A5 Tendon Failure

Topic

In birds, the wing and digital tendon system performs two distinct physical functions depending on the body region where it operates. In the wing, the tendon acts as an elastic spring involved in every wingbeat cycle; in the leg, the digital flexor tendon serves as the element that transmits and withstands the impact force of each landing. Failure of the tendon system—whether due to tendinitis, partial rupture, or complete rupture—affects these two functions differently; therefore, it is best to examine them separately before considering them together.

In the wing, the tendon functions as an elastic spring that stores elastic potential energy during one phase of the wingbeat cycle and releases it during the next, thereby reducing the net power demand placed on the pectoral muscle. In hummingbirds and swifts, this elastic spring contributes up to 40 percent of the energy for each wingbeat cycle—a portion that, in a healthy state, the pectoral muscle does not need to generate itself.

Tendinitis or a partial rupture of the wing tendon reduces the stiffness of this elastic spring and, consequently, the efficiency with which it returns stored energy during each cycle. As tendon stiffness and return efficiency decline, the elastic spring contributes a smaller fraction of the wingbeat cycle's energy, shifting a greater load back onto the pectoral muscle during each cycle. This transferred load accelerates fatigue in the pectoral muscle—the very muscle whose power-generating capacity is depleted more rapidly the larger the energy fraction it no longer receives from a healthy tendon. In the leg, however, the digital flexor tendon performs a different function: it is critical for gripping and landing, as it transmits the impact forces generated whenever the bird places its weight on the leg upon landing—forces that can exceed the bird's body weight several times over. Unlike the wing tendon, which returns energy during each wingbeat cycle, the digital flexor tendon neither stores nor returns energy; its function is purely structural—to withstand that impact force without rupturing.

If the cross-section of the digital flexor tendon is compromised—for instance, due to prior tendonitis or an unhealed partial tear—the tendon must withstand the same impact force with less load-bearing material, and the risk of a complete rupture during landing ceases to be remote. For any given condition of the digital flexor tendon, there is a threshold of impact force above which complete rupture becomes likely: the more compromised the cross-section, the lower that threshold becomes, meaning landings that a healthy tendon would withstand without issue become sufficient to cause a rupture.

Failure of the tendon system thus occurs via two completely different pathways: in the wing, through progressive degradation that shifts elastic load onto the pectoral muscle, accelerating its fatigue; and in the leg, through a sudden, complete rupture once the impact force of a landing exceeds the threshold that the compromised cross-section of the digital flexor tendon can still withstand. Evaluating these two pathways separately makes it possible to distinguish—in a bird showing signs of tendon failure—whether the issue is a gradual loss of flight energy efficiency that warrants monitoring, or an imminent risk of complete leg rupture that could incapacitate the bird during its next landing.

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