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

## TL;DR

This study shows that hamstring muscle growth in teenage alpine skiers differs by sex and is linked to biological maturation over four years.

## Contribution

The study reveals sex-specific longitudinal changes in hamstring muscle size and its association with biological maturation in adolescent skiers.

## Key findings

- Hamstring muscle size increased significantly in both sexes, with greater hypertrophy observed in males.
- Biological maturation was strongly associated with increases in femur length and total hamstring muscle size.
- ACL-injured female skiers showed smaller semitendinosus size at baseline and follow-up.

## Abstract

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.

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).

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.

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.