Hamstring muscle architecture and viscoelastic properties: reliability and retrospective comparison between previously injured and uninjured athletes

The architecture of the biceps femoris (BF) and stiffness of the hamstrings have been found to be associated with injury risk. However, less is known about the architecture of the equally voluminous semitendinosus (ST) and viscoelastic properties of both muscles in individuals with a prior injury. Methods: BF and ST of 15 athletes (previously injured, n=5; control, n=10) were assessed using ultrasonography and myotonometry. Mean architecture (muscle thickness (MT), pennation angle (PA) and fascicle length (FL)) and viscoelastic measures (stiffness, oscillation frequency and decrement) were compared between the previously injured and contralateral uninjured limb, and between the previously injured and control limbs (mean of both limbs of the control group). Control group participants returned for a duplicate measurement. Findings: Both muscles exhibited high reliability between sessions (intraclass correlation coefficient (ICC)=0.89−0.98) for architecture. BF PA was larger in the previously injured than both uninjured (+1.1^∘,d=0.65) and control (+1.51^∘,d=0.71). BF fascicles were shorter in the previously injured limb compared to the uninjured (−0.4cm,d=0.65) and control (−0.6cm,d=0.67). BF was stiffer in the previously injured compared to uninjured (+9.2Nm⁻¹,d=1.28). ST architecture and viscoelasticity were similar across limbs. Conclusion: A prior hamstring strain injury is associated with a stiffer BF characterized by larger PAs and shorter fascicles.