Sole

The sole refers to the flexor surface of the foot. It is located beneath the tarsus, metatarsus, and phalangeal regions.

The sole is not flat on the ground; instead, it forms a raised, dome-shaped structure due to the horizontal and longitudinal arches formed by the foot bones. These arches are supported and maintained by the tendons of the leg muscles and ligaments in the foot. They serve to absorb the downward force of body weight, acting as both weight-bearers and shock-absorbers.

The majority of the intrinsic muscles of the foot are found in the sole, organized into four distinct layers from superficial to deep. The tendons of extrinsic muscles, such as the flexor hallucis longus and flexor digitorum longus, pass through the second layer within the sole. They insert into the distal phalanges of the big toe and the lateral four toes, respectively. The intrinsic muscles in each layer are as follows:

• Layer 1 contains the flexor digitorum brevis, abductor hallucis, and abductor digiti minimi muscles.

• Layer 2 contains the long tendons of the flexor digitorum longus and flexor hallucis longus, as well as two intrinsic muscles: the lumbricals and flexor accessories (a.k.a. quadratus plantae).

• Layer 3 contains the adductor hallucis with its two heads, flexor hallucis brevis, and flexor digiti minimi.

• Layer 4 is the deepest and contains the interossei (plantar and dorsal) and the tendons of the tibialis posterior and peroneus longus muscles.

The neurovasculature of the sole consists of the medial and lateral plantar vessels and nerves. The vessels arise from the posterior tibial vessels, while the nerves are branches of the tibial nerve.

The deep fascia of the sole thickens to form a triangular plantar aponeurosis, which serves to (1) protects the deeper structures within the sole, (2) help maintain the longitudinal arches, and (3) provide the origin for muscles in the first layer of the sole. Other structures formed by the deep fascia include the deep transverse metatarsal ligaments that connect the metatarsophalangeal joints and the fibrous flexor sheaths of the toes, which wrap around the tendons of the flexor digitorum longus and brevis muscles.