Two-headed muscle
When studying the foot anatomy, special attention is paid to the two-headed muscle, which has a complex topography and biomechanics. This muscle is formed by two separate muscle bellies — the medial and lateral heads.
The origin point of the muscle fibers is located on the plantar surface of the cuneiform bones. At the same time, the lateral portion of the muscle has an expanded origin zone, also capturing the surface of the cuboid bone. Dense tendinous intersections play an important role in fixing the initial section.
Insertion of the muscle has a unique feature. The tendons of both heads are directed toward the base of the proximal phalanx of the first toe (hallux) of the foot. However, on their way, they pass through specialized bone structures — the lateral and medial sesamoid bones. These small bones are woven into the tendons and act as pulleys.
Functions of the two-headed muscle are strictly differentiated:
- The global task of both heads is flexion of the proximal phalanx of the great toe.
- The medial head additionally performs the function of adduction of the toe.
- The lateral head is responsible for a crucial biomechanical task — it stabilizes the sesamoid pulley mechanism, preventing tendon displacement under loads.
Innervation of the muscle is divided between two nerve trunks. The lateral part receives motor impulses from the lateral plantar nerve (n. plantaris lateralis), whose fibers originate from spinal segments S₁–S₂. The medial part is innervated by the medial plantar nerve (n. plantaris medialis, segments L₄–S₁). Blood supply to the tissues is provided by branches of the vessels of the same name: a. plantaris medialis and a. plantaris lateralis.
Opponens digiti minimi muscle
The next important structure is the opponens digiti minimi muscle (m. opponens digiti minimi). In clinical anatomy, it is often considered not as a completely independent unit, but as part of the flexor digiti minimi brevis due to their close anatomical fusion.
Origin of this muscle is localized on the lateral margin of the fifth metatarsal bone. In addition to the bony base, the muscle bundles firmly fuse with the fibrous sheath of the tendon of the peroneus longus muscle (m. peroneus longus), which creates a single functional complex on the lateral edge of the foot.
Insertion point of the fibers is distributed along the lateral margin of the fifth metatarsal bone and reaches as far as the base of the proximal phalanx of the little toe.
Biomechanics and functions of the muscle:
- The main task is slight opposition of the little toe. This movement consists of carefully bringing the fifth toe toward the median axis of the foot and its slight rotation.
- The second, but no less important function is stabilization of the lateral arch of the foot during standing and walking.
Blood supply and innervation of this zone are provided by the same lateral neurovascular bundles described for the lateral structures above (branches of the lateral plantar nerve and lateral plantar artery).
Middle (central) group: Lumbrical muscles
The central (or middle) compartment of the sole includes a group of four small but functionally indispensable muscles — the lumbrical muscles (mm. lumbricales). Their anatomical uniqueness lies in the fact that they have no rigid fixation to bones at their proximal end.
Origin point of all four muscle bellies is the mobile tendons of the flexor digitorum longus (m. flexor digitorum longus). Branching off from these tendons, the lumbrical muscles run distally.
They insert into the medial margin of the dorsal aponeurosis and also weave into the bases of the proximal phalanges of the second through fifth toes of the foot.
Functional significance of the lumbrical muscles is highly specific:
- They perform flexion at the metatarsophalangeal (MTP) joints of digits II–V.
- Simultaneously, they extend the interphalangeal (IP) joints of the same digits.
Such complex combined work (flexion in one joint and extension in others) forms the characteristic position of the toes. In anatomical practice, this movement pattern is often compared to the position of a "writing pen", which is typical for both the hand and the foot.
Innervation in this group is distributed unevenly, which is important to remember for exams: the first (most medial) lumbrical muscle is innervated by the medial plantar nerve (n. plantaris medialis). The remaining three muscles (second, third, and fourth) receive branches from the lateral plantar nerve (n. plantaris lateralis).
Blood supply to the muscle bellies is carried out by the plantar metatarsal arteries (aa. metatarseae plantares), which are direct branches of the deep plantar arch (arcus plantaris).
Briefly: answers to topical questions
Which muscles form the middle group of plantar muscles?
The middle (central) group of plantar muscles is formed by the muscles of the middle fascial compartment:
- flexor digitorum brevis muscle (m. flexor digitorum brevis);
- quadratus plantae muscle (m. quadratus plantae);
- lumbrical muscles (mm. lumbricales);
- plantar interossei muscles (mm. interossei plantares);
- adductor hallucis muscle (m. adductor hallucis).
In the central group, three layers are distinguished: superficial — flexor digitorum brevis; next — quadratus plantae and lumbrical muscles; deep — interossei muscles and the transverse head of the adductor hallucis muscle.
Which foot muscles are innervated by the medial plantar nerve?
The medial plantar nerve (n. plantaris medialis) innervates:
- muscles of the first digit (hallux);
- flexor digitorum brevis muscle (m. flexor digitorum brevis);
- first lumbrical muscle (m. lumbricalis I).
Where does the quadratus plantae originate and insert?
The quadratus plantae muscle (m. quadratus plantae) originates from the calcaneus and inserts into the tendon of the flexor digitorum longus.
- Origin — by two heads from the medial and lateral surfaces of the calcaneus.
- Insertion — lateral margin of the tendon of the flexor digitorum longus (m. flexor digitorum longus).
The innervation of the lumbrical muscles of the foot is divided by the "1 to 3" rule: the first muscle receives the medial nerve, and the remaining three receive the lateral nerve.
Class and exam questions
Where do the lumbrical muscles of the foot originate?
Unlike most muscles, they originate not from bones, but from the tendons of the flexor digitorum longus (m. flexor digitorum longus).
What function do sesamoid bones perform in the tendons of the biceps muscle?
The lateral and medial sesamoid bones act as pulleys for the tendons, and the lateral head of the muscle actively stabilizes this sesamoid apparatus.
What does the "fountain pen" sign mean when describing muscle functions?
This is a combined movement in which the lumbrical muscles flex the metatarsophalangeal (MTP) joints and simultaneously extend the interphalangeal (IP) joints of digits II–V.
Which muscle stabilizes the lateral arch of the foot?
This function is performed by the opponens digiti minimi muscle (m. opponens digiti minimi), which attaches to the fifth metatarsal bone.
What else to study on this topic
- Topography and branching of the medial and lateral plantar nerves
- Formation and branches of the plantar arterial arch (arcus plantaris)
- Structure of the dorsal aponeurosis of the foot and its connection with tendons
- Biomechanics of metatarsophalangeal (MTP) and interphalangeal (IP) joints
- The role of the tendon sheath of the peroneus longus muscle in the architecture of the foot
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