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Tendons are the elastic energy storage system of the bird's locomotor system. Unlike muscles (which convert chemical energy into mechanical energy), tendons are passive elements that store elastic potential energy when they are deformed (elongated by muscular or aerodynamic loading) and return it when they relax. This ability to store and restore energy significantly reduces the metabolic cost of flight: part of the energy of the downward flight is stored in tendons and ligaments and is recovered in the upward flight.

In addition to the tendons of the flight muscles, the furcula (E3) acts as a bony spring coupled to the tendon system. The tendon-furcula assembly forms an energy storage system that cushions the force peaks of the pectoralis and distributes them temporarily throughout the beat cycle, reducing the peak power that the muscle must generate and improving flight efficiency.



Central physical principle: U_elastic = ½ × k_tendon × x² [J]: the energy stored in a tendon is proportional to the square of the strain. The restitution efficiency ( = E_returned / E_stored) of tendon collagen is 93-97%: almost all of the stored energy is recovered in the release phase. This is fundamentally different from viscous dissipation: the tendon does not heat, it returns.

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