How can the Achilles tendon return energy during a running step?

A running step is not powered only by muscle fibers shortening. Near the back of the ankle, the Achilles tendon can stretch under load and then return some of that stored energy. The springlike part of a step is hidden beneath skin, between the calf muscles and the heel bone.

As forces load this tendon during a step, its slight elastic deformation stores mechanical energy. Later, as the runner moves toward push-off, the tendon shortens again. Its pull on the heel helps the foot push against the ground. Stretching and recoil connect two parts of the same step in time.

Muscle and tendon have different jobs in this partnership. Muscle fibers actively produce force using chemical energy. The tendon transmits force and changes length under that load. It can return energy stored earlier, but it does not manufacture a fresh supply. Calling it a spring describes that storage-and-return role, not a replacement for muscle.

Some energy is lost rather than returned, and muscles still perform substantial work. The amount stored and returned depends on how the tendon is loaded; there is no single efficiency promised by the drawing. A familiar stride combines active pulling with elastic recoil, making a living connection between the energy used now and energy briefly held from earlier in the step.