High hamstring stiffness and flexibility with comparable spinopelvic morphometry in amateur footballers: a multimodal study

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Abstract
Football’s repetitive lower-extremity biomechanics may drive functional adaptations in the hamstrings and spinopelvic alignment. We hypothesised that footballers would display greater hamstring stiffness and morphometric differences in SPPs, attributable to sport-specific loading, compared with non-athletes. This study therefore investigates the interrelationships among hamstring stiffness, flexibility, and spinopelvic alignment to clarify football-induced adaptations. This cross-sectional study included 50 licensed amateur male football players (Group A; mean age 21.74 ± 2.91 years) and 50 healthy males with no licensed sports participation (Group B; mean age 23.14 ± 2.98 years). Measurements comprised radiographic assessment of SPPs, Shear-Wave elastography (SWE) to quantify hamstring muscle stiffness, and clinical flexibility tests (CFTs) which consist of the Active Knee Extension (AKE) test, the Straight Leg Raise (SLR) test, and the Sit-and-Reach (SR) test. No significant differences were detected between the groups in SPPs. All hamstring SWE values (except for the left Biceps Femoris, p = 0.615) and all CFT values were significantly higher in Group A (p < 0.001), indicating the ‘high stiffness/high flexibility’ paradox. Within Group A, SPPs were not associated with either SWE values or CFT values (p > 0.05). Likewise, no correlations were found between SWE values and CFT values in either group (p > 0.05). Footballers exhibited higher hamstring stiffness yet showed no spinopelvic morphometric differences compared with non-athletes. The absence of SWE–CFT correlations suggests that these methods assess distinct physiological properties. The observed “high stiffness/high flexibility” paradox in footballers indicates sport-specific functional adaptations. Future longitudinal studies are needed to clarify the long-term effects of this phenomenon.

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