E5 Tendons and elasticity

Topic

Tendons serve as the elastic energy storage system within a bird's musculoskeletal apparatus. Unlike muscles, which actively convert chemical energy into mechanical energy, tendons are purely passive elements; they do not generate energy themselves but instead store elastic potential energy when deformed—stretched by the load transmitted from the muscle or by aerodynamic forces during flight—and release that same energy upon relaxation.

This capacity to store and release elastic potential energy significantly lowers the metabolic cost of flight: some of the energy generated by the pectoral muscle during the downstroke is not dissipated but is instead stored in the wing's tendons and ligaments. This stored energy is then recovered during the subsequent upstroke, thereby reducing the amount of new metabolic energy the pectoral muscle must supply during each cycle.

In addition to the tendons associated with the flight muscles, the furcula—the fused, wishbone-shaped bone formed by the bird's two clavicles—acts as a bony spring, coupled to the same tendon system that stores elastic potential energy during each wingbeat cycle.

The combined tendon-furcula assembly functions as a unified energy storage system: it dampens the force peaks generated by the pectoral muscle during each wingbeat and distributes that force across the entire cycle rather than concentrating it in a single instant. By distributing the force over time in this way, the tendon-furcula assembly reduces the peak power output required from the pectoral muscle to complete the wingbeat cycle, and this reduction in peak power improves overall flight efficiency. Together, the tendons that store and return elastic energy during each wingbeat—and the furcula, acting as a bony spring coupled to that same system—enable the bird to fly at a fraction of the metabolic cost a locomotor system lacking elastic storage capabilities would require. It is precisely this capacity to store and return energy that is lost, either wholly or in part, when a tendon is injured and ceases to function as the elastic spring it once was.

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