Tendons Are Springs

Your Achilles tendon works through a stretch-shorten cycle. As your foot lands, the tendon stretches under load and stores elastic potential energy. A split second later it snaps back, releasing that energy to help drive the next push-off.

• Passive tissue beats active muscle for repetitive loading — a spring needs no fuel to rebound
• The foot arch acts as a secondary spring, flattening on impact and recoiling at toe-off
• Kangaroos rely on the same trick — hopping faster barely costs them extra energy

This spring-loaded design is a big part of why humans excel at distance running.