November 2020

Letter to Editor: Transverse Arch Importance Overstated

By Kevin A. Kirby, DPM

I write in response to the September 2020 LER article, “Overlooked Arch in the Foot Is Key to its Evolution and Function,” by William Weir, which discusses the article, “Stiffness of the Human Foot and Evolution of the Transverse Arch,” by Venkadesan et al, published in Nature in early 2020.1

The article by Venkadesan et al discussed the possible importance of the transverse arch of the midfoot to the stiffness of the longitudinal arch of the foot. Unfortunately, both articles overestimate the biomechanical influence of the transverse arch on foot stiffness and whether Venkadesan et al’s ideas truly “have not been studied previously.”

For many years, the longitudinal arch was thought to provide most of the stiffness to the foot to improve the mechanical efficiency of human bipedal gait. In their paper in Nature, Venkadesan et al make claims that the transverse arch of the midfoot contributes “more than 40% of the longitudinal stiffness of the foot.” While the authors have done their homework in modelling the transverse arch of the midfoot to estimate its contribution to longitudinal arch stiffness, the authors omitted important factors regarding the biomechanical importance of the longitudinal arch in increasing the mechanical efficiency of gait within the human bipedal animal. In other words, the estimate made by these authors that the midfoot transverse arch contributes “more than 40% of the longitudinal stiffness of the foot” appears to be wishful thinking on the part of these researchers.

Specifically, these researchers did not take into consideration how the various tension load-bearing components of the longitudinal arch work together to reinforce each other to “load-share” and thereby increase the stiffness of the longitudinal arch of the foot. In 2017, I first published the “Longitudinal Arch Load-Sharing System of the Foot,” which describes how the plantar fascia, plantar intrinsics, deep flexors, peroneus longus and plantar ligaments all work together to contribute to longitudinal arch stiffness.2

Unfortunately, Venkadesan et al appear to under-emphasize the importance of the central nervous system (CNS)-controlled plantar intrinsic muscles and CNS-controlled posterior tibial peroneus longus, flexor hallucis longus or flexor digitorum longus muscles and how they may also increase the stiffness of the longitudinal arch. These strong muscles certainly provide very significant stiffening to the longitudinal arch of the foot. In addition, these authors seem to over-emphasize the importance of the slight curve of the midfoot bones within the frontal plane.

Second, the authors did not reference other well-known authors who have also speculated on the importance of the midfoot transverse arch. Fifty years ago, Kapandji modelled the midfoot transverse arch as being a part of the “vault” of the human foot.3 Kapandji even used a nearly identical “folded sheet” model to demonstrate the increased stability of the foot from the transverse midfoot arch that has also now been used, a half-century later, by Venkadesan et al in their paper in Nature. I would have thought that Kapandji’s well-known reference should have been included within the paper as the first reference to suggest how the transverse arch may increase the longitudinal arch stiffness of the foot. One would think that an idea introduced 50 years ago with multiple references in the indexed medical and scientific literature would rate even a brief mention in a paper that claimed, somehow, the same idea was their original.

“…the Nature authors omitted important factors regarding the biomechanical importance of the longitudinal arch in increasing the mechanical efficiency of human bipedal gait…”

Third, there was little mention of the biomechanical significance of a higher longitudinal arch height and its role in increasing the stiffness of the longitudinal arch. In 1998, Arangio et al demonstrated that raising the longitudinal arch of the foot from a flatter to higher-arched structure increased the stiffness of the longitudinal arch by over two-fold.4 Longitudinal arch height has a huge effect on longitudinal arch stiffness, a fact which was never mentioned by Venkadesan et al.

In conclusion, I doubt that the transverse arch of the midfoot contributes “more than 40% of the longitudinal stiffness of the foot”, as the Nature authors claim. The potential for the longitudinal arch to greatly increase the sagittal plane stiffness of the forefoot seemed to me to be marginalized within this paper, so that the transverse arch of the midfoot came out, in the view of the authors, as being a dominant factor in longitudinal arch stiffness. The overall literature does not support this.

Kevin A. Kirby, DPM is an Adjunct Associate Professor, Department of Applied Biomechanics California School of Podiatric Medicine at Samuel Merritt College and has a private practice in Sacramento, CA 95825.

REFERENCES
  1. Venkadesan M, Yawar A, Eng CM, et al. Stiffness of the human foot and evolution of the transverse arch. Nature. 2020; 579:97-100.
  2. Kirby KA: Longitudinal arch load-sharing system of the foot. Revista Española de Podología. 2017;28(2):e18-e26.
  3. Kapandji IA. The Physiology of the Joints. Volume 2. Lower Limb. 2nd Ed. New York, NY: Churchill Livingstone; 1970.
  4. Arangio GA, Chen C, Salathe EP: Effect of varying arch height with and without the plantar fascia on the mechanical properties of the foot. Foot Ankle Int. 1998;19:705-709.

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