First Layer of Sole

Introduction

Introduction to the First Layer of the Sole

The muscles of the sole are arranged in four layers, from superficial to deep. The first layer, lying immediately deep to the plantar aponeurosis, is the most superficial muscular layer. It consists of three intrinsic muscles: the abductor hallucis, the flexor digitorum brevis, and the abductor digiti minimi. These muscles form the bulk of the sole and are primarily responsible for the gross movements of the great and little toes, as well as assisting in supporting the longitudinal arches of the foot. All three are innervated by branches of the tibial nerve: the abductor hallucis and flexor digitorum brevis by the medial plantar nerve, and the abductor digiti minimi by the lateral plantar nerve. Their blood supply is from the medial and lateral plantar arteries. Knowledge of this layer is fundamental for understanding foot biomechanics, arch support, and clinical conditions such as entrapment neuropathies and muscle strains.

Study

1. Arrangement and Compartments

The sole is divided into three compartments by the medial and lateral intermuscular septa extending from the plantar aponeurosis. The medial compartment contains the abductor hallucis (and deeper, the flexor hallucis brevis and the tendon of flexor hallucis longus). The central compartment contains the flexor digitorum brevis (and deeper, the flexor digitorum longus tendons, lumbricals, and the neurovascular bundle). The lateral compartment contains the abductor digiti minimi (and deeper, the flexor digiti minimi brevis). In the first layer, the muscles correspond to these compartments, each lying directly beneath the corresponding portion of the aponeurosis.

2. Abductor Hallucis

Abductor hallucis is the most medial muscle of the first layer. Origin: medial process of the calcaneal tuberosity, the flexor retinaculum, and the plantar aponeurosis. Insertion: medial side of the base of the proximal phalanx of the great toe (hallux). Action: abducts the great toe (away from the second toe) and assists in flexion of the metatarsophalangeal joint; also supports the medial longitudinal arch. Innervation: medial plantar nerve (S1, S2). Blood supply: medial plantar artery.

It forms the medial border of the sole and is palpable when the great toe is actively abducted against resistance. In gait, it helps control the alignment of the hallux and contributes to the windlass effect indirectly through its fascial connections.

3. Flexor Digitorum Brevis

Flexor digitorum brevis is the central muscle of the first layer, lying in the middle of the sole. Origin: medial process of the calcaneal tuberosity and the plantar aponeurosis. Insertion: by four tendons to the middle phalanges of the lateral four toes (digits 2–5). Each tendon splits to allow passage of the flexor digitorum longus tendon (forming the digital chiasma) before inserting onto the sides of the middle phalanx. Action: flexes the proximal interphalangeal joints of toes 2–5; assists in supporting the longitudinal arches. Innervation: medial plantar nerve (S1, S2). Blood supply: medial and lateral plantar arteries.

It is analogous to the flexor digitorum superficialis in the hand, but unlike in the hand, it is an intrinsic muscle of the sole. Its tendons are ensheathed with the long flexor tendons within the fibrous flexor sheaths.

4. Abductor Digiti Minimi

Abductor digiti minimi is the lateral muscle of the first layer. Origin: lateral process of the calcaneal tuberosity and the plantar aponeurosis. Insertion: lateral side of the base of the proximal phalanx of the little toe (fifth digit). Action: abducts the little toe (away from the fourth toe) and assists in flexion at the metatarsophalangeal joint; supports the lateral longitudinal arch. Innervation: lateral plantar nerve (S1, S2, S3). Blood supply: lateral plantar artery.

It forms the lateral border of the sole. It is the only muscle of the first layer innervated by the lateral plantar nerve. Its nerve supply, a branch from the lateral plantar nerve, courses close to the calcaneus and may be compressed in Baxter's neuropathy (inferior calcaneal nerve entrapment), causing lateral heel pain.

5. Neurovascular Supply of the First Layer

The first layer muscles receive innervation from the two terminal branches of the tibial nerve. The medial plantar nerve (the larger terminal branch) supplies the abductor hallucis and flexor digitorum brevis. It enters the sole deep to the abductor hallucis and runs forward between the first and second layers, giving off muscular branches. The lateral plantar nerve supplies the abductor digiti minimi and all other intrinsic muscles not supplied by the medial plantar nerve. The blood supply is from the medial and lateral plantar arteries, branches of the posterior tibial artery. The medial plantar artery is smaller and accompanies the medial plantar nerve; the lateral plantar artery is larger and forms the deep plantar arch with the deep plantar branch of the dorsalis pedis artery.

6. Clinical Relevance

Plantar fasciitis: While primarily a disorder of the plantar aponeurosis, the first layer muscles lie directly deep to the aponeurosis. Inflammation can irritate the underlying flexor digitorum brevis and abductor hallucis, causing associated myofascial pain. In chronic cases, contracture of the first layer muscles may contribute to pain.

Baxter's neuropathy: The first branch of the lateral plantar nerve (Baxter's nerve) passes between the abductor hallucis and quadratus plantae to innervate the abductor digiti minimi. Entrapment of this nerve at the medial calcaneal tuberosity or under the deep fascia can cause isolated atrophy and weakness of the abductor digiti minimi, presenting as lateral heel pain that mimics plantar fasciitis.

Abductor hallucis strain: Runners and dancers may strain this muscle, causing medial arch pain. It is often associated with overpronation and can be diagnosed by tenderness along the muscle and pain with resisted great toe abduction.

Flexor digitorum brevis dysfunction: Repetitive impact (e.g., in marathon runners) may cause tendinopathy or partial tears of its tendons, presenting as pain in the mid-sole and pain with resisted toe flexion.

Compartment syndrome: The first layer muscles are within the medial, central, and lateral compartments. Crush injuries can cause compartment syndrome, requiring fasciotomy through the plantar aponeurosis to decompress these muscles.

Summary

Summary of the First Layer of the Sole

The first layer of the sole consists of three muscles: abductor hallucis (medial), flexor digitorum brevis (central), and abductor digiti minimi (lateral). All arise from the calcaneal tuberosity and plantar aponeurosis. They abduct/flex the great and little toes and flex the lateral four toes, respectively, while supporting the arches. The medial plantar nerve supplies the first two muscles, and the lateral plantar nerve supplies abductor digiti minimi. Blood supply comes from the medial and lateral plantar arteries. Clinically, these muscles are involved in plantar fasciitis, Baxter's neuropathy, and medial arch strains.