Location and Compartment Boundaries
The muscle is listed among the structures of the deep layer of the sole and topographically belongs to the medial fascial compartment of the sole.
The boundaries of this fascial space include:
- Inferiorly and medially — the fascia propria (fascia propria), which is a continuation of the thinned plantar aponeurosis.
- Laterally — the medial intermuscular septum (septum intermusculare mediale).
- Superiorly — at the tarsal level, the compartment is bounded by the navicular and medial cuneiform bones, and at the metatarsal level by the first metatarsal bone.
Within this compartment at the metatarsal level (Metatarsus), the flexor hallucis brevis is located alongside the abductor hallucis muscle (m. abductor hallucis).
Topography of the Deep Plantar Layer
In anatomical listings of the deep plantar layer, along with the m. flexor hallucis brevis, the following structures are included:
- Adductor hallucis muscle (m. adductor hallucis) — oblique (caput obliquum) and transverse (caput transversum) heads.
- Quadratus plantae muscle (m. quadratus plantae) and plantar interossei muscles (mm. interossei plantares).
- Tendons: flexor digitorum longus, flexor digitorum brevis, flexor hallucis longus, and peroneus longus and brevis tendons.
- Vessels and Nerves: medial and lateral plantar arteries and nerves (a., n. plantaris medialis et lateralis), deep plantar arch (arcus plantaris profundus), plantar metatarsal arteries, and the plantar metatarsal nerve.
Relationship with Sesamoid Bones and Tendons
The sesamoid bones of the big toe are embedded within the tendons of the m. flexor hallucis brevis and form a groove or canal for the tendon of the flexor hallucis longus (tendo m. flexoris hallucis longi, FHL).
On the sole, the flexor hallucis longus tendon crosses superficial to or deep depending on reference, classically forming the plantigrade master knot of Henry with the flexor digitorum longus tendon at the level of the navicular and cuneiform bones: the FHL tendon passes deep to the FDL tendons toward the hallux.
Landmarks in Anatomical Dissections
In the topographical anatomy of the vessels and nerves of the medial leg and foot, muscular landmarks include the abductor hallucis muscle (m. abductor hallucis), partially reflected in dissections, and the flexor hallucis brevis muscle (m. flexor hallucis brevis).
On the sole, the posterior tibial artery bifurcates into the medial and lateral plantar arteries (a. plantaris medialis et a. plantaris lateralis).
Do Not Confuse: Flexor Pollicis Brevis (Hand)
In upper limb anatomy, there is a similarly named muscle in Russian terminology — m. flexor pollicis brevis. It is located in the thenar eminence and acts on the thumb.
- Structure and Origin: has superficial and deep heads. The superficial head originates from the flexor retinaculum (retinaculum mm. flexorum), while the deep head originates from the trapezium and capitate bones (os trapezoideum, os capitatum). The tendon of the flexor pollicis longus passes between these heads.
- Insertion: base of the proximal phalanx of the thumb laterally via a sesamoid bone.
- Function: flexes the proximal phalanx of the thumb; assists in thumb opposition.
- Innervation: superficial head by the median nerve (n. medianus), deep head by the ulnar nerve (n. ulnaris).