REF-1217 · Walther et al., 2026 · International Journal of Sports Physiology and Performance · Paper · PubMed

Does a big-volume camp week build more endurance than the same hours spread out?

Not in 20 junior skiers: one 160% week then lighter weeks matched even weeks for VO2max and threshold speed, with lower mood.

B+moderate evidenceMeasured on trained to highly trained athletes

Walther and colleagues asked whether block periodization of low-intensity training improves endurance more than the same volume spread evenly. Twenty junior cross-country skiers, matched by VO2max and sex, were randomly assigned to start with BLOCK or EVEN and then crossed over; each period lasted 4 weeks, with a 1-week break between.

In BLOCK the skiers trained 1 week at 160% of their baseline endurance volume and then had weeks of reduced volume; in EVEN volume stayed constant; total training volume was identical. VO2max, maximal aerobic speed (Vmax) and submaximal indices were tested before and after each period, and stress and recovery were rated daily.

There was no treatment-by-time interaction for any performance variable — VO2max (P = .83, d = −0.1), Vmax (P = .15, d = −0.4) and speed at 3 mmol/L lactate (P = .55, d = 0.2). Both conditions produced some significant within-group improvements, not different from each other.

BLOCK came with significantly lower mood and perceived recovery during the high-volume week and the recovery week after it. The authors conclude that concentrating low-intensity volume is no more effective than an even distribution, call the big week well tolerated and safe, and suggest that for low-stress training consistency rather than concentration drives adaptation.

Our reading: no evidence of a benefit from a big-volume week, with a measurable short-term cost; small, short and juniors only.

Protocol

Design: crossover; matched pairs by VO2max and sex, random assignment to the starting condition; 4 weeks per condition, 1-week break.
Participants: 20 junior cross-country skiers, described as well-trained; numbers of each sex, age and VO2max not given in the abstract.
BLOCK: 1 week at 160% of baseline endurance volume, then weeks of reduced volume (how many and by how much is not given in the abstract).
EVEN: constant weekly volume.
Volume: total training volume identical between conditions; the intensity of the extra volume is described only as low-intensity training.
Tests: VO2max, maximal aerobic speed and submaximal performance (speed at 3 mmol/L lactate) before and after each period; daily subjective stress and recovery markers. The test mode (treadmill running, roller-skiing) is not given in the abstract.
Funding and interests: not given in the abstract.

Key Results

OutcomeInteraction (BLOCK vs EVEN)Effect size
VO2maxP = .83d = −0.1
Maximal aerobic speed (Vmax)P = .15d = −0.4
Speed at 3 mmol/L lactate (V3mmol/L)P = .55d = 0.2
Mood and perceived recoverysignificantly lower in BLOCK during the high-volume week and the following recovery weeknot given
Within-group: "both groups demonstrated significant within-group improvements in some performance variables"; which variables and by how much are not given in the abstract.
Direction of d: the abstract does not say which condition a negative d favours.
Abstract tension: the conclusion calls the high-volume week "well-tolerated and safe" while the Results report significantly lower mood and perceived recovery in that week and the next; both statements can be true (a transient dip without injury or illness), but "well-tolerated" should be read with the dip.

Mechanisms

Measured: no performance advantage; a subjective cost during and after the concentrated week.
Authors' interpretation: for training with a lower physiological stress profile, consistent application, rather than concentration, is the main driver of adaptation; concentrating low-intensity volume does not add a stimulus.
Inference: a single 160% week is a modest overload for athletes already training high volumes, and the lighter weeks that followed give back the extra; a 4-week window may be too short for a volume effect on VO2max or threshold speed to show in either condition.

Practical Application

Does a big-volume camp week build more endurance than the same hours spread out? Not in 20 junior skiers: one 160% week then lighter weeks matched even weeks for VO2max and threshold speed, with lower mood.
Plan a camp for its other reasons: terrain, weather, coaching, technique or time off work; do not expect extra aerobic adaptation from piling the easy hours into one week (B+ from the abstract, juniors, 4 weeks).
Expect a dip and plan the week after: mood and perceived recovery were lower in the big week and in the lighter week after it; lower mood and recovery scores in that fortnight are what the group means predict, but that does not make an individual's low reading safe to ignore; illness, injury and the base's readiness rules still apply (inference; group means from an abstract).
Even is the default for easy volume: at equal totals, the steady distribution did as well and felt better; consistency week to week is the safer bet (SYNTH-42 consistency gate).
Not about hard sessions: concentrating HIT into a loading week is a different question with its own mixed evidence (REF-1216 null in junior skiers; REF-1214, REF-1215 positive from one laboratory).

Limitations

Small and short: 20 juniors, 4-week periods; a 1-week break may not wash out the first period, and no period or carry-over analysis is reported in the abstract.
Abstract only: the size of the volume reduction after the big week, the number of reduced weeks, weekly hours, sexes, ages, VO2max, test mode, the within-group gains and the direction of the effect sizes are not given.
Power: with d up to −0.4 for Vmax (P = .15), a moderate difference in either direction cannot be ruled out; "no evidence of a difference", not "no difference".
One pattern: a single 160% week; larger or repeated overloads, or concentration of moderate or high intensity, were not tested.
Funding and conflicts: not given in the abstract.
Written from the abstract and record metadata (full text not read).
How this entry was checked and changed
Why this level

controlled crossover, n = 20 (the rubric's B+ for a crossover of 15 or more); read from the abstract (closed access; no open copy via Europe PMC or OpenAlex). Within B+ a weak one: 4-week periods with a 1-week break, and the within-group gains, the sex split, ages and period or carry-over analysis are not given in the abstract