Fact-checked by Grok 2 weeks ago

Flexor digitorum brevis muscle

The flexor digitorum brevis is an intrinsic muscle of the foot located in the first (most superficial) layer of the plantar muscles, lying immediately deep to the and superficial to the tendons of the flexor digitorum longus. It originates from the medial tubercle of the , the central portion of the , and adjacent intermuscular septa, forming a broad, fleshy belly that spans the medial aspect of the . The muscle divides distally into four tendons that pass to the lateral four toes (digits 2–5), each tendon splitting around the deeper flexor digitorum longus to insert on the middle phalanges via expansions on both sides. Innervated by the (a branch of the , arising from spinal levels S1–S2), the flexor digitorum brevis receives its blood supply primarily from branches of the medial and lateral plantar arteries, including contributions from the plantar arch and digital arteries. Its primary function is to flex the proximal interphalangeal (PIP) and metatarsophalangeal (MTP) joints of toes 2–5, facilitating toe flexion during and weight-bearing activities such as walking, climbing stairs, or navigating uneven terrain. Additionally, it contributes to maintaining the longitudinal arch of the foot and stabilizing the toes against the ground for propulsion and balance. Clinically, the flexor digitorum brevis plays a role in coordinating toe flexion with the deeper flexor digitorum longus to ensure proper ground contact during ambulation, and its tendons are sometimes harvested for reconstructive procedures, such as heel-pad reconstruction or diabetic foot flap surgery, due to their robust structure and vascularity. Weakness or injury to this muscle can lead to impaired toe flexion, potentially affecting joint stability and contributing to conditions like hammertoe deformities or altered gait mechanics.

Anatomy

Origin and insertion

The flexor digitorum brevis muscle originates from the medial tubercle of the , the central portion of the , and adjacent intermuscular . This attachment allows the muscle belly to lie firmly united with the underlying throughout its course in the mid-sole. The muscle narrows distally into four tendons, one for each of the lateral four toes, which pass forward on the plantar surface of the foot. At the level of the , each tendon bifurcates into two slips that spiral around the corresponding tendon of the deeper , forming a chiasma before reuniting to insert onto the plantar aspects of the middle phalanges of the second through fifth toes. This permits the passage of the flexor digitorum longus tendon through the interval to reach the distal phalanges. As a key component of the first (superficial) layer of the plantar foot muscles, the flexor digitorum brevis occupies a central position in the , immediately deep to the central part of the plantar aponeurosis and inferior to the flexor digitorum longus tendons.

Structure and relations

The flexor digitorum brevis is a flat, broad muscle situated in the superficial layer of the plantar foot, forming a fleshy central structure that spans the mid- region. Its overall dimensions in adults average approximately 10 cm in length and 4 cm in width, providing a substantial for flexion support. The muscle belly divides distally into four distinct slips or partial bellies, each corresponding to the lateral four toes (digits 2–5), with short tendons extending about 1–2 cm before bifurcating into slips that envelop the deeper flexor digitorum longus tendons. Positioned in the central compartment of the plantar foot, the flexor digitorum brevis lies superficially, directly deep to and plantar , which covers and protects it as the most superficial intrinsic muscle layer. Deep to the muscle resides the second plantar layer, including the quadratus plantae and the tendons of the flexor digitorum longus, over which the flexor digitorum brevis tendons pass and interact during movement. Laterally and medially, it is bordered by the abductor digiti minimi and abductor hallucis muscles, respectively, forming the medial and lateral limits of the first plantar layer. Proximate to the muscle's medial aspect runs the neurovascular bundle comprising the (a branch of the ) and accompanying vessels, which course along the border without direct embedding but in close spatial relation to facilitate innervation and supply. This arrangement positions the flexor digitorum brevis centrally within the plantar architecture, integrating it seamlessly with adjacent soft tissues while maintaining its superficial accessibility.

Innervation

The flexor digitorum brevis muscle receives its primary motor innervation from the , a terminal branch of the originating from segments S1 and S2. This nerve provides motor fibers to all four bellies of the muscle, enabling coordinated flexion of the lateral four toes. The enters the sole of the foot via the and courses distally, passing deep to the before reaching the flexor digitorum brevis from its medial aspect. Upon entering the muscle, it distributes branches that innervate the central muscle belly and its tendinous slips, ensuring precise control over proximal interphalangeal and metatarsophalangeal joint flexion. In addition to motor supply, the conveys sensory fibers, including proprioceptive afferents, to the flexor digitorum brevis and the surrounding plantar structures, facilitating sensory feedback during and . These proprioceptive components help maintain foot arch stability and toe positioning relative to ground contact. The neural supply to the flexor digitorum brevis was first described in detail by 16th-century anatomist in De Humani Corporis Fabrica, where he outlined the course of plantar nerves based on cadaveric dissections. Modern electrophysiological studies, including , have confirmed this innervation pattern by recording muscle activation during isolated toe flexion exercises, demonstrating peak activity in the flexor digitorum brevis when flexing the proximal interphalangeal joints without distal involvement.

Blood supply

The flexor digitorum brevis muscle receives its primary arterial supply from the medial plantar artery, which arises as one of the two terminal branches of the after it bifurcates beneath the flexor retinaculum. This artery courses distally along the medial aspect of the , providing muscular branches that penetrate the muscle's superficial layer to nourish its central belly. Additional arterial contributions come from the lateral plantar artery, the other terminal branch of the , through anastomotic connections formed by the plantar arterial arch. These interconnections, typically via the deep plantar branch of the completing the arch, ensure collateral circulation to the muscle, particularly in the lateral portions of its extent across the plantar surface. Such anastomoses enhance vascular redundancy, supporting the muscle's role in dynamic foot loading. Venous drainage of the flexor digitorum brevis parallels its arterial supply, primarily via the medial plantar vein, which collects blood from the muscle and adjacent structures before uniting with the lateral plantar vein to form the posterior tibial veins proximal to the ankle. This deep venous pathway integrates with the plantar venous plexus, facilitating efficient return of deoxygenated blood during by leveraging the foot's pumping mechanism. At the microvascular level, perforating branches from the medial plantar artery enter the muscle belly predominantly from the medial side, traversing between the abductor hallucis and to supply the intrinsic vascular network. These perforators arborize within the muscle to maintain oxygenation and nutrient delivery, critical for sustaining contractile function amid repetitive plantar pressure.

Function

Primary actions

The flexor digitorum brevis muscle primarily functions to flex the proximal interphalangeal () joints of the second through fifth toes, enabling curling of the middle phalanges toward the proximal phalanges. This action is facilitated by its tendons inserting on the sides of the middle phalanges after splitting around the deeper flexor digitorum longus tendons. In addition to its dominant role at the joints, the muscle provides secondary assistance in flexing the metatarsophalangeal (MTP) joints of the same toes, particularly when the toes are extended, helping to stabilize and draw the proximal phalanges downward. The muscle arises from a single broad belly in the midfoot but divides into four distinct slips that form independent tendons, one for each of the lateral four s, permitting selective or differential control of individual toe flexion without necessarily affecting the others. This multi-tendon arrangement enhances precise toe manipulation during activities requiring isolated movement. Cadaveric and studies indicate that the flexor digitorum brevis contributes to overall lateral toe flexion strength of around 51 N across the four toes.

Contribution to foot movement

The flexor digitorum brevis muscle acts synergistically with the flexor digitorum longus and quadratus plantae to enhance grip during the stance phase of , allowing coordinated flexion of the lateral four s to maintain ground contact and distribute forces across the forefoot. The quadratus plantae adjusts the oblique pull of the flexor digitorum longus to align more directly with the line of pull of the intrinsic flexors, including the flexor digitorum brevis, thereby improving the of flexion and grip in activities. This muscle stabilizes the forefoot by flexing the toes against the ground, which aids during the push-off of , contributing to forward through elastic energy return in its tendinous tissues. By anchoring the toes, it helps transmit propulsive forces from the posterior muscles to the ground, with the muscle generating approximately 13% of body weight in forces during this . The flexor digitorum brevis also contributes to arch support by helping maintain the longitudinal arch of the foot through indirect tensioning of the , from which it originates, thereby resisting arch collapse during dynamic loading in locomotion. Electromyographic studies indicate peak activity of the flexor digitorum brevis during the stance phase of the cycle (0-60% of the cycle), with sustained activation through toe-off to support stability and propulsion; in flat-footed individuals, this activity is generally increased compared to those with normal arches, likely as a compensatory mechanism for reduced passive support.

Clinical significance

Associated conditions

The flexor digitorum brevis muscle is commonly implicated in , a condition characterized by heel pain due to or degeneration at the origin. The muscle's superficial attachment to the allows tension from its contraction to contribute to fasciitis , particularly through altered at their interface, potentially exacerbating pain during activities. Studies indicate that involvement of the flexor digitorum brevis may heighten the risk of persistent heel pain, as its interphase with the can become a site of mechanical stress or secondary . In hammertoe and claw toe deformities, the flexor digitorum brevis plays a key role in the imbalance between intrinsic and extrinsic foot muscles, leading to flexion at the proximal interphalangeal () joint and hyperextension at the metatarsophalangeal (MTP) joint. This muscle, which primarily flexes the joint via its insertion on the middle phalanx, becomes taut in chronic MTP hyperextension, pulling the into a flexed posture at the interphalangeal joints and contributing to deformity progression through unopposed extensor action. Such imbalances often stem from intrinsic or overload, resulting in instability and altered mechanics that perpetuate the condition. Traumatic injuries to the flexor digitorum brevis typically manifest as strains or from acute overload or repetitive microtrauma, particularly in runners engaging in high-impact activities. Overuse can lead to muscle , degeneration, and , presenting with localized swelling, during toe flexion, and cramping in the midfoot or arch, often mimicking but originating from intrinsic muscle pathology. The flexor digitorum brevis is associated with ulcers through neuropathy-induced of intrinsic foot muscles, which impairs flexion and , elevating forefoot pressure points and promoting ulceration under the metatarsal heads. Reduced muscle function disrupts the windlass mechanism and distribution, fostering deformities like claw that increase shear forces and ulcer risk at pressure sites. In neuropathic feet, this contributes to joint instability, with flexor sometimes required to offload ulcer-prone areas by addressing imbalanced flexion.

Diagnostic and treatment approaches

Diagnosis of issues involving the flexor digitorum brevis (FDB) muscle typically begins with clinical palpation to identify tenderness over the medial plantar surface of the foot, which can indicate strain or trigger points in the muscle. Ultrasound imaging is employed to assess tendon integrity, effectively detecting partial tears and measuring muscle cross-sectional area with high validity when compared to MRI, allowing visualization of abnormalities such as atrophy or disruptions greater than 2 mm in tendon structure. MRI provides detailed evaluation of deep muscular relations, revealing edema, fatty atrophy, or denervation changes in cases of entrapment neuropathies affecting the medial plantar nerve. Electromyography (EMG) evaluates innervation deficits, with denervation potentials observed in nerve entrapment scenarios, confirming neuromuscular involvement through recordings from intrinsic foot muscles like the FDB. Treatment approaches prioritize for most FDB-related conditions, including the use of to support the foot arch and reduce plantar tension, alongside exercises that extend the s to alleviate muscle tightness. incorporates targeted toe flexion exercises, such as resisted bending of the lesser toes while seated or standing, to strengthen the FDB and improve foot stability without exacerbating injury. For persistent or severe cases, such as deformities contributing to claw toe, surgical interventions like FDB or release are performed to correct imbalances and restore alignment, with studies demonstrating reliable sectioning of slips to address proximal interphalangeal issues. In chronic strains or , may be necessary to remove inflamed tissue, followed by post-operative emphasizing retraining to optimize and prevent recurrence.

Anatomical variations

Prevalence

Anatomical variations in the flexor digitorum brevis (FDB) muscle are relatively common, with the absence of the fourth tendon slip (to the fifth toe) being the most frequently reported, occurring in approximately 31.3% of cases based on a meta-analysis of cadaveric dissections from multiple studies encompassing over 1,000 lower limbs. This true prevalence rate contrasts with a crude prevalence of 47%, reflecting variations in reporting methodologies across investigations, primarily derived from autopsy and surgical records. Other variations, such as a thin fourth slip or alterations in muscle origin, have been documented at lower rates, around 12-48%, but comprehensive meta-analytic data for these are less robust. Prevalence exhibits notable demographic differences, with significantly higher rates observed in populations (up to 77.3% for fourth slip ), potentially linked to genetic influences on foot muscle , compared to approximately 20% in , , , , and Turkish cohorts. also plays a role, as the odds of fourth slip absence are 1.5 times higher in females than in males, though overall differences in variation frequency are not always statistically significant across all studies. Bilateral is common among affected individuals, with 38.2% of cases showing absence on both sides, based on pooled data from bilateral dissections. Epidemiological insights stem predominantly from cadaveric examinations of adult populations, with limited evidence suggesting that age-related atrophy in individuals over 60 years may accentuate the visibility of existing variations through muscle thinning, though direct prevalence shifts with age remain understudied. No substantial gender disparities beyond the noted odds ratio have been consistently reported, emphasizing the role of population-specific genetic and environmental factors in shaping variation rates.

Types of variations

The flexor digitorum brevis muscle exhibits several morphological variations, including absence or of the muscle belly for the fifth , reported in up to 18% of dissected feet, which increases reliance on the flexor digitorum longus for flexion of the lateral . In such cases, the muscle's role in the superficial layer of the plantar foot may be partially compromised, though deeper flexors provide compensation. Accessory slips or heads are documented, such as a superficial and deep head with additional muscle bellies arising from the intermuscular septum, deviating from the standard single belly and four-tendon pattern. These may present as thin, separated muscle bellies or flat fascial structures for the fifth toe, potentially altering flexion dynamics at the metatarsophalangeal joint. Fusion with adjacent muscles has been observed, such as blending with the quadratus plantae, which can disrupt the distinct layering of plantar muscles and affect flexion independence. This typically involves shared tendinous connections, leading to a more integrated muscular structure in the central . anomalies often include absence of the fourth to the fifth or thin fascial replacements for other tendons, but normal bifurcation to accommodate the flexor digitorum longus is generally preserved in reported cases.

References

  1. [1]
  2. [2]
    Flexor digitorum brevis muscle - Kenhub
    Flexor digitorum brevis is a central plantar muscle of the foot involved in flexing the toes. Learn more about its anatomy and functions at Kenhub!
  3. [3]
    Flexor Digitorum Brevis Muscle - an overview | ScienceDirect Topics
    Anatomy. The FDB muscle lies immediately deep to the central part of the plantar aponeurosis. The muscle originates from the central part of the plantar ...
  4. [4]
    Flexor Digitorum Brevis - Physiopedia
    The Flexor Digitorum Brevis muscle originates from the medial process of calcaneal tuberosity and the central part of the plantar aponeurosis. Logo. 700+ ...
  5. [5]
    Flexor Digitorum Brevis | Complete Anatomy - Elsevier
    The flexor digitorum brevis muscle is located in the first layer of muscles that are found in the plantar part of the foot.
  6. [6]
    Flexor Digitorum Brevis - Wheeless' Textbook of Orthopaedics
    At the level of MTP joint each tendon splits into two arms which pass around the FDL, and finally join to insert into the middle phalanges of 2nd to 5th toes.
  7. [7]
    Anatomy, Bony Pelvis and Lower Limb, Foot Muscles - NCBI - NIH
    Apr 8, 2023 · Structure and Function · Origin: The flexor digitorum longus originates from the posterior surface of the tibia distal to the soleal line.
  8. [8]
    Medial plantar nerve: Course and distribution | Kenhub
    May 20, 2024 · As it enters the foot, it passes deep to the abductor hallucis muscle, running between it and the flexor digitorum brevis.<|control11|><|separator|>
  9. [9]
  10. [10]
    Evaluation of the activities of the intrinsic and extrinsic muscles of ...
    This study shows that the abductor hallucis, flexor hallucis brevis, flexor digitorum brevis muscles are more active during toe flexion without IP flexion.
  11. [11]
    Anatomy, Bony Pelvis and Lower Limb: Arteries - StatPearls - NCBI
    Dec 9, 2023 · Both the lateral and medial plantar arteries supply the lumbrical muscles. The medial plantar artery supplies the flexor digitorum brevis.
  12. [12]
    Vascular anatomy of plantar muscles - PubMed
    ... brevis muscles receive their main blood supply from the lateral plantar artery. The flexor digitorum brevis muscle receives branches from both arteries.
  13. [13]
    The venous system of the foot: anatomy, physiology, and clinical ...
    The medial plantar vein receives blood from the adjacent muscles, ie, the abductor hallucis, the flexor digitorum brevis, and the plantar quadratus muscle.
  14. [14]
    Medial Plantar Artery Dominance of the Plantar Arterial Arch - NIH
    The deep plantar arch is the dominant arterial supply to the plantar surface of the foot, formed through an anastomosis between the deep branch of the lateral ...
  15. [15]
    Anatomy, Bony Pelvis and Lower Limb, Foot Joints - StatPearls - NCBI
    ... flexor digitorum longus, popliteus, plantaris, soleus, and tibialis posterior muscles. These muscles manage plantar flexion, foot inversion, and toe flexion.
  16. [16]
    An Anatomical Variation in the Flexor Digitorum Brevis Muscle ...
    The 1st layer of the foot sole contains 3 muscles: Abductor hallucis, Flexor digitorum brevis (FDB), and Abductor digiti minimi. The FDB lies lateral to ...
  17. [17]
    Reliability of doming and toe flexion testing to quantify foot muscle ...
    Dec 8, 2017 · Average hallux flexion strength was 65.66 ± 24.5 N. Average lateral toe flexion was 50.96 ± 22.54 N. Average force values for each statistical ...
  18. [18]
    Quadratus Plantae - Physiopedia
    ... flexor digitorum brevis, quadratus plantae, abductor digiti minimi. Sensory fibres: plantar ligament, calcaneal periosteum. Common Fibular (Peroneal) Nerve ...
  19. [19]
    Flexor digitorum brevis utilizes elastic strain energy to contribute to ...
    Apr 22, 2022 · Therefore, we examined the muscle–tendon dynamics of the flexor digitorum brevis during stepping, jumping and landing tasks to see whether ...
  20. [20]
    Forces acting in the forefoot during normal gait - An estimate
    Aug 7, 2025 · The flexor digitorum longus and brevis forces are about 9% and 13% body weight, respectively. The high forces acting along the flexor tendons ...
  21. [21]
    Importance and challenges of measuring intrinsic foot muscle strength
    Nov 26, 2012 · ... flexor digitorum brevis, dorsal interossei and lumbrical muscles were all active during the stance phase of gait and continued until toe off [3] ...
  22. [22]
    Electromyography and cinematography of leg and foot (“normal” and ...
    Abductor hallucis and Flexor digitorum brevis are generally more active in flatfooted persons. An important regular pattern of inversion and eversion during ...
  23. [23]
    Understanding the role of plantar intrinsic foot muscles, one study at ...
    Mar 16, 2021 · The transverse head of adductor hallucis acts as a forefoot stabilizer during the propulsive phase of gait, neutralizing the tibialis anterior.<|control11|><|separator|>
  24. [24]
  25. [25]
  26. [26]
    Flexor Digitorum Brevis Muscle – Anatomy, Injuries & Recovery
    This pain is commonly caused by overuse, improper footwear, flat feet, plantar fasciitis, or muscle strain. Activities such as running, prolonged standing, or ...
  27. [27]
    Flexor Digitorum Brevis Muscle Dry Needling Changes Surface and ...
    Jan 13, 2021 · After flexor digitorum brevis muscle dry needling, midfoot plantar pressures (maximum and medium) and forefoot surface were increased, and rearfoot maximum ...Missing: dimensions | Show results with:dimensions
  28. [28]
    Validity of ultrasound imaging for intrinsic foot muscle cross ... - NIH
    Feb 14, 2022 · The purpose of this study was to investigate the validity and intratester reliability of US imaging in measuring intrinsic foot muscle size in comparison to ...
  29. [29]
    Ultrasound of the digital flexor system: Normal and pathological ...
    This article examines the anatomy and US appearance of the normal DFS and reviews the US findings associated with the most common disorders affecting it.
  30. [30]
    Ankle and Foot - Musculoskeletal Diseases 2021-2024 - NCBI - NIH
    Apr 13, 2021 · MR imaging can confirm the diagnosis by demonstrating denervation edema or fatty atrophy of the abductor hallucis, flexor digitorum brevis, ...
  31. [31]
    Nerve Entrapment Syndromes of the Lower Extremity Workup
    Aug 25, 2023 · If entrapment is present more proximally, the denervation is present in the anterior tibial muscle as well as in the extensor digitorum brevis.
  32. [32]
    Clinical utility of tibial motor and sensory nerve conduction studies ...
    May 24, 2011 · Tibial motor nerve recording from the flexor hallucis brevis muscle has been referenced as an alternative to abductor hallucis recording [20].<|separator|>
  33. [33]
    Flexor Digitorum Brevis - Rehab My Patient
    Jul 23, 2020 · Flexor digitorum brevis exercises. Toe flexion both sides. Standing or sitting, bend the toes downwards, towards the floor.
  34. [34]
    Flexor Digitorum Brevis Activation - YouTube
    Mar 23, 2021 · Today I want to give you an exercise to just try to get that muscle activated this is very important for people with hammer toes.
  35. [35]
    Percutaneous flexor digitorum brevis tenotomy: An anatomical study
    A percutaneous selective flexor digitorum brevis (FDB) tenotomy and a proximal interphalangeal (PIP) joint arthrolysis may correct a lesser claw toe deformity.
  36. [36]
    Focal myositis in the foot: one unique case in the plantar aspect - NIH
    Dec 30, 2022 · Treatment may include giving steroids or surgery. The injury may decline spontaneously and may develop into multiple myositis. We have ...
  37. [37]
    The fourth slip of the flexor digitorum brevis muscle of the human ...
    Meta-analytical results of fourth slip agenesis were as follows; a) a true prevalence rate of 31.3%, b) a crude prevalence rate of 47%, c) a bilateral ...
  38. [38]
    Age-Related Reduction of Foot Intrinsic Muscle Function and ... - NIH
    Jun 4, 2024 · This study aimed to investigate the effects of aging on the function of FIMs and to explore the influence of FIMs on postural control in the elderly.
  39. [39]
    A Missing Flexor Digitorum Brevis Tendon and Its Relationship to ...
    Dec 6, 2022 · This study aimed to assess the frequency of a missing flexor digitorum brevis tendon in a Hispanic population for the first time.Missing: splitting | Show results with:splitting
  40. [40]
    A Missing Flexor Digitorum Brevis and its Evolutionary and Clinical ...
    Apr 17, 2020 · The flexor digitorum brevis develops embryologically from anterior condensations of abaxial muscle cell precursors starting at 6 weeks. This ...
  41. [41]
    Some variations of the musculus flexor digitorum brevis - PubMed
    The aim of the present study was to investigate variations of the musculus flexor digitorum brevis. We examined 33 feet in total from 15 embalmed adult ...Missing: prevalence | Show results with:prevalence
  42. [42]
    Separated muscle belly of the flexor digitorum brevis for the fifth toe
    Sep 30, 2023 · This case report describes a variation of the flexor digitorum brevis (FDB) with a separated muscle belly and tendon at the fifth toe.Missing: independent action
  43. [43]
    [PDF] On the Variations of the Muscle Flexor Digitorum Brevis: Anatomical ...
    Flexor digitorum brevis is an intrinsic muscle of the sole of the foot that lies immediately superior to the central part of the plantar aponeurosis and ...<|control11|><|separator|>