Deep posterior muscles of the lower Leg
Anatomy and supply
The deep posterior muscles of the lower leg are part of the calf musculature. They are located within the deep posterior compartment. These muscles are innervated by the tibial nerve (L4-S2). They include:
- Tibialis posterior muscle: originates at the interosseous membrane, tibia and fibula and inserts at the medial border of the foot at the tuberosity of the navicular bone and the cuneiform bones as well as the bases of 2nd, 3rd, and 4th metatarsals. Some resources also mention that a tendinous band of the muscle passes laterally and a little proximally to the tip and distal margin of the sustentaculum tali.
- Flexor digitorum longus muscle: begins at dorsal side of the tibia and curves laterally around the navicular bone. There the tendon fans out into four smaller tendons which insert at the bases of the distal phalanges of the second to fifth toes. Interestingly the place of the fanning-out is the insertion of the quadratus plantae muscle at the same time.
- Flexor hallucis longus muscle: has its origin at the dorsal fibula and interosseous membrane. Then it runs under the sustentaculum tali of the calcaneus and finally inserts at the distal phalanx of the big toe. This muscle forms a tendon sheath at the foot sole as well.
The tendon of the flexor digitorum longus muscle crosses the tendon of the tibialis posterior muscle proximally to the medial malleolus (crural chiasm) and the tendon of the flexor hallucis longus muscle at the sole of the foot (plantar chiasm). The tendons of the deep posterior muscles have in common that they all run behind the medial malleolus (medial malleolar sulcus), namely in the following order (from top to bottom): tibialis posterior, flexor digitorum longus, flexor hallucis longus. There they lie within separate tendon compartments formed by the deep layer of the flexor retinaculum of the foot.
All deep posterior muscles are involved in the plantar flexion in the upper ankle joint and the inversion (supination) in the lower ankle joint. Additionally the flexor digitorum longus makes a flexion of the proximal and middle joints of the second to fifth toes. The flexor hallucis longus produces a powerful flexion of the big toe which goes along with the flexion of the second and third toe at the same time due to the crossing of the tendons at the plantar chiasm. Furthermore the tendon sheath of the tibialis posterior and flexor hallucis longus support the transversal and longitudinal arches of the foot.
The deep posterior muscles play a role in both walking and standing. Their insufficiency (e.g. caused by a compression of the tibial nerve) may clinically present with two different malpositions of the foot: Due to the weakened inversion in the lower ankle joint the eversion by the peroneal muscles may outweigh (pes valgus). The pes valgus itself usually does not cause any discomfort and can be treated with insoles if necessary. Another classic foot malposition is the talipes calcaneus which – similar to the paralysis of the triceps surae muscle – evolves from the outweighing of the anterior muscles of the lower leg. Particularly observable is the solitary paralysis of the flexor hallucis longus muscle where the rolling of the big toe is immensely hindered.