Maximum-Effort Jumping Exposes the Achilles Tendon to High Strain and Indicates Potential Overload in Adolescent Athletes

Plyometric exercise is a fundamental component of many sport disciplines and may overload the Achilles tendon (AT) by subjecting it to high strain magnitudes. AT strain profiles were investigated during countermovement, drop, hurdle, and horizontal single-leg jumps - common plyometric exercises - in order to assess exercise-induced AT loading. This was done by measuring AT elongation during the jumps using reflective markers on the skin to reconstruct the AT path, while the myotendinous junction of the gastrocnemius medialis was recorded using ultrasound in 18 adolescent athletes. Electromyographic (EMG) activity of the tibialis anterior, soleus, gastrocnemius medialis and lateralis muscles, kinematics, and ground reaction forces were also measured. Although the maximum resultant ankle joint moment differed significantly between the investigated jumps, maximum AT strain and EMG activity of the triceps surae muscles were invariant between jump types. On average, the maximum AT strains ranged from 9.0% to 10.7%, and the EMG activity ranged from 0.96 to 1.43 times the maximum EMG activity achieved during a maximum voluntary fixed-end plantar flexion contraction, across the jumps. The consistently high AT strain magnitudes across the investigated jumps highlight the substantial mechanical demand placed on the AT during maximum-effort jumping tasks, which exceed the reported effective strain range for tendon adaptation. The frequent application of plyometric exercises involving such high-magnitude strains in training practice may contribute to AT overload in adolescent athletes.