Training-induced Achilles tendon adaptation: a distinct population within adolescence

Abstract

The Achilles tendon functions as a load-bearing component crucial for movement performance and swiftly adapts to changes in body mass and strength, particularly during periods of growth spurts. Mechanical and morphological attributes of the Achilles tendon were assessed in 41 participants (20 boys and 21 girls, comprising 22 athletes and 19 nonathletes) over a span of 18 months, employing ramped isometric plantar flexion. Longitudinal variations were analyzed using linear mixed modeling. This longitudinal study aimed to explore the combined effects of sport participation, maturation, and sex on the mechanical characteristics of the Achilles tendon across various muscle-tendon lengths. While the rate of stiffness increase post peak height velocity was comparable across all groups, a noteworthy finding was that boy-athletes displayed a more pronounced and earlier rate of Achilles tendon stiffness augmentation twelve months prior to PHV. Additionally, a distinctive mechanical property alteration was identified solely in boy-athletes at the 0° ankle angle. This elevation in Achilles tendon stiffness among boy-athletes was linked to greater peak force of the plantar flexors and a reduction in elongation. In summary, our study introduces novel longitudinal evidence indicating that sport participation interacts with sex and maturation, yielding a distinct adaptive response in Achilles tendon stiffness, specifically observed at the neutral position. These findings suggest a nuanced adaptive model wherein elevated force levels preceding PHV instigate heightened mechanical loading in specific muscle-tendon lengths, thereby inducing increased stiffness throughout adolescence. Specifically, the Achilles tendon stiffness exhibited a more substantial rate of increase in athletes compared to nonathletes and girls, leading to the conclusion that boy-athletes represent a distinct population during the maturation process.