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Soleus Muscle – Origin, Nerve Supply, Functions

Soleus Muscle is a powerful muscle in the back part of the lower leg (the calf). It runs from just below the knee to the heel, and is involved in standing and walking. It is closely connected to the gastrocnemius muscle and some anatomists consider them to be a single muscle, the triceps surae. Its name is derived from the Latin word “solea”, meaning “sandal”.

Soleus muscle, a flat, broad muscle of the calf of the leg lying just beneath the gastrocnemius muscle. It arises from the upper portions of the tibia and fibula, the bones of the lower leg, and then joins with the gastrocnemius to attach via the Achilles tendon at the heel. Its major action is flexion of the ankle joint, particularly when the leg is bent at the knee, thereby extending the foot downward.

Origin and insertion

The soleus muscle arises from the soleal line on the dorsal surface of the tibia, medial border of the tibia, head of the fibula, and posterior border of the fibula. Part of the fibers arises from the tendinous arch of the soleus, which spans between the tibia and fibula and arches over the popliteal vessels and tibial nerve.
The soleus muscle runs along with the gastrocnemius muscle and together they insert onto the posterior surface of the calcaneus via the calcaneal tendon. The calcaneal tendon, commonly called the Achilles tendon, is the strongest tendon of the human body. It is easily visible and palpable at the heel.

Innervation and blood supply

The soleus is innervated by the ventral rami of S1 and S2 spinal nerves, carried by the tibial nerve into the posterior compartment of the leg
Tibial nerve, L4, L5, S1 , S2

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Blood supply

Blood supply of the soleus muscle is from peroneal artery proximally and the posterior tibial artery distally;
Muscle has a mixed-blood supply;
Vascular supply of the soleus is from popliteal, posterior tibial, & peroneal vascular pedicles to the proximal muscle, peroneal pedicles to distal lateral belly, and segmental posterior tibial pedicles to distal medial belly;
With distal pedicles from the posterior tibial artery ligated & based on proximal pedicles from the posterior tibial and peroneal arteries, a muscle can be transposed medially or laterally to cover defects in the middle third of the leg;
Proximal vasculature arises directly from the popliteal vessels and can reliably carry all but the distal 4 to 5 cm of the muscle;
Intramuscularly, the vasculature of the soleus divides into a bipenniform segmental pattern;
w/ this vascular pattern, either half of the soleus muscle can be used, leaving a functional hemisoleus muscle intact

A clinically important venous plexus is present in the soleus muscle belly. Physiologically it contributes to the muscle pump of the lower extremity. In pathological circumstances, such as thrombophilia, it constitutes a common site of onset of deep vein thrombosis (DVT). Veins follow the arteries of the same name into the popliteal vein.

Function

The action of the calf muscles, including the soleus, is plantarflexion of the foot (that is, they increase the angle between the foot and the leg).

They are powerful muscles and are vital in walking, running, and keeping balance. The soleus specifically plays an important role in maintaining standing posture; if not for its constant pull, the body would fall forward.
Also, in an upright posture, the soleus is responsible for pumping venous blood back into the heart from the periphery and is often called the skeletal-muscle pump, peripheral heart or the sural (tricipital) pump.^[rx]
Soleus muscles have a higher proportion of slow muscle fibers than many other muscles. In some animals, such as the guinea pig and cat, the soleus consists of 100% slow muscle fibers. Human soleus fiber composition is quite variable, containing between 60 and 100% slow fibers.^[rx]
The soleus is the most effective muscle for plantar flexion in a bent knee position (Hence called the first gear muscle). This is because the gastrocnemius originates on the femur, so bending the leg limits its effective tension. During regular movement (i.e., walking) the soleus is the primary muscle utilized for plantar flexion due to the slow-twitch fibers resisting fatigue.^[rx]