REF-1718 · Pecci et al., 2026 · Sports Health, · Paper · PubMed

In soccer players the Nordic hamstring exercise lengthened biceps femoris long-head fascicles (P = 0.01), with significant effects at higher volumes (more than 290 repetitions over 6-12 weeks, 57 a week) and no significant difference between training twice and once a week, in a meta-analysis of 6 studies (12 training groups, 168 players); fascicle length is a muscle-architecture marker, not an injury outcome, and the abstract gives no effect sizes and no risk-of-bias result

Main caveat: Six studies and 168 players; the abstract gives P values only, without effect sizes, intervals or heterogeneity.
limited evidenceMeasured on trained to highly trained athletesRead from the abstract only

Pecci and colleagues asked how resistance-training variables change the architecture of the biceps femoris long head in soccer players.

They searched five databases to January 2024 and included training studies in soccer players that measured muscle thickness, fascicle length and pennation angle before and after a programme: 6 studies with 12 training groups and 168 participants.

The analyses set the Nordic hamstring exercise against soccer training alone, higher against lower volume, and two sessions a week against one.

The NHE significantly lengthened biceps femoris long-head fascicles (P = 0.01); twice a week was not significantly different from once a week; and volumes above 290 repetitions, done over 6 to 12 weeks at 57 repetitions a week, gave significant effects.

The authors conclude that the NHE lengthens the fascicle, especially with sufficient volume and two days a week, and that the effects of intensity, effort and exercise choice are still unknown.

The abstract reports no effect sizes, intervals or heterogeneity, no results for muscle thickness or pennation angle, and no risk-of-bias assessment; its conclusion recommends two days a week although its own result found no significant difference between two and one.

Participants

6 studies, 12 training groups, 168 soccer players.
Age, sex, competition level and season phase: not in the abstract.

Protocol

Search: five databases (not named in the abstract) from inception to January 2024.
Inclusion: studies with training intervention groups in soccer players that measured biceps femoris long-head architecture before and after the programme.
Outcomes extracted: muscle thickness, fascicle length, pennation angle.
Comparisons: NHE against soccer interventions; training volume; training frequency (twice vs once a week).
Not in the abstract: the imaging method, the NHE protocols, programme lengths of each study, the effect measure, the risk-of-bias or quality tool, heterogeneity, and whether any study was randomised.

Key Results

ComparisonResult in the abstract
NHE vs soccer interventions, fascicle lengthFascicle length of the long head rose significantly with NHE (P = 0.01); size not given
Twice vs once a weekNo significant difference
Higher volume (more than 290 repetitions, over 6-12 weeks, 57 repetitions a week)Significant effects; size and comparator not given
Muscle thickness, pennation angleExtracted as outcomes; no result reported in the abstract
No effect sizes, confidence intervals or heterogeneity statistics are given in the abstract.

Practical Application

The Nordic hamstring exercise lengthens the biceps femoris long-head fascicles in soccer players, and this abstract links larger totals (over 290 reps across 6-12 weeks) with the clearest change. That is a change in muscle structure; whether it lowers injury risk was not tested here. The injury evidence for programmes that include the Nordic is REF-766 (about half the hamstring injury rate).
Once a week was not significantly different from twice a week in this analysis; the authors' "2 days per week" in their conclusion is not supported by the result their abstract reports, so the assistant should not quote two days a week as a finding.
Where it differs from the base: SYNTH-59 keeps the Nordic in a sprinter's week and leans on REF-887, which found low and high Nordic volumes gave similar, very large gains in eccentric strength and fascicle length after 6 weeks or more.This abstract instead reports significant fascicle effects at higher volume (over 290 reps, 57 a week) and does not say how lower volumes did or what they were compared with. REF-887 does not define its low and high volumes in reps, so the two cannot be lined up, and this result is a between-study association on a marker; the base's "a low volume kept up" stays, with this tension stated.Sets and reps remain practice or a clinician's call.
Inference, not tested here: for a runner or sprinter, the Nordic's structural effect probably transfers, since it is the same muscle and exercise, but these were soccer players and the outcome is architecture only.
This is not treatment advice for a hamstring injury: return from a strain and the dose of eccentric work during rehabilitation are decided with a physiotherapist or sports physician.

Limitations

Six studies and 168 players; the abstract gives P values only, without effect sizes, intervals or heterogeneity.
No risk-of-bias or certainty assessment is reported in the abstract, and it does not say how many studies were randomised.
The volume and frequency findings compare training groups across studies, not within one trial, so they show association, not that the dose caused the difference.
The conclusion's two-days-a-week recommendation is not matched by the reported frequency result.
Fascicle length is a marker; strength, sprint and injury outcomes were not analysed.
Results for muscle thickness and pennation angle, the NHE protocols, the players' age, sex and level, and the funding are not in the abstract; full text not read.
How this entry was checked and changed
Why this level

B+ at most because risk of bias not read (abstract only): the abstract reports no risk-of-bias or certainty assessment; one step down (rule 3) because the outcome is muscle architecture (fascicle length), a marker, not strength, sprint performance or injury