Background Female athletes have increased risk of anterior cruciate ligament (ACL) injury after puberty. It has been demonstrated that female athletes have greater valgus lower limb alignment than males during landing, and this may contribute to the gender disparity in ACL injury risks. In early teens, however, the gender difference in lower limb alignment has not been elucidated.

Objective The purpose of this study was to examine whether dynamic lower limb alignment alters during the pubertal stage in girls and boys, in a longitudinal survey.

Design Cohort study.

Settings Elementary school students with no lower extremity complaints.

Participants 33 females and 27 males aged 10 to 11 years old were evaluated and followed for three consecutive years.

Assessment of risk factor The subjects performed a drop-vertical-jump by dropping directly down off a box, and immediately performing a maximum vertical jump. The frontal view of the lower limb was recorded with a digital video camera and the images at deepest point of body (landing) were captured.

Main outcome measurement The separate distances between right and left hips and knees were measured. The knee separation distances normalised by the hip separation distance (K/H ratio) were used as a parameter representing the valgus/varus lower limb alignment.

Results At landing, K/H ratio in females was 0.41±0.10 in the first year, 0.39±0.11 in the second year, 0.36±0.10 in the third year. In males, the ratios were 0.60±0.21, 0.52±0.17, 0.55±0.19. Knee valgus alignment in females increased with maturation, with significant difference between first year and third year. However, there were no significant changes in males.

Conclusion Different changing pattern of lower limb alignment during landing was seen between boys and girls, and it may be related to the gender difference in incidence of ACL injury.