Medial Muscles of the sole of the Foot
Anatomy and supply
The plantar fascia which surrounds all muscles of the sole of the foot consists of three chambers. The muscles lying within the medial group form a bulge referred to as the 'ball' of the big toe. It contributes to the surface anatomy of the medial sole of the foot and is easy to palpate. The medial plantar muscles are innervated by motor branches of the tibial nerve (L5-S2). They are:
- Abductor hallucis muscle: has its origin at the calcaneal tuberosity, plantar aponeurosis and the superficial layer of the flexor retinaculum. Its tendon runs along the medial sesamoid and inserts at the proximal phalanx of the big toe. The muscle is innervated by the medial plantar nerve.
- Adductor hallucis muscle: has two heads: The transverse head originates at the 3rd to 5th metatarsophalangeal joint and the deep transverse metatarsal ligament. The oblique head arises at the cuboid bone, the lateral cuneiform bone and the 2nd to 4th metatarsal bones. To be precise the oblique head does not lie within the medial but central group of the plantar fascia. The common tendon runs along the lateral sesamoid and inserts at the proximal phalanx of the big toe. The innervation is supplied by the lateral plantar nerve.
- Flexor hallucis brevis muscle: originates at the cuneiform bones and plantar calcaneocuboid ligament. Its insertions are the medial (medial head) and lateral (lateral head) sesamoid. While the medial head is innervated by the medial plantar nerve the lateral head is supplied by the lateral plantar nerve (double innervation).
In addition to the three above mentioned muscles there are more structures lying in the medial group of the plantar fascia: the tendon of the flexor hallucis longus muscle and the blood vessels and nerves of the medial foot sole (medial plantar artery and vein, medial plantar nerve).
The medial muscles of the foot sole have various tasks: First of all they move the metatarsophalangeal joint of the big toe leading to the abduction (abductor hallucis muscle), adduction (adductor hallucis muscle) and flexion (both flexor hallucis brevis and adductor hallucis muscle) of the big toe. Together they support the plantar flexion. Furthermore they actively support the structure of the foot arches. While all three muscles stabilize the longitudinal arch the transverse head of the adductor hallucis muscle is the only muscle securing the transverse arch.
The insufficiency of the ligaments and muscles of the foot sole often lead to foot deformities. One of the most common is the bunion (hallux valgus) characterized by a pathological adduction of the metatarsal bone of the big toe. Normally the abductor hallucis muscle limits the lateral movement of the big toe. However in the course of the bunion a shifting of the tendon force occurs causing the muscle to even increase the adduction. Typical symptoms are callus formation at the first metatarsal bone, pain during stress and restriction of mobility. Obese women wearing inappropriate shoes (high-heeled, tight, pointy) and lacking physical exercise are particularly in risk of suffering from bunion.