Deep posterior muscles of the leg
When studying the muscles of the leg, we can examine them by four primary groupings, which are the anterior, fibular/lateral, superficial posterior and deep posterior compartments. These are defined by intermuscular septa and surrounded by the deep fascia of the leg.
The deep posterior muscles of the leg are part of what is commonly known as the 'calf' musculature. They are located deep to the triceps surae.
Anatomy and supplyThese deep muscles of the posterior compartment of the leg are innervated by the tibial nerve (L4-S2), and are individually described below.
Tibialis posterior muscle
The tibialis posterior originates from the posterior surfaces of the the tibia and fibula, as well as the interosseous membrane. It 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 to the cuboid bone, and another more proximal attachment to the tip and distal margin of the sustentaculum tali.
Flexor digitorum longus muscle
The FDL arises from the medioposterior surface of the tibia and descends inferiorly through the leg towards the ankle joing.
After passing inferiorly around the medial malleolus of the tibia, it continues towards the navicular bone of the foot, wrapping underneath it.
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
The FHL has its origin on the posterior surface of the fibula and interosseous membrane. It continues inferiorly where it passes posterior to the distal end of the tibia and travels through a groove located on the posterior aspect of the talus bone. From here, it crosses onto the calcaneus bone, and passes through another groove, located under the sustentaculum tali, or talar shelf of calcaneus.
The tendon then continues along the length of the plantar aspect, or the sole of the foot, before it inserts into the base of the distal phalanx of the great toe
Interrelations of the deep posterior muscles
The tendons of the deep posterior muscles all run posterior to the distal end of the tibia. More specifically, the tendons of the tibialis posteior and flexor digitorum longus run around the medial malleolus of the tibia (medial malleolar sulcus), with the former lying anterior to the latter. The tendon of the flexor hallucis longus passes around the posterior distal end of the tibia a little more laterally to the other two tendons. All three tendons lie within separate tendon compartments which are formed by the deep layer of the flexor retinaculum of the foot.
The tendon of the flexor digitorum longus muscle crosses the tendon of the tibialis posterior muscle proximal to the medial malleolus of the tibia (crural chiasm), and the tendon of the flexor hallucis longus muscle at the sole of the foot (plantar chiasm).
All deep posterior muscles are involved in the plantar flexion of the ankle joint and the inversion (supination) in the subtalar joint.
- Additionally, the flexor digitorum longus is involved in flexion of the second to fifth toes, occuring at the metatarsophalangeal and interphalangeal joints of the same digits.
- The flexor hallucis longus produces a powerful flexion of the big toe, also occuring occuring at the metatarsophalangeal and interphalangeal joints. This usually occurs in tandem with flexion of the second and third toe at the same time due to the crossing of the tendons at the plantar chiasm.
- Furthermore the tibialis posterior also plays an important role in supporting the transversal and longitudinal arches of the foot.
The deep posterior muscles play an imporant role in both walking (gait) 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 weakened ability to invert the foot at the subtalar joint, the opposing eversion caused by the fibular/peroneal muscles may result in an outward deviation of the foot at the subtalar joint (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 is a condition resulting from a weakened ability of the muscles involved in plantarflexion of the foot. This causes the opposing dorsiflexion forces created by the anterior muscles of the leg to become dominant, with the toes are pointing upwards and the foot resting on the heel.
Another condition resulting from amage to the tendon of the tibialis posteior muscle is 'flat foot' or 'pes planus', which is caused by failure of the medial longitudinal arch of the foot. This is often treated by means of corrective insoles.