A Six-Day High-Intensity Interval Training Microcycle Improves Selected Indicators of Endurance Performance in Well-Trained Female Cross-Country Skiers and Biathletes
European journal of sport science · 2026
A 6-day HIT block nudged lactate-threshold and VO2max velocity in female skiers, but the actual time-trial result wasn't statistically significant.
Part of: Training load and injury risk: what the evidence says · Endurance performance and physiology: training the aerobic engine · Female athlete health: the evidence base
The paper
Controlled training study, n=17 well-trained female cross-country skiers and biathletes, comparing a 6-day HIT microcycle plus 5-day recovery (HITblock) against continued regular training (REG).
What they found
HITblock produced larger gains than REG in velocity at VO2max (3.6% vs 1.2%, p=0.024) and velocity at 4 mmol/L lactate (4.4% vs 0.0%, p=0.023); a composite index also favoured HITblock (4.5% vs 0.9%, p=0.017), but the 15-min time-trial velocity, arguably the most race-relevant measure, did not differ significantly between groups (5.4% vs 1.3%, p=0.083).
Study design
Small parallel-group controlled training intervention (not clearly described as randomised), n=17 split across two arms, in well-trained female XC skiers and biathletes over a roughly 11-day intervention window with pre/post roller-ski testing. This sits low on the hierarchy, closer to a controlled cohort comparison than a definitive RCT.
Methodology
With only ~8-9 athletes per arm, the study is underpowered for anything but large effects, and the p-values for the 'positive' outcomes (0.023, 0.024) sit close to the 0.05 threshold where a couple of individual results could flip significance. No blinding is possible for a training intervention, intensity was self-regulated via RPE rather than objectively verified, and four separate outcome comparisons (plus an exploratory composite) raise multiple-comparison concerns without correction mentioned. Menstrual cycle phase, a known confounder in female endurance research, isn't mentioned as controlled. A clinician would ascertain the key claim by checking whether the 15-min time trial, the most ecologically valid outcome, reached significance, and it did not.
The appraisal
The statistically significant findings are modest in absolute size (3-4%) and apply to lab-based threshold and VO2max velocities, not to the whole-effort time trial that best proxies competitive performance, which missed significance (5.4% vs 1.3%, p = 0.083) in a sample of only 17 athletes. This is a prospective controlled comparison of two training arms rather than a passive cohort, so it can support a reasonably causal read on the physiological outcomes measured, but the abstract doesn't state whether group assignment was randomised, and with 17 athletes split two ways, pre-existing differences in training history or HIT-responsiveness can't be excluded. The exploratory composite index is built from the same underlying measures rather than an independent outcome, so it shouldn't be weighted as confirmatory evidence on top of the individual velocity results.
The gap
It's unclear whether these short-term threshold and VO2max velocity gains translate into actual competition performance or persist beyond the immediate post-intervention window, and the non-significant time-trial result is the elephant in the room.
Landmark context
This extends the broader block-periodization-of-HIT literature in endurance sport (work in this space has mostly involved male cyclists and runners) into a female cross-country ski and biathlon population, an evidence gap the authors are explicitly addressing rather than overturning an established landmark finding.
What to do Monday
Not yet actionable as a standard prescription change; a coach might consider a short HIT-concentration block with reduced overall load before a key phase as a reasonable experiment with an individual athlete, but shouldn't assume it reliably improves race-day performance based on this data alone.
In practice
For endurance athletes already handling 3-4 HIT sessions a week (XC skiers, biathletes, and by extension distance runners, cyclists or rowers at a similar training age), a short 5-7 day HIT-concentration block with reduced overall load and a built-in easy tail is a reasonable periodisation experiment before a key phase, tracked with the same field markers used here: velocity at a fixed blood lactate threshold and terminal speed on an incremental test, not just subjective readiness. In the S&C setting, don't sell this as a guaranteed race-day fix, since the whole-effort time trial didn't significantly improve; treat the block as a threshold-nudging tool, re-test on a fixed protocol before and after, and revert to habitual training if the athlete shows early overreaching signs (disrupted sleep, elevated resting heart rate, flat pace or power) given that recovery volume is deliberately cut during the block. In the physiotherapy clinic, this evidence is mainly useful as context when an endurance athlete reports unusual fatigue after a self-imposed HIT block: it shows concentrated HIT with planned recovery is tolerated by well-trained athletes without implying a performance guarantee, so fatigue persisting beyond the expected recovery window still warrants the usual overreaching work-up rather than reassurance drawn from this paper alone.
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