Anatomical Characteristics and Functional Importance of Intrinsic Muscles
Intrinsic muscles are a unique group of muscles that have both proximal and distal attachments within the foot itself.
On the dorsal side of the foot, there is only one intrinsic muscle, the extensor digitorum brevis, which is innervated by the deep branch of the common fibular nerve.
The extensor digitorum brevis is located proximally on the dorsolateral surface of the calcaneus, near the calcaneocuboid joint.
This muscle consists of four individual tendons:
- One tendon extends to the dorsal surface of the hallux, often referred to as the extensor hallucis brevis.
- The other three tendons blend with the tendons of the extensor digitorum longus, inserting into the dorsal digital expansion of the second, third, and fourth toes.
The primary function of this muscle is to assist the extensor hallucis longus and extensor digitorum longus in toe extension.
All other intrinsic foot muscles are located on the plantar surface, where they form four distinct layers.
A notable feature is that the plantar fascia lies superficially to the first-layer muscles.
First Layer of the Plantar Intrinsic Muscles
The first layer consists of three main muscles:
- Flexor digitorum brevis
- Abductor hallucis
- Abductor digiti minimi
These muscles function as a unit and have proximal attachments on the calcaneal tuberosity, lateral process, and surrounding connective tissue.
Flexor Digitorum Brevis (FDB)
- Inserts distally on the bilateral plantar surfaces of the middle phalanges of toes 2–5.
- Each tendon bifurcates proximally to allow passage of the flexor digitorum longus (FDL) tendons, a structure similar to the relationship between the flexor digitorum superficialis and profundus in the hand.
- The primary function is to assist FDL in toe flexion.
Abductor Hallucis
- A medial plantar muscle that forms a key anatomical passageway for neurovascular structures traveling to the plantar foot.
- Inserts on the medial aspect of the proximal phalanx of the hallux, in conjunction with the medial head of the flexor hallucis brevis.
- It primarily abducts the hallux and contributes to toe flexion.
Abductor Digiti Minimi
- Located laterally, forming part of the outer contour of the foot.
- Inserts on the lateral side of the proximal phalanx of the fifth toe.
- Primarily abducts the little toe, with a secondary role in toe flexion.
Second Layer of the Plantar Intrinsic Muscles
The second layer contains two muscles:
- Quadratus plantae (Flexor digitorum accessorius)
- Lumbricals
Both muscles have a close anatomical association with the flexor digitorum longus tendons.
Quadratus Plantae (Flexor Digitorum Accessorius)
- Arises from the plantar surface of the calcaneus with two distinct heads.
- Inserts distally at the lateral border of the common FDL tendon, altering its line of pull.
- Primary function:
- Stabilizes the FDL tendon
- Prevents excessive medial deviation of the tendons during toe flexion.
Lumbricals
- Originates from the FDL tendons proximally.
- Each muscle courses medially along the toes to insert onto the extensor digital expansion.
- Functionally identical to the lumbricals of the hand, they:
- Flex the metatarsophalangeal joints
- Extend the interphalangeal joints
This layered structure of intrinsic foot muscles plays a crucial role in:
- Supporting the arches
- Maintaining dynamic foot stability
- Assisting movement in walking, running, and balancing activities.