In my final year of physical therapy school I was asked by fellow classmates to give a speech at the annual Virginia Physical Therapy Association’s student conclave, because I often took advantage of class presentations to talk about primates while I had a captive, albeit disgruntled audience of fifty. While this may sound insane when viewed out of context, shoehorning apes into presentations was not as difficult, nor as inappropriate as one might expect. So, having been armed with an ape-related topic and the irresistible compulsion to torment students from other schools, I agreed emphatically. I fashioned a speech about the anatomy of ape and human feet with particular regard to the outer (lateral) arch of the foot, and if you’re wondering how the two might relate in a manner that is relevant to students of rehabilitative medicine and the general public, I would urge you to read further. However, if I’ve failed to inspire your curiosity, I’ll probably just write future posts about monkeys and dismantle your skepticism with persistence.
To begin, I must take you back to my first full time clinical internship. During this time I briefly observed a patient being treated for pain in a joint that helps comprise the outer arch of the foot, known as the calcaneocuboid joint. You may see this referred to as “cuboid syndrome.”

Cuboid syndrome is primarily caused by ankle sprains, but in more rare cases, like that of the patient I observed, it may result from overuse and repeated tissue strain while walking. Normally as we walk, the bones of our mid-foot lock in the shape of an arch to aid in pushing off from the ground. If the cuboid bone cannot lock properly to maintain that arch, as may be the case in a pronated or “flat” foot, a bending stress may over-load the soft tissues supporting this bone. This repeated process may then lead to increased mobility and ultimately, pain. But while I stood beside that patient, I realized that I had seen this concept of hyper-mobility in the outer foot before, only to a much greater degree. I saw it in the image below, and no, this is not my foot.

This is the foot of a bonobo ape, and the bend it displays is known as the “mid-tarsal break.” Essentially that means the outer part of the foot is bending in the middle. This phenomenon is found in all ape feet, and is not considered pathological. This mobility then prevents an ape’s foot bones from locking in the same manner as ours do before push-off while stepping forward. Instead, the heel leaves the ground first, followed by the middle of the foot, and lastly, the toes.

Their feet function in this manner because apes move on four limbs, and would not benefit from arched feet in the same manner as humans. Their arches would also be rendered useless in the jungle canopy, which is precisely where we find the benefit of a flexible foot. The “mid-tarsal break” better enables apes to vertically scale trees. It allows them to more effectively grasp a trunk with their toes and simultaneously push up from that same foot. We humans, however, do not live in the trees. So why would this information be relevant to us?
Well, after I left the clinic that day I opened an old textbook to find this diagram.

You can see the flexible ape foot beside that of a human, and below it reads, “The mid-tarsal break is not seen in the more rigid human foot.” Imagine my surprise then, when an internet search uncovered this image – an uncanny mid-tarsal break, occurring in a human foot.

This is a phenomenon once thought not to occur in people at all. But as this photo might suggest, that is beginning to change. A recent study in Boston found that eight percent of a sample of museum goers displayed this foot type. The researchers also showed that the main source of this bend was the same as in apes, the joints involving the cuboid bone. And not only that, a follow-up study also correlated the break with more rounded bases of their two outer metatarsal bones (the long bones just before the toes) – meaning that their bones were more ape-like.

As Bostonians are not renowned tree climbers, why would their feet resemble apes for this particular feature? A physical therapist might ascribe it at first to abnormal stresses in the foot. For instance, a therapist might suspect that over-pronation and low arches play a role in this, especially given that I introduced them earlier as causes of hyper-mobility in the foot. But these factors alone cannot fully explain it. Because although this phenomenon was found to be significantly correlated with lower arches, excessive pronation, and high BMI, it was still found in many people who were average for these measures. Participants were also excluded if they had any known history of foot pathology. Therefore, it is likely that this foot type is a design with which some people can function perfectly fine. The outer arch of the foot is simply less uniform than we once thought, and we have found in Homo sapiens a spectrum of mobility in the arch.
Now, students of physical therapy learn about ways in which people differ from what is considered normal. Often times they are called deformities, which make it tempting to see variation like that shown above as something going wrong. But what I have presented thus far indicates that an ape-like foot can be both a normal and natural occurrence. In fact, variation exists across most if not all aspects of human anatomy, and it is not limited to shapes and sizes. Some individuals have entire muscles uncommon to most others, called the pyramidalis or the levator claviculae. A number of us also have asymptomatic scoliotic spines and extra bones in the foot. So too do some of us have 4 vertebrae in the lumbar spine (low back) while others have 6, and 5 vertebrae are most common.
Furthermore, similar variation is found in ape feet. Although their feet are typically more flexible than ours, they too can be more mobile, or more rigid than what is usual. Not only that, while four lumbar vertebrae are common to chimpanzees, they too can have (at lower frequencies) 3 or 5 lumbar vertebrae! What we have then is a bell shaped curve, where one feature is predominant but others exist at lower frequencies, such as the number of vertebrae in the spine.

Now, not all of this variation is the consequence of mere error in development, and the reason for that lies in adaptation. Variation exists because it is essential to adaptation. It affords a species the potential to exploit new environments and resources. A flexible foot was once important to our ancestors, but through the generations, likely in consequence of the demands of our environment, our bell curve shifted what would have been considered “normal” to its current position today.

But in this process of change, not all ancient genes are lost or purged. Old features may persist, like subtle reminders of where we came from. And like I stated previously, it is not just in our feet. There is a beautiful world of variation within our bodies, and as a physical therapist I have the pleasure of treating that variation and how it interacts with the environment. In the patient I observed with cuboid syndrome, slight mobility was abnormal. Other people, however, may exist perfectly well with a more flexible foot. But it is important to remember that these individuals, and these feet, are still representative of the human race; even if their bodies differ from the anatomical diagrams of muscles and bones that you might see in clinical settings. We may not all be completely represented by such diagrams, and that is okay, because it is not solely our resemblance to these images that makes us human, it is also our deviation from them. It is our variation.

If there is one thing I hope to be gleaned from this post, it is that structural deviations are not sentences to lives of eternal pain and suffering. In fact, if you have foot pain, having a flatter foot may not be playing a role at all. You may simply require more regular movement, or the issue may stem from the spine or hips. It is ultimately up to your doctor and therapist to determine if that is the case. Just know that if you possess a feature like those described above, your body is beautiful, and should be celebrated for the complexity it affords us as a people.

Written by: Dr. Thomas Meeusen, PT, DPT

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