Midtarsal Break: Why Some Human Feet Bend in the Middle

The midtarsal break is a visible fold or bend across the middle of the foot that happens during walking, where the bones of the midfoot flex upward as the heel lifts off the ground. It is a hallmark of how non-human primates walk and climb, and for decades it was treated as something humans simply do not have. That story turns out to be incomplete, and recent research has revealed surprising variation in how the human midfoot actually behaves.

What the Midtarsal Break Looks Like and Where It Happens

When you watch a chimpanzee or macaque walk, there is a moment during each step when the middle of the foot visibly bends upward, almost as if the foot were folding in half. That fold is the midtarsal break. The name suggests it occurs at a single joint, but the motion is actually distributed across two joint areas in the outer part of the foot. Data from X-rays, dissections, video of captive primates, and skeletal specimens show that roughly two-thirds of total midfoot bending happens at the joint between the cuboid bone and the bases of the fourth and fifth metatarsals, with the remaining third happening at the joint between the heel bone (calcaneus) and the cuboid.¹ The shape of those joints matters: the bases of the outer metatarsals are convex and sit inside concave facets on the cuboid, creating a socket that permits upward bending rather than locking against it.

In non-human primates, this flexibility is an advantage for gripping branches and navigating uneven arboreal surfaces. The bone density in their calcaneus-cuboid joint reflects this: compressive loading concentrates along the top (dorsal) edge of the joint, exactly where forces peak during the midtarsal break phase of support.² The entire architecture of the outer midfoot in these species is built to accommodate repeated bending under load.

Why the Human Foot Was Thought to Be Different

The textbook explanation for why humans walk the way we do rests on an idea called the mobile adaptor-rigid lever paradigm. The foot supposedly starts each step as a loose, flexible structure that molds to the ground, then stiffens into a rigid lever just before push-off, so that calf muscle force transfers efficiently into forward propulsion. The midtarsal break’s absence in humans was cited as key evidence for this rigidity. Non-human primates, with their floppy midfoot, were said to waste energy because they could not form a stiff lever.

This model was enormously influential in podiatry, orthopedics, and evolutionary biology. The problem, as a 2023 narrative review laid out, is that the rigid-lever model was never experimentally tested when it was first proposed. When researchers finally measured foot stiffness during actual walking and running with modern equipment, they found that the foot does not actually increase its stiffness just before or during push-off the way the model predicts.³ The human foot behaves in a more versatile, non-linear way, adjusting its stiffness continuously to match the demands of each step rather than toggling between two fixed states. This does not mean the midfoot is as floppy as a chimpanzee’s, but it does mean the old dichotomy between “rigid human” and “flexible ape” was drawn too sharply.

Some Humans Do Have a Midtarsal Break

Perhaps the most surprising finding in this area came from pressure-plate studies of modern humans. In one study of 398 adults, about 8 percent showed both elevated pressure under the outer midfoot and visible midfoot bending during walking that matched the definition of a midtarsal break.⁴ These were not people with known foot pathology or flat feet. They were otherwise normal walkers whose midfoot just happened to be more flexible than average.

A larger follow-up study of 671 adults and children confirmed that midfoot flexibility in humans exists on a broad spectrum, with some individuals overlapping the range seen in apes.⁵ This variation does not appear to cause pain or dysfunction on its own. People with more midfoot flexibility just load their feet differently, and their skeletons reflect it in subtle changes to joint surface shape.

This finding was important beyond clinical foot care. It means the sharp line researchers had drawn between “human-like” and “ape-like” feet in the fossil record was based on an incomplete picture of living humans. If 8 percent of modern people walk with a visible midtarsal break, then finding signs of midfoot flexibility in a fossil hominin does not automatically mean that individual was still climbing trees or had not yet evolved “modern” bipedalism.

What Keeps the Midfoot Stiff in Most People

Given that the bones of the outer midfoot have some capacity to bend, the question becomes: what prevents that bend from happening in most human steps? The answer involves a combination of ligaments, tendons, and the geometric architecture of the arch itself.

The plantar fascia, a thick band of connective tissue running along the sole from heel to toes, is the most well-known stabilizer. But cadaver experiments show that cutting the plantar fascia alone decreases arch height without causing total collapse. It takes cutting all four major plantar ligaments, including the long plantar ligament, to produce a complete loss of the longitudinal arch.⁶ A separate study confirmed this redundancy: a stable arch was maintained as long as either the plantar fascia or the long plantar ligament was intact. Total collapse required removing both.⁷ The system is designed with backup.

Muscles add a dynamic layer on top of those passive ligaments. The posterior tibial tendon, which runs along the inner ankle and attaches to the midfoot, increases arch stiffness when loaded. Meanwhile, the peroneus longus tendon, crossing the sole from the outer ankle, actually decreases arch stiffness when loaded.⁸ The interplay between these two muscles gives the foot a tunable range of stiffness: the posterior tibial muscle can stiffen the arch for forceful push-off, while the peroneus longus can allow more compliance for shock absorption or adapting to rough terrain. The foot is not simply stiff or flexible; it is adjustable.

The transverse arch, running side to side across the midfoot, also plays a structural role. It allows the longitudinal arch to behave as a flexible lever in some loading conditions while acting as a stiff spring lever in others.⁹ Think of it like the curvature of a sheet of paper: a flat sheet bends easily along its length, but curving it side to side makes it much harder to fold. The transverse arch provides that cross-curvature to the foot.

Sex Differences in Midfoot Flexibility

Midfoot mechanics are not identical between men and women. During running, women showed significantly greater peak midfoot bending and a larger range of motion in the sagittal plane compared with men, with large effect sizes.¹⁰ A separate study looking at drop-jump landings found similar patterns: women had significantly more midtarsal joint bending immediately after heel contact, along with higher angular velocity at that joint. In the late landing phase, midtarsal bending in women averaged about 11 to 12 degrees compared with 7 to 8 degrees in men.¹¹

These differences have practical implications for injury risk and shoe design. A midfoot that bends more absorbs impact differently, shifting load patterns across the plantar fascia and the metatarsal heads. Women’s running shoes and athletic footwear are often just scaled-down men’s models, but the biomechanical evidence suggests the midfoot in particular behaves differently enough to warrant distinct design considerations rather than simple size adjustments.

How Footwear Changes the Midfoot Over Time

The kind of shoes you grow up wearing appears to reshape how stiff your midfoot becomes. A study comparing habitually minimally-shod populations with conventionally-shod populations found that people who spent most of their lives in minimal footwear had arches that were about 9 percent higher and roughly 27 percent stiffer than those in conventional shoes. The conventionally-shod group, meanwhile, showed about 27 percent more midfoot bending during walking.¹²

This is a striking finding because it suggests the degree of midfoot flexibility you walk around with is not purely genetic or anatomical. It is partly a product of the mechanical environment your feet developed in. Supportive shoes and arch supports may reduce the demand placed on the intrinsic muscles of the foot during growth, leading to weaker muscles and a more compliant midfoot in adulthood. The minimally-shod feet had to do more work with every step, and they adapted by becoming stronger and stiffer.

This does not mean everyone should throw away their shoes. Transitioning abruptly from supportive footwear to barefoot or minimal shoes carries its own injury risks, especially for adults whose feet have already adapted to decades of conventional shoes. But it does suggest that the stiffness of the human midfoot, including how close a given person comes to displaying a midtarsal break, is influenced by lifelong mechanical loading and is not a fixed species-wide trait.

Energy Storage in the Arch

One reason midfoot flexibility matters beyond academic interest is its role in energy efficiency. The longitudinal arch deforms under load during each step, storing elastic energy in its stretched ligaments and tendons, then releasing that energy during push-off. Researchers tested this by comparing running in normal minimal shoes versus shoes fitted with custom insoles that restricted arch deformation. The insoles that prevented the arch from flexing increased the metabolic cost of running by about 6 to 7 percent, closely matching the predicted energy that the arch would have stored and returned if it were free to deform.¹³

This means the foot’s slight flexibility is not a design flaw or a leftover from tree-climbing ancestors. It is an active contributor to efficient locomotion. A foot that is too rigid misses out on this free energy return. A foot that is too flexible wastes energy because the lever arm collapses during push-off. The human midfoot sits in a sweet spot, and the variation across individuals in midtarsal break expression may represent different positions along that stiffness-efficiency trade-off.

Reading Midfoot Flexibility in the Fossil Record

The midtarsal break matters intensely to paleoanthropologists because it is one of the few foot functions that can leave traces in both fossil bones and preserved footprints. The Laetoli footprints in Tanzania, made about 3.66 million years ago by an early hominin (likely Australopithecus afarensis), have been analyzed extensively. Topographic statistical analysis of these prints, combined with experimental footprint-formation studies and computer simulation, indicates that most features of modern human-like foot function were already present in the Laetoli trackmaker, though less strongly expressed than in living humans.¹⁴

Before the discovery that living humans vary in midtarsal break expression, any sign of midfoot flexibility in a fossil foot was interpreted as primitive, meaning the hominin had not yet evolved a fully modern gait. Now the interpretation is more nuanced. A fossil foot that shows some midtarsal mobility might belong to a capable biped who simply fell on the more flexible end of normal human variation. This recalibration has implications for how we reconstruct locomotion in species like Homo naledi, whose foot has a mix of human-like and more flexible features. The question is no longer “did they have a midtarsal break or not?” but rather “where on the spectrum of midfoot flexibility did they fall, and what does that tell us about their habitual locomotion?”

How Researchers Measure Midfoot Motion

Studying the midtarsal break in living people is technically challenging because the bones of the midfoot are small, tightly packed, and overlapping on standard X-rays. Skin-mounted markers, the standard tool in gait labs, slide over the underlying bones and can exaggerate or mask true joint motion. Newer techniques using biplane fluoroscopy, where two X-ray sources capture the foot simultaneously from different angles, allow researchers to track individual bones directly during movement. Validation work has demonstrated that these systems can accurately register foot bone positions and rotations using model-based tracking algorithms.¹⁵

Pressure plates are another common tool, and they were what first revealed midtarsal break variation in humans. When someone walks across a plate embedded in the floor, sensors record where and how hard the foot presses at each instant. A person with a midtarsal break shows elevated pressure under the outer midfoot late in stance, when the heel has already lifted but the midfoot is still bending rather than acting as a rigid strut. This is a low-cost way to screen large populations, though it measures the consequence of midfoot bending (pressure patterns) rather than the bending itself.

Computational modeling rounds out the toolkit. Finite element analysis can simulate how stress distributes across the bones of the foot under different loading conditions. In healthy feet, stress concentrates on the lateral metatarsal, talus, and navicular bones, with peak stress in the fourth metatarsal reaching about 5.4 megapascals during the stance phase.¹⁶ These models help researchers predict how changes in midfoot stiffness, whether from anatomy, injury, or surgical intervention, would redistribute forces through the rest of the foot.

What Chimpanzee Feet Reveal About the Midfoot’s Versatility

Chimpanzees, our closest living relatives, are often held up as the exemplar of the midtarsal break in action. But even in chimps, the midfoot does not behave as simply as the old models suggested. Kinematic studies of chimpanzee ankles and feet during both terrestrial and arboreal locomotion found that while chimps used high midfoot inversion angles (rolling the foot inward) during tree-based movement, their midfoot bending in the forward-backward plane was actually similar across all locomotor modes.¹⁷ In other words, chimps do not reserve midfoot flexibility for climbing and shut it off on the ground. The midfoot bends about the same amount regardless of surface; what changes is how much the foot rolls and twists to grip different substrates.

This matters because it suggests the midtarsal break is not simply a climbing adaptation that evolution “turned off” in humans. It is more like a default mechanical property of primate feet that humans have partially constrained through changes in joint shape, ligament strength, and arch geometry, but never fully eliminated. The variation we see in living humans, where some people show a clear midtarsal break and most do not, reflects how tightly that constraint operates across individuals, not whether the underlying capacity exists at all.