Deep posterior muscles of the leg
Grossly speaking, there are three major compartments of the leg: anterior, lateral, and posterior. The posterior group is strongly developed due to the consistent resistance to gravity while standing, walking, jumping, and so on.
For all of those actions, the muscles of the posterior compartment need to be strong and numerous. For this reason, the posterior compartment is subdivided further into superficial and deep muscle groups.
This article will discuss the anatomical properties that the deep posterior muscles of the leg share, and as well as the clinical states related to them.
Origin: Lateral femoral condyle, Posterior horn of lateral meniscus of knee joint
Innervation: Tibial nerve (L5-S2)
|Flexor hallucis longus||
Origin: (Distal 2/3 of) Posterior surface of fibula, Interosseous membrane
Innervation: Tibial nerve (S2, S3)
|Flexor digitorum longus||
Origin: Posterior surface of tibia, (inferior to soleal line)
Innervation: Tibial nerve (S1, S2)
Origin: Posterior surface of tibia, Posterior surface of fibula, Interosseous membrane
Innervation: Tibial nerve (L4, L5)
The deep muscles are separated from the superficial layer by a deep layer of fascia. The posterior muscles are natural antagonists to the anterior muscle group. Generally, their main functions are plantarflexion, inversion of the foot, and flexion of the toes. Additionally, the natural tension of these muscles, especially the tibialis anterior, supports the medial arch of the foot. Namely, the deep flexor muscles of the leg are:
- Popliteus muscle
- Flexor hallucis longus muscle
- Flexor digitorum longus muscle
- Tibialis posterior muscle
It is common that muscles of a certain area will share innervation and vascularization. In this case, all muscles of this compartment are innervated by the tibial nerve, which is a branch of the sciatic nerve. When it comes to the vascularization, all of them except for the popliteus muscle, are supplied by the posterior tibial artery, which is a branch of the popliteal artery. A small exception here is the popliteus muscle, which is vascularized directly from the popliteal artery (a branch of the femoral artery).
The popliteus muscle is the smallest of all four muscles in the deep compartment. It is also the most superior to all others, with its origin in the lateral condyle of the femur, in addition to the posterior horn of the lateral meniscus of the knee joint. The muscle extends downwards and medially over the posterior aspect of the tibia and inserts just above the soleal line on the posterior side of the tibia.
The muscle forms the floor of the popliteal fossa and its tendon separates the lateral meniscus from the lateral ligament of the knee joint. This is the only muscle of the deep posterior compartment that acts exclusively in the knee joint.The popliteus muscle is essential for the processes of walking, standing up, and sitting down. During the closed chain gait cycle (with the foot in contact with the ground), it unlocks the knees by laterally rotating the femur on the tibia. On the other hand, during open chain movements (where the foot is not in contact with the ground), the muscle medially rotates the tibia on the femur.
Flexor Hallucis Longus Muscle
This muscle originates from the distal two-thirds of the posterior surface of the fibula and adjacent portion of the interosseous membrane.
The muscle fibers extend through the posterior compartment of the leg and converge to form a solid tendon that passes behind the distal end of the tibia just before the ankle. The tendon then passes through a groove on the posterior surface of the talus, continues through the groove of the inferior surface of the sustentaculum tali, to finally reach its insertion on the plantar surface of the base of the distal phalanx of the hallux. The contraction of this muscle causes plantarflexion of the foot, as well as the flexion of the hallux. It also assits in inversion of the foot.
Flexor Digitorum Longus Muscle
The flexor digitorum muscle has its origin on the medial side of the posterior surface of the tibia, inferior to the soleal line. The muscle fibers descend through the medial portion of the posterior compartment of the leg.
The tendon of the muscle arises just above the ankle and enters the plantar portion of the foot by passing through the groove on the medial malleolus of the tibia. After this, the tendon deviates laterally, reaches the medial portion of the foot, and divides into four terminal tendons which insert on the plantar surfaces of the bases of the distal phalanges of the lateral four toes. The flexor digitorum longus performs flexion of the toes II to IV.
Tibialis Posterior Muscle
This muscle originates from the posterior surfaces of the interosseous membrane and adjacent portions of the tibia and fibula.
The fibers of the muscle descend through the posterior compartment of the leg, mostly covered with the flexor digitorum muscle laterally and the flexor hallucis longus muscle medially. The tendon of the muscle passes through the groove on the medial malleolus under the tendon of the flexor digitorum longus muscle. Then, it extends anteriorly through the medial plantar side of the foot and finally inserts onto the plantar surfaces of the navicular and cuneiform bones, the bases of the second through fourth metatarsal bones, and variably the cuboid bone. The tibialis posterior causes inversion and plantarflexion of the foot. It also provides support for the medial longitudinal arch of the foot.
The most common injury seen in this compartment are strains. Due to a sudden exaggerated effort put upon the muscles, they can suffer a straining injury. This condition is usually transitory and the affected muscle can be back to normal within a few days.
Since fascia separates the deep and superficial groups of the posterior muscles, this region is prone to compartment syndrome. It is a condition in which the muscle tissue is inflamed and the process of inflammation compromises the function of the blood vessels that supply the muscle. Because of that, the plasma and proteins enter the extracellular space and cause the muscle to swell. Since the space is strictly limited by the thick fascia, the swelling will compress the sensory neurons and cause sensation of pain and discomfort.
It is not uncommon that roots of the sciatic nerve become compressed by spinal vertebrae and cause paralysis distal to the place of the compression. Since the tibial nerve is a branch of the sciatic nerve, the compression typically causes decreased function in the affected muscles accompanied with pain. The insufficiency of the muscles may clinically present in two ways:
- Due to inability to invert the foot at the subtalar joint, the predominance of the fibular muscles may cause an outward deviation of the foot at the subtalar joint, which is a condition called pes valgus.
- If the weakened ability of plantarflexion is dominant, the resulting condition is referred to as the talipes calcaneus, and is clinically presented as a dorsiflexion of the foot with the toes pointing upwards.
Also, damage of the tibialis posterior tendon initiates a condition called flat foot which is caused by the failure of the medial longitudinal arch of the foot since it is normally supported by the tibialis posterior muscle.