British professor calls ankle-foot complex 'a masterpiece of engineering'

Five-hundred years ago, the great artist and engineer Leonardo da Vinci wrote, "The foot is a masterpiece of engineering and a work of art."

Stuart Burgess, a professor of engineering design at Bristol University in the United Kingdom, agrees. In a recent article in the journal BIO-complexity, Burgess reviewed in detail the complex form and function of the ankle-foot complex, and refutes the arguments of those who claim that human feet have a "bad design."

Burgess has nearly 40 years' experience in engineering design and research. He carried out research on the ankle-foot complex as part of a visiting research fellowship at Cambridge University in 2021.

The article outlines the main functions of the foot, including flexibility, strength and rigidity, plantar dorsiflexion, pronation-supination and balance. These are basically movements that humans make every day without thinking about foot design--walking, running, standing on one foot or on both feet.

Burgess reviewed the three anatomical sections of the human foot, the hindfoot, midfoot, and forefoot, and the lower leg bones (fibula and tibia) that connect to the foot. He also described the 50 ligaments and 10 muscles that hold together the seven bones of the ankle-foot complex, and provides detailed illustrations to help the reader visualize how the complex system works together. 

He then elaborated about how the ankle-foot design is optimally engineered to perform the main functions required of the foot.

The `bad design' arguments

Burgess summarizes the `bad design' arguments put forward by critic Nathan Lents in his 2008 book Human Errors. Lents makes five specific claims. These include that most of the ankle bones are pointless, there's no reason for the paired lower leg bones, a single fused structure of solid bone and no ligaments would function better,  the ankle malfunctions because of its bad design and that no engineer would have so many separate parts in a joint.

Burgess notes Lents' arguments assume that because humans evolved from a four-legged ape-like creature, evolution couldn't efficiently create an optimal design for the two-legged human. 

As Burgess puts it, "Lents essentially argues that since evolution could not produce an optimal layout of bones in the ankle-foot complex, the ankle-foot complex must therefore be assumed to be a very sub-optimal design."

`Bad design' arguments refuted

Burgess shows that all the bones of the ankle-foot complex play a role in the foot's functioning and are essential from an engineering standpoint.

As for the argument that a fused ankle structure would work better and stronger, Burgess notes that ankle fusions performed for medical reasons have been shown to degrade ankle functioning. 

This is because, first, the loads on the joint are higher when the "shock-absorbing flexibility of individual joints" is removed. Second, Burgess says, "a solid beam structure is much weaker than an elemental arch for a material like bone."

Burgess says that critics who say that the ankle joint malfunctions often, don't take into account that failure is often due to misuse. He uses an analogy to a car, noting, "Most modern cars are well designed and very reliable when in good condition and used properly. However, despite the high quality of design, a modern car will fail if overloaded or neglected or if it is simply old."

As for the argument that engineers would not design a joint with so many parts, Burgess counters that designers of robotic feet and prosthetic feet are advised by engineers to "copy the design of the foot to produce better functionality." He also notes that it's also recommended that ankle replacements follow the anatomy of human ankles.

Appreciating the complex anatomy

As bipeds human beings carry out all the functions that Burgess describes without thinking much about the movements involved. His article provides a useful and well-illustrated guide to the specific mechanics of everyday movement.

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Stuart Burgess. "Why the Ankle-Foot Complex Is a Masterpiece of Engineering and a Rebuttal of `Bad Design' Arguments." BIO-complexity, Nov. 28, 2022.

doi:10.5048/BIO-C.2022.3.