Hamstring fascicle behavior during leg curl exercises performed at varying knee joint angles and loading intensities in individuals with a history of unilateral hamstring injury: a randomized cross-over design study
BMC sports science, medicine & rehabilitation · 2026
Small acute ultrasound study finds knee angle and load affect hamstring fascicle length, but shows no real difference between previously injured and healthy limbs — not ready to guide rehab.
The paper
Randomized crossover mechanistic study, n=14 male university athletes with a history of unilateral hamstring strain injury (HSI).
What they found
Biceps femoris long head (BFlh) mid-region fascicle operating length varied significantly by knee angle (p=0.002) and contraction intensity (p<0.01), with generally longer fascicles at 30° than 60°/90°. Neither the main effect of limb nor any limb interaction was significant (p>0.05); the normalized between-limb difference was ~10%.
The appraisal
This is a small (n=14), single-session acute physiology study, not an intervention trial — there is no injury outcome, and it cannot show whether fascicle behavior actually differs between previously injured and healthy limbs; the authors' own null interaction result argues against a between-limb difference, though with n=14 the study is underpowered to rule one out. The 'longer fascicle at 25% vs 100% MVIC at 30° in the uninjured limb only' finding is an exploratory post-hoc comparison the authors themselves flag as not supported by a significant interaction, so it shouldn't be treated as a real limb-specific effect. Crossover/repeated-measures design and ultrasound fascicle tracking are appropriate methods, but retrospective trial registration and small sample limit confidence in effect estimates.
The gap
There's no chronic/longitudinal data linking this acute fascicle behavior to actual re-injury risk or to structural fascicle length adaptation — an acute biomechanics snapshot in 14 people can't tell you whether training at 30° knee flexion changes HSI risk.
Landmark context
This sits downstream of the architectural risk-factor literature (e.g., Timmins et al.'s work linking short BFlh fascicle length to HSI risk) and the eccentric/Nordic-curl fascicle-lengthening literature (Bourne, Opar and colleagues); it probes the acute mechanical conditions that might eventually inform such interventions rather than testing one.
What to do Monday
No — this doesn't change Monday's practice. It's hypothesis-generating for future rehab/prevention exercise prescription (e.g., testing 30° knee flexion loading), not evidence to alter current hamstring rehab protocols.
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