Biceps femoris long head muscle and aponeurosis geometry in males with and without a history of hamstring strain injury

Objectives
Hamstring strain injuries (HSIs) commonly affect the proximal biceps femoris long head (BFlh) musculotendinous junction. Biomechanical modeling suggests narrow proximal BFlh aponeuroses and large muscle-to-aponeurosis width ratios increase localized tissue strains and presumably risk of HSI. This study aimed to determine if BFlh muscle and proximal aponeurosis geometry differed between limbs with and without a history of HSI.

Methods
Twenty-six recreationally active males with (n?=?13) and without (n?=?13) a history of unilateral HSI in the last 24?months underwent magnetic resonance imaging of both thighs. BFlh muscle and proximal aponeurosis cross-sectional areas, length, volume, and interface area between muscle and aponeurosis were extracted. Previously injured limbs were compared to uninjured contralateral and control limbs for discrete variables and ratios, and along the relative length of tissues using statistical parametric mapping.

Results
Previously injured limbs displayed significantly smaller muscle-to-aponeurosis volume ratios (p?=?0.029, Wilcoxon effect size (ES)?=?0.43) and larger proximal BFlh aponeurosis volumes (p?=?0.019, ES?=?0.46) than control limbs with no history of HSI. No significant differences were found between previously injured and uninjured contralateral limbs for any outcome measure (p?=?0.216?1.000, ES?=?0.01?0.36).

Conclusions
Aponeurosis geometry differed between limbs with and without a history of HSI. The significantly larger BFlh proximal aponeuroses and smaller muscle-to-aponeurosis volume ratios in previously injured limbs could alter the strain experienced in muscle adjacent to the musculotendinous junction during active lengthening. Future research is required to determine if geometric differences influence the risk of re-injury and whether they can be altered via targeted training.