported in the abstract, and the trial comes from the same laboratory as the other positive block trials.

Protocol

Design: randomized, two parallel groups, five weeks "approaching the competition period" (abstract); the citing meta-analysis scored it 1 for random allocation on PEDro, total 5/10 (REF-946, full text, Table 2), and describes the pooled block studies as pre-season.
Participants: 19 elite cross-country skiers, BP 10 and TRAD 9; mixed sex (REF-946 table: "Mixed"); age, VO2max and the women's number are not given in the abstract.
BP: HIT sessions per week 5, 1, 3, 1, 1 (weeks 1-5).
TRAD: HIT sessions per week 2, 2, 3, 2, 2.
Total HIT: 11 sessions in each group (our arithmetic); similar total LIT (abstract).
Session content (from [REF-946], Mølmen 2019, full-text table; not in this abstract): uphill cross-country skiing intervals at 88-100% of HRmax, 30 min of HIT per session; groups matched "by total and intensity-specific training volume"; HIT replaced the usual hard sessions; outcomes also included fractional utilization of VO2max and work economy.
Outcomes in the abstract: peak power output, power output at 4 mmol/L lactate and VO2max (incremental test). Test timing after the last hard session, weekly hours and the date of the first race are not given in the abstract.
Funding and interests: not given in the abstract.

Key Results

Outcome (5 weeks)BP (5-1-3-1-1 HIT)TRAD (2-2-3-2-2 HIT)Between groups
Peak power output+4 ± 4%−3 ± 6%P < 0.01
Power at 4 mmol/L lactate+11 ± 10%+2 ± 4%P < 0.01
VO2max+2 ± 2% (P < 0.05)no changenot reported in the abstract; "not greater" per REF-1216
Effect sizes BP vs TRADmoderate (abstract)values not given in the abstract; REF-946 (full text) gives ES = 1.26 for 4-mmol power, "large" on its own scale
Citing full texts: neither BP nor TRAD changed skiing economy over the five weeks (REF-946); the block group improved power at the lactate marker more than TRAD (REF-946); "improvements in VO2max were not greater following block- versus evenly-distributed HIT" (REF-1216).
A drop in the comparison arm: TRAD's peak power fell 3 ± 6% over five weeks of two HIT sessions a week, so part of the 7-point peak-power gap is a loss in TRAD; the abstract does not say whether that fall was significant within the group.
Authors' conclusion: block periodization "have superior effects on several endurance and performance indices compared with traditional organization."

Mechanisms

Measured: no mechanism; the abstract reports test outcomes only.
Authors' interpretation: not given in the abstract beyond the conclusion; the block rationale is overload followed by a lighter period (as REF-1216's Introduction puts it, citing Issurin 2010 REF-1225).
Inference: BP's last two weeks held one HIT session each and TRAD's two, so BP came to the post-test with less recent intensity; a fresher test, and not only a larger adaptation, may explain part of the gap. The abstract does not give the days between the last session and the test.
Inference: a second, smaller loading week (3 sessions in week 3) after one easier week is a "wave" rather than a single block; the trial tests this specific 5-1-3-1-1 pattern, not blocking in general.

Practical Application

Can a block of intervals close to the season still pay? In 19 elite skiers, 5 weeks approaching competition, front-loading the same 11 HIT sessions raised peak power 4% vs −3% when spread. One small trial, one lab.
Close to the season, a block is not ruled out by the evidence: the 5-week pattern (5, 1, 3, 1, 1 HIT sessions a week) raised threshold power and peak power in high-level athletes who were approaching competition; the house default of "no new lever" under ~8 weeks (SYNTH-42; SYNTH-76's "no new build" concerns the 3-4-week window between two A-races) rests on practice; this trial does not test it, because it redistributed HIT the skiers already did and measured no race.This block redistributed hard sessions the skiers already did; it was not a new kind of stimulus.
Do not promise a VO2max jump: within-group +2 ± 2%, and the between-group VO2max comparison is not reported here; the threshold and peak-power gains are the robust part.
Weigh it: a randomized but small trial from the laboratory that ran the other positive block trials (REF-1214, REF-136); independent crossover trials in junior skiers found no block advantage (REF-1216, HIT; REF-1217, easy volume).
Keep intensity flowing after the loading week: both groups kept at least one HIT session every week; REF-1216's block arm, which had none in its last week, showed no gain (inference across trials).

Limitations

Small: 10 vs 9.
One laboratory: the same group produced the other positive block trials and the meta-analysis that pooled them; no independent replication of a block advantage in skiers.
Abstract gaps: no VO2max between-group test, no effect-size values, no sex split, no ages, no test timing, no race date, no performance (race or time-trial) outcome. The record's MeSH terms include Adolescent and Female, so some skiers were probably juniors and some women (inference from indexing, not stated in the abstract).
The comparison arm lost peak power, which inflates the between-group difference relative to the block's own gain.
Specific pattern: a 5-1-3-1-1 wave over five weeks; whether a single loading week or a different spacing would do the same was not tested.
Funding and conflicts: not given in the abstract.
Written from the abstract and record metadata, with protocol details from two citing full texts (labelled).
How this entry was checked and changed
Why this level

parallel RCT, n = 19 (10 block vs 9 traditional), below the rubric's 20 for B+; read from the abstract (closed access; no open copy via Europe PMC or OpenAlex). Protocol details below that the abstract lacks come from two citing full texts and are labelled