Comparison of Hamstring Functional and Stiffness Characteristics Between Adolescent Female Soccer Players and Age-Matched Controls: A Cross-Sectional Study

This cross-sectional study compared the functional and stiffness properties of the hamstring muscles between adolescent female soccer players and age-matched controls. Cluster analysis split the soccer group into 28 participants aged ≥170 months (~14.2 years) and 15 participants < 170 months. The control group was similarly divided (older: 14 participants; younger: 13 participants). Unilateral isometric peak torque and isotonic peak power of the knee flexors and eccentric knee flexor strength measured during the Nordic hamstring exercise (NHE strength) for each leg were determined. The semitendinosus shear modulus was also measured in both legs using ultrasound shear wave elastography. Across all age groups, the soccer group (39-117 Nm) demonstrated significantly greater NHE strength bilaterally (control: 20-80 Nm) (<i>p</i> < .001). Conversely, hamstring stiffness was comparable between the groups, with a notable increase observed in the younger age cohort (older: 2.3-6.3 kPa; younger: 3.1-6.7 kPa) (<i>p</i> = .013). Moreover, soccer players' NHE strength did not align with conventional knee flexor strength measures. These results demonstrate that adolescent female soccer players possess a distinct hamstring muscle profile, notably exhibiting significantly greater NHE strength compared to controls, with semitendinosus shear modulus being largely comparable between the groups. This unique profile is commonly associated with a reduced risk of hamstring strain injury, a finding potentially relevant for injury prevention strategies in this population. Such findings offer practical guidance for injury screening, but their interpretation is limited by the cross-sectional design and the control group's activity heterogeneity.