The human foot isn’t as stiff as originally thought. In fact, many of its features have been found to be common to the great apes as a whole.

One of the tools researchers use to pinpoint the presence of walking humans in fossil records, and distinguish them from those of apes, is the shape of the human foot. The human foot, anterior to the heel, is a flat and flexible plate, with arches on the lateral (little-toe) and medial (big-toe) side, and one running across the foot from lateral to medial. In the 1930s, two papers, by anatomists Herbert Elftman and John Manter, argued that it was quite distinct from that of other apes in having a stiff arch on the lateral side, stabilised in bone, and very often compared to an architectural arch.

This design, Elftman and Manter argued, provided a degree of stabilisation that was missing in other apes, which had feet that flexed in the mid-foot and made ground contact there. The stiff arch on the lateral side was thought to be especially unique by archaeologists and podiatrists alike. However, the uniqueness was never put to the test until recently.

In a paper shortly to be published in Proceedings of the Royal Society, Dr. Karl Bates and Prof. Robin Crompton, from the Department of Musculoskeletal Biology, University of Liverpool, and their colleagues report a recent study that measured about 25,000 footfalls recorded at the University’s Gait Laboratory. Forty-five people, aged 18-68, were made to walk on a treadmill, rigged with pressure-sensitive sensors, at 1.1 m/s for 5 minutes. Using a pixel-by-pixel statistical comparison of foot-pressure levels exerted on the whole surface, Prof. Crompton & co. found that some human feet were relatively stiff and some flexible, just like they found in measurements they’d made in other other apes.

“Most normal humans produce footfalls in which there is ground contact and hence flexion in the lateral mid-foot. So, there is no clear dividing line between human foot function and that of other apes,” Prof. Crompton said about the results of the study.

It was thought that the stabilising lateral arch prevented the human foot from making ground contact in the lateral mid-foot region, and turned it into a permanently stiff lever. However, all the arches now appear to be spring-like structures that can absorb, store and release energy, and their springiness can be actively adjusted by muscles to suit ground conditions. The Liverpool group suggest that the flexibility was retained from our tree-dwelling ancestors even as we left the trees, as they enabled the foot to accommodate soft and hard ground much better than can horses, dogs and most other ground-dwelling animals.

“Contrary to received wisdom, the human foot is not unique,” Prof. Crompton argues. “It is relatively stiff, certainly, but this stiffness is substantially transient and adjustable.”

The conclusions defy the conventional wisdom derived from the studies of Elftman and Manter, including a more recent model used in diagnoses of diabetic foot and osteoarthritis. Between 1971 and 1978, Merton Root, an American podiatrist, created a unified reference system for understanding foot biomechanics based on eight criteria of normalcy. Not meeting one of these would mean it was a possible case of diabetic foot, for instance. A key criterion for normalcy in this so-called ‘Root model’ is the absence of substantial flexion and ground contact under the lateral mid-foot.

However, data from the Liverpool study shows that 67 per cent of the healthy individuals in his study recorded at least one footfall in both feet that violated this criterion, and indeed exceeded clinical criteria for diagnosis of possible diabetic foot or osteoarthritis. “This suggests that some patients may be getting unnecessary and actually harmful treatment,” Prof. Crompton added. In fact, his research adds to a growing body of literature and studies asking for the Root paradigm to be overturned.

Understanding the evolution of human walking is another field impacted by the study, all of which have much to gain from understanding how the complex muscle-bone system of the foot works. Many studies start by comparing the human foot to that of the chimpanzee – on the assumption that just because the two are close genetic cousins, human feet evolved from something like that of the chimpanzee.

However, “they do not for example consider that chimpanzees have been evolving just as long as humans and so are probably just as specialized, if in a different direction,” Prof. Crompton ventured. In fact, some other apes have feet remarkably human-like in proportions, such as the lowland gorilla. Interestingly, it is probably the most tree-dwelling of the African apes, a fact which supports the idea of a tree-dwelling origin for modern human foot function.

"Our ancestors probably first developed flexibility in their feet when they were primarily tree-dwelling, and moving on bendy branches, but as time passed and we became more and more ground-dwelling animals, some new features evolved to enable us to move quickly on the ground,” said Dr. Bates in a press release.