The flexor digitorum brevis (FDB) is a short, broad muscle that sits on the sole of your foot, directly beneath the tough plantar fascia, and its primary job is to curl the second through fifth toes downward at the middle joint. That description makes it sound like a minor player, but the FDB turns out to be central to how your foot stores and releases energy when you walk, jump, or land, and it is one of the muscles surgeons turn to when they need living tissue to rebuild a damaged heel. It is also surprisingly variable from person to person, with many people missing part of it entirely and never noticing.
Where It Sits and How It Works
The FDB is the most superficial intrinsic muscle on the underside of the foot. It originates from the calcaneal tuberosity (the underside of your heel bone) and from the plantar aponeurosis, the thick band of connective tissue that runs along the sole. From that single origin it fans out into four separate tendons, one heading toward each of the lateral four toes. Each tendon splits into two slips at around the base of the proximal phalanx, creating a small tunnel through which the tendon of the longer flexor muscle (flexor digitorum longus, coming from the calf) passes on its way to the tip of the toe. The two slips then reunite and attach to the sides of the middle phalanx.1National Journal of Clinical Anatomy. Morphology of flexor digitorum brevis muscle in northern Tamil Nadu region- an anatomical study with phylogenetic perspective This split-and-reunite design is sometimes called a perforated tendon, and it is the FDB’s signature structural feature: it lets the short flexor bend the middle joint of each toe while the long flexor, passing through the tunnel, reaches the tip.
Nerve supply comes from the medial plantar nerve, a branch of the tibial nerve. In rare cases, dual innervation from both the medial and lateral plantar nerves has been documented, but this is the exception rather than the rule.2PubMed Central. Comparative anatomy of the flexor digitorum brevis in hominids Blood supply is shared: the FDB receives branches from both the medial and the lateral plantar arteries, making it somewhat unusual among the sole’s intrinsic muscles, most of which are fed predominantly by one artery or the other.3Annals of Plastic Surgery. Vascular Anatomy of Plantar Muscles
The Fifth-Toe Tendon That Often Is Not There
If you were to dissect a group of cadavers and check the FDB in each foot, you would find that the tendon heading to the little toe is frequently small, weird, or missing altogether. This is one of the best-documented anatomical variations in the foot, and reported absence rates vary widely depending on the study population. One study found the fifth-toe tendon absent in about 18% of specimens and notably small in another 36%. Another found it absent in roughly 27%, while a third reported absence rates as high as about 72%, with bilateral absence being common.4PubMed Central. Separated muscle belly of the flexor digitorum brevis for the fifth toe: a case report The spread in those numbers partly reflects differences in how “absent” is defined (truly gone versus merely vestigial) and partly reflects real variation between populations.
When the fifth-toe slip is absent, adjacent muscles appear to compensate, and there is no significant effect on walking or balance. Research in a Hispanic population found no meaningful relationship between the presence or absence of the fifth-toe tendon and sex or ethnicity, and speculated that because losing this tendon carries essentially no functional penalty, its absence could become more common over evolutionary time.5Anatomia. A Missing Flexor Digitorum Brevis Tendon and Its Relationship to Sex and Ancestry: Evaluation in Hispanic Population
Other variations exist beyond simple absence. In some individuals the fifth-toe slip originates as a separate, deep muscle belly rather than as a branch of the main muscle, or it arises partly from the tendon of the long flexor rather than from the calcaneus. One cadaveric study found such accessory muscle bellies in four human specimens, with the extra slips inserting into the fourth and fifth digits and receiving their nerve supply from the medial plantar nerve just like the main muscle.6PubMed Central. Comparative anatomy of the flexor digitorum brevis in hominids These variations matter mostly to surgeons and to researchers trying to trace the evolutionary history of the foot. For the average person, the FDB works fine whether it sends four tendons or three.
Elastic Energy Storage in the Arch
Walking is often described as controlled falling, and every step involves a brief compression and recoil of the foot’s longitudinal arch. The plantar aponeurosis and the ligaments spanning the arch have long been recognized as passive springs that store and return energy during this cycle. More recent work has shown that the FDB actively participates in this spring-like mechanism, and it does so in a surprisingly clever way.
During movements like step-ups and jumps, the arch compresses and the FDB’s tendinous tissue stretches, banking elastic strain energy. When the arch recoils, that stored energy is released. The muscle fascicles themselves shorten more slowly than the tendons recoil, a decoupling that lets the tendon do the fast, springy work while the muscle fibers control the process at a more measured pace. In decelerating movements like step-downs and landings, the pattern reverses: the tendinous tissue elongates more and faster than the muscle fascicles, absorbing energy as the foot cushions impact.7PubMed Central. Flexor digitorum brevis utilizes elastic strain energy to contribute to both work generation and energy absorption at the foot The upshot is that your nervous system can exploit the natural compression of the arch to either add or remove mechanical energy from the body, depending on whether you are speeding up or slowing down. The FDB is not just a toe-curling muscle; it is part of the foot’s built-in shock absorber and catapult system.
Diabetic Neuropathy and Muscle Wasting
Because the FDB is innervated by a relatively long peripheral nerve, it is vulnerable to the same damage that diabetes inflicts on nerves throughout the lower limb. In people with early-stage diabetic peripheral neuropathy, intrinsic foot muscles shrink measurably even before patients notice major balance or walking problems. One study found that the combined volume of the four intrinsic foot muscles (including the FDB) was about 28% smaller in people with diabetic neuropathy compared to controls without diabetes.8PubMed Central. Early-Stage Diabetic Neuropathy Reduces Foot Strength and Intrinsic but Not Extrinsic Foot Muscle Size The larger extrinsic muscles (those originating in the calf with tendons running into the foot) were not yet significantly affected at the same disease stage, suggesting that the intrinsic muscles are among the earliest casualties of nerve damage in the foot.
This early atrophy matters because the intrinsic muscles contribute to toe grip strength, arch stiffness, and the fine postural adjustments you make while standing. As the FDB and its neighbors waste away, the foot gradually loses its ability to distribute pressure evenly, which increases the risk of skin breakdown and ulceration, particularly on the ball of the foot and under the metatarsal heads. Clinicians managing diabetic foot care are increasingly paying attention to intrinsic muscle health as a marker of nerve damage that might otherwise go undetected until more serious complications arise.
Minimalist Shoes and Muscle Growth
The idea that modern cushioned footwear may let foot muscles weaken from disuse has gained traction in sports science. Several studies have tested whether switching to minimal shoes (thin-soled, flexible, without arch support) can reverse that deconditioning. In a 12-week trial with adult runners, those who transitioned to minimal shoes showed a significant increase in muscle volume of the FDB, alongside growth of the abductor digiti minimi.9Journal of Sport and Health Science. The effect of minimal shoes on arch structure and intrinsic foot muscle strength
Whether these gains extend to children is less clear. A randomized trial in children who wore moderate minimalist shoes found increases in FDB cross-sectional area and toe flexion strength, but the changes were moderate in size and did not reach statistical significance.10PubMed. The long-term effects of wearing moderate minimalist shoes on a child’s foot strength, muscle structure and balance: A randomised controlled trial Children’s feet are still developing and may respond differently to footwear changes than adult feet, so the adult results should not be assumed to apply across all ages. Still, the adult evidence suggests that people who want to strengthen their intrinsic foot muscles can do so by spending more time in shoes that ask the foot to do more work, provided they transition gradually enough to avoid overuse injuries.
Short Foot Exercises and Flatfoot Rehabilitation
Outside of footwear changes, targeted exercises can also build the FDB and its neighboring muscles. Short foot exercises, sometimes called “foot doming,” involve pulling the ball of the foot toward the heel without curling the toes, which shortens the arch and activates the intrinsic foot muscles. A narrative review of the evidence concluded that this type of exercise can improve arch height, enhance dynamic foot support, improve foot morphology, and alleviate pain associated with flatfoot.11PubMed Central. Short foot exercises for flatfoot therapy: Status and prospects
In practice, these exercises are simple enough to do sitting at a desk. You press the tips of your toes gently into the floor, then try to shorten the arch by drawing the metatarsal heads toward the heel. The toes should stay relatively flat rather than clawing. Holding each contraction for several seconds and repeating for a few sets is a common protocol. Physical therapists often pair short foot exercises with single-leg balance drills or heel raises to build both strength and the coordination needed to use these muscles during walking and running. For people with flexible flatfoot or mild arch pain, this approach gives the FDB a direct training stimulus that cushioned shoes and sedentary habits tend to remove.
Measuring the FDB in Research and Clinical Settings
One reason the FDB has attracted research attention is that it is accessible to measurement. It sits close to the skin surface, so ultrasound can image it clearly, and electromyography (EMG) can pick up its electrical activity. A scoping review of EMG studies on intrinsic foot muscles found that the vast majority used surface electrodes rather than the needle-based fine-wire type. For the FDB specifically, fine-wire electrodes were needed in several studies because, despite being the most superficial intrinsic muscle, it still sits beneath the plantar fascia, and surface electrodes can pick up contaminating signals from neighboring muscles.12Revista Médica ClÃnica Las Condes. ELECTROMYOGRAPHIC ASSESSMENT OF FOOT INTRINSIC MUSCLES IN THERAPEUTIC EXERCISE. A SCOPING REVIEW
For routine clinical work, ultrasound and MRI are used to assess muscle size and detect atrophy. MRI can quantify muscle volume precisely and has been the tool of choice in studies comparing diabetic and non-diabetic feet. Ultrasound is quicker, cheaper, and can be done in an office visit, making it useful for tracking changes over time, for example during a rehab program. In either modality, a noticeably small or absent FDB on the fifth-toe side is a normal variant rather than a sign of disease, which is worth knowing to avoid unnecessary alarm.
The FDB as a Surgical Flap
The dual blood supply from both plantar arteries makes the FDB an attractive option for reconstructive surgery. When the weight-bearing surface of the heel is damaged by trauma, burns, or chronic wounds, surgeons need tissue that can withstand repeated pressure and provide some protective sensation. The FDB can be detached from its origin, rotated on its blood supply, and laid into the defect as a muscle flap, then covered with a skin graft.
Early reports described this approach as producing durable, sensate coverage of the heel pad that held up to walking without breakdown over follow-up periods of six to eight months.13Journal of Plastic, Reconstructive & Aesthetic Surgery. Reconstruction of extensive plantar heel defects with skin-grafted flexor digitorum brevis muscle flaps Proponents of the technique have argued that because the FDB is local tissue with matching nerve supply, it offers better sensation and biomechanical compatibility than tissue imported from distant sites like the latissimus dorsi or the free radial forearm flap.14PubMed. Reconstruction of the heel pad by flexor digitorum brevis musculocutaneous flap transfer The trade-off is the loss of toe flexion power in the affected foot, but given that many people lack the fifth-toe slip naturally and the remaining intrinsic muscles can compensate for flexion at the second through fourth toes, the functional cost is generally considered acceptable when weighed against having a nonhealing wound on the sole of the foot.
Comparative Anatomy and Evolutionary Context
The FDB offers a small window into how the human foot diverged from that of other primates. In non-human hominids (great apes), the intrinsic foot muscles retain configurations suited to a grasping foot that spends time in trees. In humans, the FDB has become more streamlined, reflecting the shift to obligate bipedalism and the need for a rigid, spring-like arch rather than a prehensile platform. Comparative dissection work across hominid species has traced these changes, noting that the accessory muscle bellies and variable tendon arrangements occasionally seen in human feet resemble the typical arrangement in other apes more closely than the standard human pattern.15PubMed Central. Comparative anatomy of the flexor digitorum brevis in hominids
The progressive loss of the fifth-toe slip fits this evolutionary narrative as well. The little toe contributes less to balance and propulsion in humans than it does in species that grip branches, so the muscular hardware serving it has become expendable. Whether the fifth-toe tendon continues to disappear from the population at a measurable rate is speculative, but the wide range of absence rates documented across different populations suggests that the trait is genuinely in flux rather than fixed. The FDB, in other words, is still being edited by the slow, messy process of human evolution, one tendon at a time.