How the Achilles Tendon Stores Energy and Adapts to Load

Peter explains how energy storage tendons differ from positional tendons such as tibialis anterior. The Achilles is compliant, rich in glycosaminoglycans and viscoelastic, and the interfascicular matrix lets fascicles slide and twist like a spring. During walking and running the stiff calf muscle works close to isometrically while the tendon stretches and recoils, which saves energy. The stretch-shortening cycle, shown by a counter movement jump beating a squat jump, needs both an elastic tendon and a strong muscle. Achilles forces reach three to four times body weight in walking and six or more in running, and unpublished data from his PhD student show plyometric tasks load the tendon far more than calf raises. On mechanotransduction, tenocytes sense load and respond. Heavy slow resistance training increases stiffness in healthy tendons, while plyometric training does not, probably because fast loading strains the tendon less. Heavy slow work adapts the tendon; hopping builds power and hopping efficiency.