The tendons that hold your skeleton together are diverse, tightly knit, and overlapping in complex ways. The optimization of tendons for function extends from the macroscale to the nanoscale.
Anatomists have understood the look and feel of tendons ever since the early days of dissection, but only recently have scientists been able to study their nanostructure. A new paper in Nature’s open-access journal Scientific Reports reveals that “In tendons, differing physiological requirements lead to functionally distinct nanostructures.” They begin with a summary of what tendons do:
While collectively referred to as tendons, the physiological functions served by the tissues that connect muscle to bone vary considerably within certain animals, including humans. Tendons like the digital extensors and flexors of the hand transmit forces in such a way that fingers can be moved with great precision. Other tendons, like the Achilles, function as springs that enable locomotive activities such as running and jumping to be performed efficiently by storing energy during deceleration, and then releasing it to help power acceleration. [Emphasis added.]
Already we’re learning amazing things about tendons to earn our respect. In his series “The Designed Body” in these pages two years ago, Howard Glicksman described how tendons are attached to muscles so as to handle tension and prevent strain.
Next, the researchers talk physiology. How do tendons function in activities of life? Some tendons function just by virtue of their position. Others are highly dynamic, acting as load-bearing springs. Notice how robust and resilient tendons are, able to perform repetitive actions many times over a lifetime of basketball, walking, or playing a piano. Marathon runners should pay attention to this paragraph:
Tendons that store and release energy face differing functional demands to those that are primarily positional in nature. Unlike positional tendons, energy storing tendons must be able to withstand large forces applied in a highly repetitive manner. When running, for example, tension in the Achilles tendon during ground contact exceeds 12 times body weight; in the case of a marathon, the tendon must endure this loading about 25,000 times without rest. In other tissues such as bone, demanding mechanical loading regimes are dealt with via remodeling, where fatigue damage occurs, but its excessive accumulation is prevented by turnover of the damaged components. Surprisingly, despite undergoing high stress cyclic loading, energy storing tendons do not undergo appreciable remodeling.Rather than continually repair accrued damage, these specialized tendons appear to have evolved highly fatigue resistant structures.
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Source: Evolution News