Longitudinal changes in hamstring muscle morphology in adolescent competitive alpine skiers are sex-specific: a 4-year cohort study

Background: The aim of this study was to investigate changes in hamstring architecture and size over a 4-year period, explore sex-specific differences, and examine associations with biological maturation in adolescent competitive alpine skiers. Materials and methods: 59 competitive alpine skiers (27 females, 32 males; baseline age: 14.9 ± 0.7 years) competing at regional to national level were examined at baseline and follow-up. BMI was calculated and biological maturation estimated using the sex-specific Mirwald equation. Ultrasound imaging assessed biceps femoris long head architecture and maximal anatomical cross-sectional area (ACSAmax) of all hamstring muscles. ACL injuries were retrospectively recorded and verified using medical records and MRI. Participants were grouped as uninjured (n = 53) and ACL-injured (n = 6). Results: Mixed ANOVA revealed significant main and interaction effects for hamstring muscle size, but not for architecture. Post hoc analyses showed that hamstring muscle size increased in both sexes (all p < 0.001), except for the semitendinosus in females (p = 0.499). Biological maturation was associated with changes in femur length (R² = 0.32, p < 0.001) and total hamstring muscle size (R² = 0.52, p < 0.001). As a secondary observation, female skiers who later sustained an ACL injury (n = 5) descriptively showed smaller semitendinosus size across both time points. Conclusions: This study provides new insights into hamstring development in adolescent alpine skiers and highlights the role of biological maturation. Hamstring size increased, with greater hypertrophy in males. The smaller semitendinosus size in ACL-injured females may be relevant for injury prevention.