Development of Muscle-Tendon Adaptation in Preadolescent Gymnasts and Untrained Peers: A 12-Month Longitudinal Study

The current study investigated the effects of long-term athletic training on the development of the triceps surae muscle-tendon unit in preadolescence.

Eleven preadolescent untrained children and a group of 21 artistic gymnastics athletes of similar age (9 ± 1.7 yr) and maturity (Tanner stages I and II) participated in the study. The measurements were conducted every 3 months for 1 yr, and training volume and duration of the athletes were documented. Plantar flexor muscle strength, Achilles tendon stiffness, maximum tendon strain, and gastrocnemius medialis morphometrics were measured by integrating kinematics, ultrasonography, and dynamometry. A linear mixed-effects model was used to analyze the investigated parameters.

We found greater muscle strength (P < 0.001) in athletes compared with nonathletes but no differences in Achilles tendon stiffness (P = 0.252), indicating a training-induced imbalanced adaptation of muscle strength and tendon stiffness in preadolescent athletes. Although pennation angle (P = 0.490), thickness (P = 0.917), and fascicle length (P = 0.667) did not differ between groups, we found higher fluctuations in pennation angle and muscle strength over 1 yr in athletes. The imbalanced adaptation of muscle strength and tendon stiffness together with greater fluctuations of muscle strength resulted in greater tendon strain fluctuations over 1 yr (P = 0.017) and a higher frequency of athletes with high-level tendon strain (≥9%) compared with nonathletes.

The findings indicate an increased mechanical demand for the tendon in preadolescent athletes that could have implications for the risk of tendon overuse injury. Therefore, we recommend the implementation of individual training approaches to preserve a balanced adaptation within the triceps surae muscle-tendon unit in preadolescent athletes.