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Stiffness of the human foot and evolution of the transverse arch

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Stiffness of the human foot and evolution of the transverse arch

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The stiff human foot enables an efficient push-off when walking or running, and was critical for the evolution of bipedalism1–6. The uniquely arched morphology of the human midfoot is thought to stiffen it5–9, whereas other primates have flat feet that bend severely in the midfoot7,10,11. However, the relationship between midfoot geometry and stiffness remains debated in foot biomechanics12,13, podiatry14,15 and palaeontology4–6. These debates centre on the medial longitudinal arch5,6 and have not considered whether stiffness is affected by the second, transverse tarsal arch of the human foot16. Here we show that the transverse tarsal arch, acting through the inter-metatarsal tissues, is responsible for more than 40% of the longitudinal stiffness of the foot. The underlying principle resembles a floppy currency note that stiffens considerably when it curls transversally. We derive a dimensionless curvature parameter that governs the stiffness contribution of the transverse tarsal arch, demonstrate its predictive power using mechanical models of the foot and find its skeletal correlate in hominin feet. In the foot, the material properties of the inter-metatarsal tissues and the mobility of the metatarsals may additionally influence the longitudinal stiffness of the foot and thus the curvature–stiffness relationship of the transverse tarsal arch. By analysing fossils, we track the evolution of the curvature parameter among extinct hominins and show that a human-like transverse arch was a key step in the evolution of human bipedalism that predates the genus Homo by at least 1.5 million years. This renewed understanding of the foot may improve the clinical treatment of flatfoot disorders, the design of robotic feet and the study of foot function in locomotion. The transverse tarsal arch, acting through the inter-metatarsal tissues, is important for the longitudinal stiffness of the foot and its appearance is a key step in the evolution of human bipedalism.
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Nature | Vol 579 | 5 March 2020 | 97
Article
Stiffness of the human foot and evolution of
the transverse arch
Madhusudhan Venkadesan1 ✉, Ali Yawar1, Carolyn M. Eng1,10, Marcelo A. Dias2,3,4,10,
Dhiraj K. Singh5,9, Steven M. Tommasini6, Andrew H. Haims6,7, Mahesh M. Bandi5 ✉ &
Shreyas Mandre8 ✉
The sti human foot enables an ecient push-o when walking or running, and was
critical for the evolution of bipedalism1–6. The uniquely arched morphology of the
human midfoot is thought to stien it5–9, whereas other primates have at feet that
bend severely in the midfoot7,10,11. However, the relationship between midfoot
geometry and stiness remains debated in foot biomechanics12,13, podiatry14,15 and
palaeontology4–6. These debates centre on the medial longitudinal arch5,6 and have
not considered whether stiness is aected by the second, transverse tarsal arch of
the human foot16. Here we show that the transverse tarsal arch, acting through the
inter-metatarsal tissues, is responsible for more than 40% of the longitudinal stiness
of the foot. The underlying principle resembles a oppy currency note that stiens
considerably when it curls transversally. We derive a dimensionless curvature
parameter that governs the stiness contribution of the transverse tarsal arch,
demonstrate its predictive power using mechanical models of the foot and nd its
skeletal correlate in hominin feet. In the foot, the material properties of the inter-
metatarsal tissues and the mobility of the metatarsals may additionally inuence the
longitudinal stiness of the foot and thus the curvature–stiness relationship of the
transverse tarsal arch. By analysing fossils, we track the evolution of the curvature
parameter among extinct hominins and show that a human-like transverse arch was a
key step in the evolution of human bipedalism that predates the genus Homo by at
least 1.5million years. This renewed understanding of the foot may improve the
clinical treatment of atfoot disorders, the design of robotic feet and the study of foot
function in locomotion.
When walking and running, people use the ball of the foot to apply
forces that exceed bodyweight17. Because of these forces, the midfoot
experiences large sagittal-plane torques that bend the foot. A stiff
midfoot reduces the loss of propulsive work due to foot deformation
and helps to efficiently utilize the mechanical power generated by the
ankle during push-off2–4.
The unique arch shape of the human midfoot is thought to underlie
the higher stiffness of human feet compared to other primate feet5,6,9,18
(Extended Data Table1). However, stiffness is not a static quantity and mus-
cle activity can modulate midfoot stiffness in both humans and apes
13,19,20
.
The static stiffness due to the passive structures of the foot forms the
baseline around which muscles with similar mechanical action as the pas-
sive tissues are likely to modulate stiffness. Therefore, understanding the
morphological features underpinning the static stiffness is crucial for
both static and dynamic conditions (Supplementary Information 1.1–1.3).
The human midfoot has two pronounced arches: the extensively
studied medial longitudinal arch (MLA)5,6,20 and the less-studied
transverse tarsal arch (TTA) (Fig.1a). The MLA stiffens the midfoot
in part through a bow-string arrangement with the stiff longitudinal
fibres of the plantar fascia7,9 and a windlass-like mechanism due to toe
dorsiflexion just before push-off
8,21
. In addition to the plantar fascia,
the longitudinally oriented long plantar, short plantar and calcaneona-
vicular ligaments are essential for the static midfoot stiffness in humans
and other primates
9,18
. However, in contrast to the plantar fascia, the
contribution of these ligaments does not depend on the height of the
MLA, as shown by their nearly equal relative contributions in both
arched human feet9 and flat monkey feet18 (Extended Data Table1 and
Supplementary Information 1.4).
The relationship between the height or curvature of the MLA and
midfoot stiffness remains controversial5,20. Some people have no dif-
ficulty walking with a heel-to-toe style despite having little to no MLA
12
.
Conflicting evidence also emerges in foot disabilities11,22 and surgical
reconstruction of the MLA
15
when correlating MLA height with foot
flexibility, and casts further doubt on the relationship between the
https://doi.org/10.1038/s41586-020-2053-y
Received: 29 March 2018
Accepted: 23 January 2020
Published online: 26 February 2020
Check for updates
1Department of Mechanical Engineering and Materials Science, Yale University, New Haven, CT, USA. 2School of Science, Aalto University, Espoo, Finland. 3Nordic Institute for Theoretical
Physics (NORDITA), Stockholm, Sweden. 4Department of Engineering, Aarhus University, Aarhus, Denmark. 5Nonlinear and Non-equilibrium Physics Unit, OIST Graduate University, Onna,
Japan. 6Department of Orthopaedics and Rehabilitation, Yale University, New Haven, CT, USA. 7Department of Radiology and Biomedical Imaging, Yale University, New Haven, CT, USA.
8Mathematics Institute, University of Warwick, Coventry, UK. 9Present address: Engineering Mechanics Unit, Jawaharlal Nehru Centre for Advanced Scientiic Research, Bangalore, India.
10These authors contributed equally: Carolyn M. Eng, Marcelo Dias. e-mail: m.venkadesan@yale.edu; bandi@oist.jp; shreyas.mandre@warwick.ac.uk
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... The longitudinal ligaments comprise of the short and long plantar ligaments and the spring ligament complex (plantar calcaneonavicular ligament) [9,13]. This appears reasonable, as the midfoot experiences large sagittal-plane torques bending the foot during walking [27,28]. The ligaments thereby provide a static stiffness, and the muscle activity modulates the midfoot stiffness [27]. ...
... This appears reasonable, as the midfoot experiences large sagittal-plane torques bending the foot during walking [27,28]. The ligaments thereby provide a static stiffness, and the muscle activity modulates the midfoot stiffness [27]. Moreover, the plantar ligaments, contrary to the herein observed lateral and dorsal ligaments, are constantly present in humans and apes. ...
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Background This study aims to analyze the ligaments of the dorso-lateral calcaneo-cuboid joint and to assess the biomechanical relevance of the bifurcate ligament. Methods 16 specimens were analyzed for their ligamentous anatomy of the dorso-lateral calcaneo-cuboid joint and side-alternating assigned to two groups with varying ligamentous dissection order. The Chopart joint was stressed in plantar, medial, and lateral direction measuring the displacement by an 3D motion tracker for every dissection step. Results 37.5% of specimens had all ligaments (lateral calcaneo-cuboid, dorsal calcaneo-cuboid, bifurcate calcaneo-cuboid, bifurcate calcaneo-navicular), 37.5% were lacking bifurcate´s calcaneo-cuboid-portion, and 25% presented without dorsal calcaneo-cuboid. Biomechanical testing revealed no significant displacement within the calcaneo-cuboid or talo-navicular joint for any stressed state except for axial compression with dissected dorsal talo-navicular joint capsule in Group 2. Conclusion Broad morphological variability and missing significant displacement regardless of its integrity, make the bifurcate ligament appear of limited biomechanical relevance.
... The plantar muscles (Farris et al., 2019;Kelly et al., 2014), ligaments (Bates et al., 2013;Bojsen-Møller, 1979;Ker et al., 1987;Lovejoy et al., 2009), and fascia (Ker et al., 1987;Sichting et al., 2020) work in tandem with the shape and posture of the bones in the medial (Hicks, 1955;H. Elftman and Manter, 1935;Bojsen-Møller, 1979), lateral(McNutt et al., 2018, and transverse arches (Venkadesan et al., 2020) to increase the intrinsic stiffness that enables foot leverage. Specifically, the static height of the medial arch is thought to appreciably influence the arch's intrinsic stiffness, despite the generally weak relationship between arch height and arch vertical mobility (Cornwall and McPoil, 2011;Zifchock et al., 2006). ...
... show evidence of more pronounced medial and transverse structural arches, likely supported by arch-spanning ligaments and muscles (Susman, 1983;Venkadesan et al., 2020). As a result, Homo could better take advantage of arch mobility and its effects on ankle posture and the contractile state of the triceps surae muscles in propulsion. ...
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Developing the ability to habitually walk and run upright on two feet is one of the most significant transformations to have occurred in human evolution. Many musculoskeletal adaptations enabled bipedal locomotion, including dramatic structural changes to the foot and, in particular, the evolution of an elevated medial arch (H. Elftman and Manter, 1935). The foot's arched structure has previously been assumed to play a central role in directly propelling the centre of mass forward and upward through leverage about the toes (Herbert Elftman and Manter, 1935) and a spring-like energy recoil (Hicks, 1955). Paradoxically, these roles seemingly require either arch rigidity (for the former) or mobility (for the latter). However, it is unclear whether or how the mobility and height of the medial arch support its propulsive lever function. Here we show, using high-speed biplanar x-ray, that regardless of intraspecific differences in medial arch height, arch recoil enables a longer contact time and favourable propulsive conditions for walking upright on an extended leg. This mechanism presumably helped drive the evolution of the longitudinal arch after our last common ancestor with chimpanzees, who lack this mobility during push-off. We discovered that the previously overlooked navicular-medial cuneiform joint is primarily responsible for this mobility in human arches, suggesting that future morphological investigations of this joint will provide new interpretations of the fossil record. Our work further suggests that enabling the mobility of the longitudinal arch in footwear and surgical interventions is critical for maintaining the ankle's natural propulsive ability.
... In addition, the medial longitudinal arch (MLA) and the transverse tarsal arch (TTA) are calculated as planar angles. These arches are a representation of the midtarsal joint poses due to active and passive foot structures, and are believed to play a major role in stiffening the foot during gait [45]. The medial longitudinal arch is defined as the angle between a proximal and distal 3D vector. ...
... The methodology to determine the marker placement sensitivity has been described in detail previously [36]. In short, ten adults (6 females, 26.8 ± 2.6 years, 176.4 ± 8.1 cm, 67.2 ± 8.5 kg, EU shoe size: 41 ± 2, range [38][39][40][41][42][43][44][45] and nine children (5 female, 10.7 ± 1.9 years, 147.7 ± 12.8 cm, 41.1 ± 10.9 kg, EU shoe size: 36 ± 2, range: 31-38) with asymptomatic feet were included. A set of 21 markers was placed on the right foot and shank, which included all AFM, OFM and RFM markers. ...
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... Of these factors, the one most affected by running technique is running economy (Barnes & Kilding, 2015;Saunders et al., 2004), defined as distance travelled per unit of energy expended. Because of our evolutionary history, all humans share key anatomical traits that minimise energy expenditure during running, such as long lower limbs, long Achilles tendons, foot arches and short toes (Bramble & Lieberman, 2004;Holowka & Lieberman, 2018;Rolian et al., 2008;Venkadesan et al., 2020). Nevertheless, running economy can vary substantially between individuals and groups (Barnes & Kilding, 2015), particularly between people who run habitually vs. infrequently (Bransford & Howley, 1977;Morgan et al., 1995). ...
... These adaptations almost certainly originated long ago in the human evolutionary lineage, very likely among early members of Homo during the Early Pleistocene (Bramble & Lieberman, 2004). Examples of such adaptations can be found across diverse systems in our bodies including the musculoskeletal system (Bramble & Lieberman, 2004;Eng et al., 2015;Holowka & Lieberman, 2018;Lieberman et al., 2006;Rolian et al., 2008;Venkadesan et al., 2020), cardiopulmonary system (Bramble & Carrier, 1983;Callison et al., 2019;Shave et al., 2019), thermoregulatory system (Carrier, 1984;Kamberov et al., 2018;Lieberman, 2015) and nervous system (Raichlen et al., 2012;Wallace et al., 2018b). Thus, from a broad evolutionary perspective, regardless of variation in running techniques, humans are in general 'good' runners, or at least we have an evolved natural capacity to run well. ...
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... For example, devices that add stiffness to the foot (e.g., shoes and/or insoles) can enhance the ankle plantar flexor force production via a slower contractile velocity [16], which can afford whole-body metabolic energy savings in some locomotion tasks, such as during fast walking [16]. Moreover, foot structures have inherent mechanisms that may regulate its stiffness (and, hence, the lever function), including structural (e.g., the curvature of the transverse arch [17]) and neuromechanical (e.g., the activation of intrinsic foot muscles [11]) features. All of these studies in young adults may offer a theoretical basis for examining the foot's lever function and its consequences on locomotor performance in older adults. ...
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... The third layer was two-thirds length (around the transverse arch). This probably enhanced the support of the transverse arch by increasing the arch height [37]. ...
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Modern humans are characterized by a highly specialized foot that reflects our obligate bipedalism. Our understanding of hominin foot evolution is, although, hindered by a paucity of well-associated remains. Here we describe the foot of Homo naledi from Dinaledi Chamber, South Africa, using 107 pedal elements, including one nearly-complete adult foot. The H. naledi foot is predominantly modern human-like in morphology and inferred function, with an adducted hallux, an elongated tarsus, and derived ankle and calcaneocuboid joints. In combination, these features indicate a foot well adapted for striding bipedalism. However, the H. naledi foot differs from modern humans in having more curved proximal pedal phalanges, and features suggestive of a reduced medial longitudinal arch. Within the context of primitive features found elsewhere in the skeleton, these findings suggest a unique locomotor repertoire for H. naledi, thus providing further evidence of locomotor diversity within both the hominin clade and the genus Homo.
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