What Is the Medial Longitudinal Arch and How Does It Work?

The medial longitudinal arch is the highest of the foot’s arches, running along the inner edge from heel to big toe, and it serves as the primary spring and shock absorber that makes upright walking and running efficient. Formed by bones, ligaments, tendons, and the tough plantar fascia on the sole, this arch is not a rigid structure but a flexible one that flattens under load and rebounds to propel you forward. Its design is so central to human locomotion that restricting its natural movement during running increases energy cost by about six percent. Understanding what holds it up, how it develops, and what goes wrong when it fails explains a surprising amount about foot pain, gait problems, and even why your shoe size can change over a lifetime.

What the Arch Is Made Of

The medial longitudinal arch is built from a chain of bones: the calcaneus (heel bone) at the back, the talus sitting on top of it, the navicular and three cuneiform bones in the midfoot, and the first three metatarsals leading to the toes. These bones are stacked and angled so that the middle of the chain sits well above the ground, creating the visible curve on the inner side of your foot. The bones alone would collapse under body weight, so they are bound together by ligaments and reinforced by the plantar fascia, a thick band of connective tissue that runs along the sole from heel to toe bases. Together, these create what one anatomy reference calls “an elastic and adaptive base that can support the entire body.”¹

Among the ligaments, one stands out as the arch’s most critical passive support: the spring ligament, formally called the plantar calcaneonavicular ligament. It connects the calcaneus to the navicular bone and cradles the head of the talus, which sits directly on top of it. If the spring ligament stretches or tears, the talus drops downward. That downward shift pulls the heel into an outward tilt and flattens the arch, producing the classic flatfoot deformity.² The spring ligament is considered the primary static stabilizer of the medial longitudinal arch.³

Active Stabilizers That Keep It From Collapsing

Ligaments and fascia provide passive support, but they are not enough on their own. The tibialis posterior muscle, which runs behind the inner ankle bone and attaches across multiple midfoot bones, is the arch’s most important active stabilizer. Its tendon pulls upward on the underside of the midfoot during each step, dynamically propping the arch when body weight is loading it. A cadaveric study that simulated repeated walking cycles found that without tibialis posterior activity, the arch flattened significantly more than when the muscle was active, and the passive structures alone could not maintain arch height.⁴ Clinical experience confirms this: when the tibialis posterior becomes deficient, the arch progressively collapses.⁵

The small muscles on the sole of the foot, the plantar intrinsic muscles, also contribute. When researchers electrically stimulated these muscles under controlled loads, the arch shortened in length and increased in height, showing that they can actively stiffen the arch.⁶ Their role during actual walking and running, however, appears to be more nuanced. A study that blocked these muscles with a nerve injection found that arch stiffness during the high-load phase of ground contact was not significantly altered. Instead, the intrinsic muscles seem to matter most at push-off, stiffening the front of the foot so it can act as a rigid lever.⁷ So the tibialis posterior handles the heavy lifting of arch support during stance, while the smaller sole muscles fine-tune the foot’s behavior at the end of each step.

The Arch as an Energy-Recycling Spring

The medial longitudinal arch does far more than prevent your foot from collapsing. It functions as a mechanical spring. When your foot hits the ground, the arch flattens slightly, stretching the plantar fascia and elastic tendons and storing energy. As you push off, those structures recoil, returning energy that helps propel you forward. Research estimates that the human foot can recycle up to about 17% of the energy needed to power a stride through this mechanism.⁸

The intrinsic foot muscles contribute to this energy recycling in an interesting way. Rather than shortening and lengthening like typical muscles, they contract nearly isometrically (holding their length) while their tendons stretch and recoil. They effectively act as stiff struts that let the tendons do the elastic work, similar to how the calf muscles behave during efficient running.

When arch compression is artificially restricted, running gets measurably harder. Researchers who limited arch compression by roughly 80% during moderate-speed running found that metabolic cost rose by about six percent, regardless of whether runners landed on their forefoot or heel.⁹ That is a substantial penalty for losing what amounts to a few millimeters of arch movement, and it illustrates why stiff, overly supportive footwear can work against the foot’s natural efficiency.

How the Arch Develops in Children

Babies and toddlers look flat-footed, and they are. The medial longitudinal arch has not yet formed, and a pad of fat fills the space where it will eventually appear. Parents and pediatricians often worry about this, but research shows it is entirely normal. The arch develops gradually, with the most dramatic changes happening between ages four and six. One study found that children aged three and four had a high prevalence of low arches, ranging from 36% to 86% depending on how it was measured, and that a significant shift toward an adult-like arch shape occurred between ages four and five.¹⁰

A cross-sectional and longitudinal study of Czech children found that the most pronounced phase of arch formation is largely complete by around six years of age in both sexes, after which the rate of change drops sharply. Only subtle, clinically minor changes continued up to nine years old. The researchers specifically warned against labeling a child’s foot as “flat” before around age six, because the arch is still actively developing. They recommended classifying young children’s feet as “formed” or “unformed” rather than “flat” or “normal,” to avoid misdiagnosing a healthy developing foot as pathological.¹¹

There is also considerable individual variation during these years. A longitudinal study that followed hundreds of children with flat feet found that only a portion developed arches over the 1.5-year follow-up period, and those who did tended to have milder flatness to begin with and better single-leg balance.¹² In other words, some children develop arches earlier than others, and the timeline is not uniform.

When the Arch Falls in Adults

Adult-acquired flatfoot deformity is a progressive condition, and it usually begins with degeneration of the posterior tibialis tendon, the same muscle that serves as the arch’s primary dynamic stabilizer. The tendon can develop inflammation, thicken, stretch out, or tear. As it weakens, the arch loses its active support and begins to sag. Initially the foot is still flexible, and you can recreate the arch by standing on your toes. Over time, though, the dropping arch stretches the spring ligament and other supporting ligaments, and the deformity becomes rigid.¹³

When the spring ligament’s compromise is severe enough and other ligaments have also failed, imaging shows the talus bone collapsing downward through the arch, with the navicular sliding off to the side.¹⁴ The cascade matters clinically because it means the problem is rarely just one structure failing. Multiple stabilizers have to give way before the characteristic deformity sets in, which explains why treatment sometimes needs to address several structures at once.

Does Arch Height Predict Pain?

Here is where common assumptions break down. Many people believe that flat feet automatically mean foot pain and that high arches are similarly problematic. The reality is messier. A study examining arch height and its relationship to pain and dysfunction in people with lower-limb problems found that arch height alone did not predict either pain or dysfunction.¹⁵ Plenty of people with very flat or very high arches go through life with no foot complaints.

That said, arch shape does affect biomechanics in ways that can predispose certain people to problems. A study of young adults found that those with flat feet had significantly thicker plantar fascia, higher foot dysfunction scores, and reduced ankle range of motion compared to those with normal arches.¹⁶ High arches bring their own risks: a rigid, high-arched foot absorbs impacts poorly, which has been linked to elevated risk of stress fractures, lateral ankle sprains, and Achilles tendon problems in runners.¹⁷

A large cross-sectional study of over 1,000 university students quantified how much arch deformity affects the foot’s energy-storage function. Severe flatfoot reduced the arch’s elastic energy index by roughly 39% compared to a normal arch, and high arches also showed diminished energy storage, though to a lesser degree.¹⁸ So the arch is not just about shape for its own sake. Both extremes compromise the foot’s ability to store and return energy efficiently, which can increase fatigue and injury risk over time, especially with high-volume activity like distance running.

Exercises Versus Orthotics for Flat Feet

If you have flexible flat feet and want to do something about them, two main options exist: strengthening exercises and arch-support insoles. Research suggests exercises may be the more effective long-term strategy. “Short foot” exercises, where you try to shorten the foot by pulling the ball of the foot toward the heel without curling the toes, specifically target the plantar intrinsic muscles. One study found that eight weeks of this training reduced the amount the arch dropped under load and increased arch height, offering preliminary evidence that the muscles can be trained to support the arch better during functional tasks.¹⁹

A comparison study found that short foot exercises were more effective than arch-support insoles at improving both the medial longitudinal arch and dynamic balance in people with flexible flat feet.²⁰ A randomized controlled trial confirmed that while insoles improved foot posture, they were less effective than exercise alone or exercise combined with insoles.²¹

Orthotics still have a role, especially for symptom relief and for people who cannot commit to a daily exercise program. Foot orthoses with arch support do reduce strain on the tibialis posterior muscle and improve rearfoot alignment. But there is a ceiling effect: increasing the hardness of the arch support beyond a moderate level does not keep making things better. A biomechanical analysis found that very hard arch supports actually increased knee stress and shifted loading to the forefoot joints, potentially creating new problems.²² Similarly, finite-element modeling showed that while increasing arch support height reduced peak pressures in some areas, it increased pressure in the midfoot and shifted plantar fascia stress to different regions rather than simply eliminating it.²³ The practical takeaway: a moderate arch support tends to be better tolerated and more effective than a maximally rigid one.

How Footwear Shapes the Arch Over Time

Most shoes, especially conventional ones with thick cushioning, elevated heels, and narrow toe boxes, effectively do the arch’s job for it. The muscles that actively stabilize the arch get less work when external support is always present, and over decades of shoe-wearing, those muscles can weaken. Minimalist shoes, which have thin, flexible soles and little to no arch support, have been studied as a potential countermeasure. A systematic review found that wearing minimalist shoes led to significant increases in intrinsic foot muscle size and strength, with strength gains ranging from 9% to 57% and muscle size increases of 7% to 11%.²⁴ The reviewers noted that minimalist shoes may be a more convenient way to strengthen foot muscles than structured physical therapy programs, since the strengthening happens passively during daily activity.

That does not mean everyone should immediately switch to barefoot-style shoes. Transitioning too quickly can overload tissues that are not prepared for the additional stress, particularly the metatarsals and Achilles tendon. A gradual transition, slowly increasing the proportion of time in minimalist footwear over weeks or months, is generally recommended.

Pregnancy and Permanent Arch Changes

One life event that can permanently alter the medial longitudinal arch is pregnancy. The combination of increased body weight and hormonal changes that loosen ligaments can flatten the arch in a way that does not fully reverse after delivery. A study measuring foot structure before, during, and after pregnancy found significant decreases in arch height and arch rigidity, along with increases in foot length. These changes persisted months postpartum and appeared to be permanent. The first pregnancy seemed to produce the most significant changes, with subsequent pregnancies having a smaller additional effect.²⁵

This explains why many women find their shoe size increases after having children and never goes back down. The foot is literally longer because the arch has dropped. It is not swelling or weight gain persisting; it is a structural change in the arch itself. Awareness of this can help women plan for appropriate footwear during and after pregnancy and consider foot-strengthening exercises to maintain arch integrity.

An Evolutionary Perspective

Humans are unusual among primates in having a well-defined medial longitudinal arch. Great apes have much flatter feet, which work well for grasping branches but are less efficient for bipedal walking. Biomechanical modeling comparing human and ape feet during bipedal standing found that the proportions of the human foot, and the height of its medial arch, reduce the total forces required in joints and muscles compared to ape feet. The human arch height appears to be optimized for bipedal standing and was likely shaped by natural selection.²⁶

The fossil record suggests that the longitudinal arch did not emerge as a single event in hominin evolution. Analysis of Australopithecus sediba, a hominin that lived roughly two million years ago, found that its rearfoot was indistinguishable from modern African apes, strongly suggesting it lacked a longitudinal arch despite being bipedal. Other fossil hominins show varying degrees of arch-related traits, indicating that there was diversity in foot morphology among our ancestors during the period when bipedalism was evolving.²⁷ The modern human arch, in other words, was not the inevitable endpoint of bipedal evolution but one successful solution among several that early hominins experimented with.

When Surgery Becomes Necessary

For severe flatfoot that has not responded to conservative treatment, surgical options exist. The specific procedure depends on the stage of the deformity and which structures have failed. One common approach is a medial displacement calcaneal osteotomy, where the heel bone is cut and shifted inward to realign it under the ankle. Cadaveric research found that this osteotomy reduces the Achilles tendon’s arch-flattening effect and plays an important role in slowing the progression of flatfoot deformity.²⁸

For more severe cases, particularly in adolescents with large talonavicular angles and pronounced heel valgus, a double calcaneal osteotomy combining two different cuts to the heel bone has shown strong ability to restore both hindfoot and midfoot alignment while reducing strain on the medial ligaments.²⁹ A lengthening calcaneal osteotomy is another option that has shown dramatic improvements in the alignment between the talus and navicular bones.³⁰ These procedures are typically combined with soft-tissue repairs, such as tendon transfers or spring ligament reconstruction, depending on the extent of damage. Surgery is generally reserved for cases where months of conservative management have failed and pain or functional limitation significantly affects daily life.

The Windlass Mechanism and Toe Extension

There is one more piece of arch mechanics worth knowing about because it explains something you can feel in your own foot. The plantar fascia wraps around the base of the toes. When you extend your big toe upward, the fascia winds around the toe joint like a cable around a winch, pulling the heel and forefoot closer together and raising the arch. This is the windlass mechanism, and research has confirmed it is active during normal walking whenever the big toe extends before push-off.³¹ You can test it right now: sit down, put your foot flat on the floor, and lift just your big toe. You should see and feel the arch rise. This mechanism is one reason that shoes with stiff soles or cramped toe boxes can interfere with natural arch function. If the big toe cannot extend freely, the windlass cannot fully engage, and the arch loses one of its tightening mechanisms at a critical moment in the gait cycle.