Flexor Hallucis Brevis (Bicipital Plantar Muscle)
Special attention is paid to the flexor hallucis brevis, which features a complex topography and biomechanics. This muscle is formed by two distinct muscular bellies — medial and lateral heads.
Origin fibers arise from the plantar surface of the cuneiform bones. Additionally, the lateral portion has an expanded origin extending to the surface of the cuboid bone. Dense tendinous bands play an important role in anchoring the proximal attachment.
Insertion has a unique feature. The tendons of both heads travel toward the base of the proximal phalanx of the hallux (first digit). Along their path, they encompass specialized bony structures — the lateral and medial sesamoid bones. These small bones are embedded within the tendons and act as pulleys.
Functions of the flexor hallucis brevis are strictly differentiated:
- The global action of both heads is flexion of the proximal phalanx of the great toe.
- The medial head additionally performs adduction of the digit.
- The lateral head carries out a critical biomechanical task — it stabilizes the sesamoid complex, preventing tendon displacement under load.
Innervation is split between two nerve trunks. The lateral part receives motor input from the lateral plantar nerve (n. plantaris lateralis), originating from spinal segments S₁–S₂. The medial part is innervated by the medial plantar nerve (n. plantaris medialis, segments L₄–S₁). Blood supply is provided by branches of the corresponding vessels: a. plantaris medialis and a. plantaris lateralis.
Opponens Digiti Minimi
Another key structure is the opponens digiti minimi (m. opponens digiti minimi). In clinical anatomy, it is often viewed not as a completely isolated entity, but as part of the flexor digiti minimi brevis due to their close anatomical fusion.
Origin localizes to the lateral margin of the fifth metatarsal bone. Beyond bony attachment, the muscle fibers firmly merge with the fibrous tendon sheath of the fibularis longus (m. peroneus longus), creating a unified functional complex on the lateral border of the foot.
Insertion extends along the lateral margin of the fifth metatarsal and reaches as far as the base of the proximal phalanx of the fifth digit.
Biomechanics and functions:
- The primary role is slight opposition of the fifth digit. This movement combines gentle medial abduction toward the midline of the foot with a small degree of rotation.
- The secondary, equally vital function is stabilization of the lateral arch of the foot during standing and walking.
Blood supply and innervation are provided by the same lateral neurovascular bundles described for the lateral structures above (branches of the lateral plantar nerve and lateral plantar artery).
Central Group: Lumbrical Muscles
The central compartment of the sole contains four small yet functionally indispensable muscles — the lumbricals (mm. lumbricales). Their anatomical uniqueness lies in the fact that they lack a direct proximal bony attachment.
Origin of all four muscle bellies arises from the mobile tendons of the flexor digitorum longus (m. flexor digitorum longus). Emerging from these tendons, the lumbrical muscles course distally.
Insertion attaches to the medial margin of the dorsal digital aponeurosis and blends into the bases of the proximal phalanges of digits II through V.
Functional significance of the lumbricals is highly specific:
- They flex the metatarsophalangeal (MTP) joints of digits II–V.
- Concurrently, they extend the interphalangeal (IP) joints of the same digits.
This complex combined action (flexion at one joint and extension at others) produces a characteristic finger and toe posture. In anatomical practice, this movement pattern is often compared to holding a pen ("writing position"), seen in both the hand and the foot.
Innervation within this group is unevenly distributed: the first (most medial) lumbrical 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 delivered via the plantar metatarsal arteries (aa. metatarseae plantares), which are direct branches of the deep plantar arch (arcus plantaris).