Anatomy and supply
The central muscles of the foot sole lie within the central compartment between the muscles of the big and little toe. The compartment comprises numerous short foot muscles in different layers. Together they form the central surface of the foot sole. Branches of the tibial nerve (L5-S2) are responsible for the innervation of the central muscles. In detail, they are:
- Flexor digitorum brevis muscle: originates at the calcaneal tuberosity and plantar aponeurosis. Distally it divides into four tendons moving towards the second to fifth toe. At the proximal phalanges these tendons separate further into two smaller tendons which finally insert medially and laterally at the middle phalanges. [innervation: medial plantar nerve]
- Quadratus plantae muscle: runs from the calcaneus to the tendon of the flexor digitorum longus muscle. [innervation: lateral plantar nerve]
- Lumbrical muscles (I-IV): originate at the tendons of the flexor digitorum longus muscle and insert medially at the proximal phalanges of the second to fifth toe and dorsal aponeurosis. [innervation: medial plantar nerve (I), lateral plantar nerve (II-IV)]
- Plantar interossei muscles I-III: extend from the third and fourth metatarsal bones to the medial side of the proximal phalanx of the respective toe. [innervation: lateral plantar nerve]
- Dorsal interossei muscles I-IV: are two-headed muscles arising from the adjacent sides of two neighbouring metatarsal bones. Their tendons insert laterally at the proximal phalanges of the second to fourth toe and medially at the second toe. [innervation: lateral plantar nerve]
The flexor digitorum brevis muscle lies relatively superficially under the plantar aponeurosis and marks the largest muscle in the central compartment. The quadratus plantae muscle runs above it. The origins of the lumbrical muscles are located at the distal end of the quadratus plantae muscle. In comparison the plantar and dorsal interossea muscles lie further at the top.
In addition to the above mentioned muscles the central compartment of the foot sole comprises the oblique head of the adductor hallucis muscle, the posterior tibial artery and vein and the tibial nerve. However the vessels and nerves divide quite early into a medial and lateral bundle entering the medial and lateral compartment of the foot sole. Distally the branches meet again in the central compartment forming an arterial and venous arcade (plantar arch and plantar venous arch).
The muscles of the central compartment fulfill various tasks. The flexor digitorum brevis muscle does a flexion of the second to fifth toe and actively supports the longitudinal arch of the foot. The quadratus plantae muscle does not move any joints but
has a rather special function: By pulling at the tendon of the flexor digitorum longus muscle it shifts the tendon’s force effect to the longitudinal direction increasing the effectiveness of the plantar flexion of the muscle. Both the lumbrical and plantar interossei muscles flex and adduct the toes (closing the spread toes). The dorsal interossei muscles flex the toes as well but in contrast cause an abduction (spreading of the toes).
The plantar aponeurosis passively contributes to the longitudinal arch of the foot sole. If it is chronically strained (e.g. in athletes and elderly people) a plantar fasciitis may develop. This is a degenerative change of the plantar aponeurosis at the origin at the calcaneus. Typically the X-ray shows a dense ossification (calcaneal or heel spur). Being overweight, metabolic and rheumatic diseases are the most important risk factors. The affected people experience a slow and subtle, stress-dependent pain at the heel and the sole of the foot. In addition to orthopedic insoles, physiotherapy and pain medication (if required) good results are achieved with shock wave therapy.