BIO-Complexity, Vol 2022

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Why the Ankle-Foot Complex Is a Masterpiece of Engineering and a Rebuttal of “Bad Design” Arguments

Stuart Burgess


To perform agile bipedal movement, human feet must meet extremely demanding requirements in terms of compactness, flexibility, strength, joint movements, actuation, and control. These requirements are met through very sophisticated engineering solutions. This paper describes four highly specialised mechanical features of the ankle-foot complex that show a very high degree of complexity and fine-tuning: i) a multi-arched structure; ii) a multifunctioning midfoot; iii) elastic hinge joints; and iv) a fibula linkage mechanism. Engineering insight reveals a close relationship between form and function in the ankle, a relationship seen in its multiple bones and the layout of those bones. In particular, it is shown that the five midfoot bones are needed to form the optimal kinematic and structural interface between the hindfoot and forefoot. The relatively poor performance of prosthetic feet, robot feet, and replacement ankles confirms that bipedal movement involves extremely demanding requirements and that the human foot is an example of masterful engineering. The claims of bad design in human feet by authors such as Nathan Lents are shown to clearly contradict scientific evidence.

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