In a peer-reviewed paper published in BIO-Complexity, Bristol University engineering professor Stuart Burgess explains “Why the Ankle-Foot Complex Is a Masterpiece of Engineering and a Rebuttal of ‘Bad Design’ Arguments.” Brian Miller has previously covered Professor Burgess’s arguments in a lecture, but those are framed as a response to arguments from ID-critics such as Jeremy DeSilva and Nathan Lents. Those critics claim that the human foot-ankle complex is sub-optimal because it reflects an unguided process where evolution attempted to convert a skeletal structure adapted for quadrupedal locomotion to bipedalism. Burgess argues in response that the ankle-foot complex “show a very high degree of complexity and fine-tuning” and “masterful engineering.” Moreover, “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” and the “five midfoot bones are needed to form the optimal kinematic and structural interface between the hindfoot and forefoot.”

Burgess observes that many who have studied the foot without a preconceived bias have recognized its “excellent design.” He quotes Leonardo da Vinci who called the human foot “a masterpiece of engineering and a work of art,” and more modern researchers who observe the “nearly effortless human gait” or who note that various foot structures “work in perfect synchronisation” because it is “superbly constructed for ambulation.” 

A Contrast with “Bad Design”

In contrast, “bad design” proponents believe that most of the seven anklebones are pointless, poorly coordinated, and fundamentally a bad design because a “fused structure” would work better than “a joint with so many separate parts.” Burgess answers arguments that the ankle-foot performs poorly for bipedalism because it was originally evolved for quadrupedal locomotion by observing that such arguments “are based on circular reasoning and assumptions about what evolution could or could not do in the past.” He believes that “A better scientific approach to assessing the quality of design is to study the actual biomechanics and functions of the foot.”

Burgess observes that “The requirements for agile bipedal movement are extremely demanding.” After all, the foot must be “a compact multifunctioning precision device” which has to fulfill multiple requirements which are sometimes contradictory:

1. Act as a strong and stiff lever to propel the body forwards in walking and running. Joint movement is plantarflexion.

2. Act as a flexible platform to absorb shocks and adapt to uneven ground. Joint movements include dorsi-flexion, pronation, and supination.

3. Provide 3-point contact with the ground to allow standing on one or two legs and to enable controlled push-off from the ball of the feet. The control must involve fine adjustment of direction as well as power.

Difficult and Contradictory Demands

Yet Burgess further notes that “The requirements of a stiff lever and flexible platform are difficult to achieve because they are contradictory. To achieve these two requirements the foot must have stiffness and flexibility in just the right places. In addition, the foot must have the ability to adjust stiffness through precise control of muscles.” The foot is able to accomplish this because it “has three interconnecting flexible arches that perform multiple functions in particular three-point contact with the ground, stiff lever for take-off and flexibility for shock absorption.” Burgess notes how well-designed these arches are:

There are several features that maintain the integrity of the arches: (i) foot arches segmented like a Roman arch, which induces compressive forces, particularly the bone that forms the keystone to the arch; (ii) short ligaments that tie adjacent bones together; (iii) longer ligaments (like the spring ligament) that tie the arch across multiple bones; (iv) muscle-tendon groups that act like a sling, pulling the arches upwards; and (v) muscles that stiffen the arch.

He further notes that the bones of the midfoot allow it to perform five main sub-functions, including providing a “flexible transverse arch,” “Load bearing structure during pronation,” “Kinematic interfaces for pronation-supination,” “Structural interface for longitudinal loads,” and “Stiffening of the medial arch.” 

Burgess also finds that “Another specialised design feature in the ankle-foot complex is the elastic hinge joints,” as some 17 of such joints allow “significant flexibility” in the foot and also aid in shock absorption. In fact, Burgess reports that these elastic hinge joints have at least five sub-functions, including “(i) flexibility; (ii) load-bearing; (iii) energy storage; (iv) failsafe design; and (v) ultra-low friction.” 

Bad-design proponents have asked why there are paired bones at the bottom of the leg above the ankle instead of a single bone. Burgess notes there are good reasons for this as fibula is “well known to provide stability to the ankle joint” via “a type of linkage system with multiple bars.” He cites two specific advantages to having a fibula bone:

One advantage of the fibula is that it increases the moment arm (mechanical advantage) of muscles acting on the ankle-foot complex. A second advantage is that the fibula increases the attachment area for muscles and therefore allows more muscle to act on the joint.

Answering Bad Design

After providing this review of the engineering functions of the ankle-foot complex, Burgess is able to address claims that many foot and angle bones are functionless. In reality, “this paper has shown that all the bones of the ankle-foot complex have very important roles in the specialized design features. In particular, the five bones of the midfoot have multiple functions.” He also definitively shows that the fibula bone is necessary because it “provides essential stability to the ankle joint during pronation by forming a multi-bar linkage mechanism.” Burgess shows that a fused ankle structure would not function better because “It is well known in the medical field that ankle fusions lead to a degradation of ankle performance” and relative movement of ankle bones affords various functions, including shock absorption. 

A major anti-design argument is that ankles are prone to sprains or other injuries, but Burgess notes that this confuses misuse with bad design:

The importance of this differentiation can be illustrated by analogy with a modern car. 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 very old. Therefore, when considering malfunctions in joints it is important to check why there was a malfunction. If the ankle-foot complex malfunctions due to overload, neglect, or health issues, this does not mean the design can be judged as bad.

Burgess ends with four conclusions:

1. There are four highly specialised design features in the ankle-foot complex

2. The ankle-foot complex is superior to human-engineered joints

3. Lents’s bad design arguments are contrary to scientific evidence

4. Engineering insight explains form and function

This last point is crucial because it shows that the very design and structure of the ankle-foot complex must exist to for it to perform its functions. According to Burgess, the system exhibits “very sophisticated engineering design.”