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How do tendons act like springs during running?

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Diagram showing the structure of human striated or skeletal muscle, with labeled cross-sections from whole muscle to fascicle, muscle fiber, myofibril, and sarcomere, highlighting actin and myosin filaments. — muscle The structure of striated, or skeletal, muscle. Striated muscle tissue, such as the tissue of the human biceps muscle, consists of long fine fibers, each of which is in effect a bundle of finer myofibrils. Within each myofibril are filaments of the proteins myosin and actin; these filaments slide past one another as the muscle contracts and expands. On each myofibril, regularly occurring dark bands, called Z lines, can be seen where actin and myosin filaments overlap. The region between two Z lines is called a sarcomere; sarcomeres can be considered the primary structural and functional unit of muscle tissue.

Tendons in the legs of mammals function as springs to reduce the metabolic energy cost of running. When a foot strikes the ground, the body decelerates, and the energy associated with this impact is stored within the tendons as elastic strain energy. This stored energy is subsequently released through elastic recoil, assisting in movement and locomotion.1

This mechanism is possible because tendons possess inherent elastic properties. While tendons primarily serve to transmit the mechanical force of muscle contraction to bones, their ability to store and return energy allows for more efficient movement during activities like running.12