Sole of foot

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Regions of the lower limb seen from the anterior and posterior views.

The sole of foot refers to the inferior or bottom surface of the foot, which contacts the floor when standing barefoot. It is also called the plantar region of foot, and is opposite to the dorsum of foot, which refers to the superior surface.

The sole of foot has specific names for various areas. For instance, the area beneath the heads of the first two metatarsal bones on the medial side is called the ball of foot, and the section directly under the calcaneus is referred to as the heel or heel region.

The sole of foot is not flat, but instead bears three foot arches: medial longitudinal, lateral longitudinal, and transverse. The bones of the foot, along with several ligaments, muscles and muscle tendons, work together to form and maintain these arches. This unique design allows it to support the weight of the body and to perform complex movements necessary for walking, running, and jumping, while also absorbing the impact forces generated during these activities.

The sole is composed of several layers. Starting superficially or externally and moving deeper, it includes the skin, subcutaneous tissue (superficial fascia), deep fascia, muscles, tendons, ligaments and bones, as well as the neurovasculature that supply these structures.

A thick layer of skin covers the sole of foot, particularly over the major weight-bearing regions such as the heel, lateral margin, and ball of the foot. The skin of the sole is devoid of hair and abundant in sweat glands. Beneath the skin lies the subcutaneous tissue, a thick layer of fat and fibrous connective tissue septa that provides cushioning and insulation.

Beneath the subcutaneous tissue lies the deep fascia of the sole of foot, formed by the plantar aponeurosis. This strong, fibrous band extends from the heel to the toes. It supports the longitudinal arches of the foot and plays a crucial role in foot mechanics.

Below the deep fascia, the sole of foot houses 14 individual muscles, which are grouped into four distinct layers. These plantar muscles function collectively to reinforce the arches of the foot and support the weight of the body when standing and moving.

• The first layer includes the abductor hallucis, flexor digitorum brevis, and abductor digiti minimi muscles. These muscles support the arches of the foot and aid in toe movement.
• The second layer contains two muscles: the quadratus plantae which assists in toe flexion and the lumbricals that extend the toes at the interphalangeal joints and flexes them at the metatarsophalangeal joints.
• The third layer, consisting of the flexor hallucis brevis, adductor hallucis, and flexor digiti minimi brevis, primarily facilitate the adduction and flexion of the big and little toes.
• The fourth layer comprises the plantar and dorsal interossei muscles, which abduct and adduct the toes, respectively. These also assist with maintaining balance and distributing weight across the foot.

Sandwiched between the first and second layers are the tendons of the flexor digitorum longus, a muscle located in the deep posterior compartment of the leg. Two neurovascular planes also exist between the sole's muscle layers: a superficial plane located between the first and second layers, and a deep plane positioned between the third and fourth layers.

The deepest structures of the sole of foot are the undersurfaces of the foot bones, along with various ligaments and extrinsic tendons that play a crucial role in forming and supporting the arches of the foot.

The foot comprises seven tarsal bones, five metatarsal bones, and 14 phalangeal bones, with several joints between them. The major ligaments in this region that contribute to the formation of the foot arches are the spring ligament (calcaneonavicular) and the short plantar (plantar calcaneocuboid) ligament. Additionally, this area contains several extrinsic tendons, which originate in the leg and insert into the plantar aspect of the foot bones, supporting the arches and enabling foot movements. These include the tendons of the tibialis posterior and fibularis (peroneus) longus muscles.

The sole of foot receives its innervation from the medial and lateral plantar nerves, both branches of the tibial nerve. These nerves provide sensation to the skin and motor function to the muscles of the sole.

The blood supply to the sole of foot is primarily from the posterior tibial artery, which divides into the medial and lateral plantar arteries that run alongside their respective nerves. Venous drainage in the sole is by two pathways: the superficial plantar veins within the subcutaneous tissue, draining to the medial and lateral marginal veins, and the deep venous network, which drains blood from the deeper structures of the sole into branches that run alongside the medial and lateral plantar arteries.

Superficial lymphatic drainage of the sole occurs via the medial superficial vessels, which accompany the great saphenous vein, and lateral superficial vessels, which follow the small saphenous vein to the popliteal fossa. The deep lymphatic vessels from the sole, on the other hand, accompany the major veins and drain into the popliteal lymph nodes.