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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