In humans and some other mammals, the soleus is a powerful

muscle Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscl ...

in the back part of the lower

leg A leg is a weight-bearing and locomotive anatomical structure, usually having a columnar shape. During locomotion, legs function as "extensible struts". The combination of movements at all joints can be modeled as a single, linear element ca ...

(the

calf Calf most often refers to: * Calf (animal), the young of domestic cattle. * Calf (leg), in humans (and other primates), the back portion of the lower leg Calf or calves may also refer to: Biology and animal byproducts * Veal, meat from calves * ...

). It runs from just below the

knee In humans and other primates, the knee joins the thigh with the leg and consists of two joints: one between the femur and tibia (tibiofemoral joint), and one between the femur and patella (patellofemoral joint). It is the largest joint in the hu ...

to the heel, and is involved in standing and walking. It is closely connected to the

gastrocnemius muscle The gastrocnemius muscle (plural ''gastrocnemii'') is a superficial two-headed muscle that is in the back part of the lower leg of humans. It runs from its two heads just above the knee to the heel, a three joint muscle (knee, ankle and subtala ...

and some anatomists consider them to be a single muscle, the

triceps surae The triceps surae consists of two muscles located at the calf – the two-headed gastrocnemius and the soleus. These muscles both insert into the calcaneus, the bone of the heel of the human foot, and form the major part of the muscle of the p ...

. Its name is derived from the Latin word "solea", meaning "

sandal Sandals are an open type of footwear, consisting of a sole held to the wearer's foot by straps going over the instep and around the ankle. Sandals can also have a heel. While the distinction between sandals and other types of footwear can ...

Structure

The soleus is located in the superficial

posterior compartment of the leg The posterior compartment of the leg is one of the fascial compartments of the leg and is divided further into deep and superficial compartments. Structure Muscles Superficial posterior compartment Deep posterior compartment Blood ...

. The soleus exhibits significant morphological differences across species. It is

unipennate Muscle architecture is the physical arrangement of muscle fibers at the macroscopic level that determines a muscle’s mechanical function. There are several different muscle architecture types including: parallel, pennate and hydrostats. Force pr ...

in many species. In some animals, such as the rabbit, it is fused for much of its length with the

. In humans, the soleus is a complex, multi-pennate muscle, usually having a separate (posterior)

aponeurosis An aponeurosis (; plural: ''aponeuroses'') is a type or a variant of the deep fascia, in the form of a sheet of pearly-white fibrous tissue that attaches sheet-like muscles needing a wide area of attachment. Their primary function is to join musc ...

from the gastrocnemius muscle. A majority of soleus muscle fibers originate from each side of the anterior aponeurosis, attached to the tibia and fibula. Other fibers originate from the posterior (back) surfaces of the head of the

fibula The fibula or calf bone is a leg bone on the lateral side of the tibia, to which it is connected above and below. It is the smaller of the two bones and, in proportion to its length, the most slender of all the long bones. Its upper extremity i ...

and its upper quarter, as well as the middle third of the medial border of the

tibia The tibia (; ), also known as the shinbone or shankbone, is the larger, stronger, and anterior (frontal) of the two bones in the leg below the knee in vertebrates (the other being the fibula, behind and to the outside of the tibia); it connects ...

. The fibers originating from the anterior surface of the anterior aponeurosis insert onto the median septum and the fibers originating from the posterior surface of the anterior aponeurosis insert onto the posterior aponeurosis. The posterior aponeurosis and median septum join in the lower quarter of the muscle and then join with the anterior aponeuroses of the gastrocnemius muscles to form the

calcaneal tendon The Achilles tendon or heel cord, also known as the calcaneal tendon, is a tendon at the back of the lower leg, and is the thickest in the human body. It serves to attach the plantaris, gastrocnemius (calf) and soleus muscles to the calcaneus (h ...

Achilles tendon The Achilles tendon or heel cord, also known as the calcaneal tendon, is a tendon at the back of the lower leg, and is the thickest in the human body. It serves to attach the plantaris, gastrocnemius (calf) and soleus muscles to the calcaneus ( ...

and inserts onto the posterior surface of the

calcaneus In humans and many other primates, the calcaneus (; from the Latin ''calcaneus'' or ''calcaneum'', meaning heel) or heel bone is a bone of the tarsus of the foot which constitutes the heel. In some other animals, it is the point of the hock. S ...

, or heel bone. In contrast to some animals, the human soleus and gastrocnemius muscles are relatively separate, such that shear can be detected between the soleus and gastrocnemius aponeuroses. The Soleus is vestigial in the horse.

Relations

The gastrocnemius muscle is superficial to (closer to the skin than) the soleus, which lies below the gastrocnemius. The

plantaris muscle The plantaris is one of the superficial muscles of the superficial posterior compartment of the leg, one of the fascial compartments of the leg. It is composed of a thin muscle belly and a long thin tendon. While not as thick as the achilles te ...

and a portion of its tendon run between the two muscles. Deep to it (farther from the skin) is the

transverse intermuscular septum The deep transverse fascia or transverse intermuscular septum of leg is a transversely placed, intermuscular septum, from the deep fascia, between the superficial and deep muscles of the back of the leg. At the sides it is connected to the margi ...

, which separates the superficial posterior compartment of the leg from the deep posterior compartment. On the other side of the fascia are the

tibialis posterior muscle The tibialis posterior muscle is the most central of all the leg muscles, and is located in the deep posterior compartment of the leg. It is the key stabilizing muscle of the lower leg. Structure The tibialis posterior muscle originates on th ...

, the

flexor digitorum longus muscle The flexor digitorum longus muscle is situated on the tibial side of the leg. At its origin it is thin and pointed, but it gradually increases in size as it descends. It serves to flex the second, third, fourth, and fifth toes. Structure The fle ...

, and the

flexor hallucis longus muscle The flexor hallucis longus muscle (FHL) is one of the three deep muscles of the posterior compartment of the leg that attaches to the plantar surface of the distal phalanx of the great toe. The other deep muscles are the flexor digitorum longus an ...

, along with the

posterior tibial artery The posterior tibial artery of the lower limb is an artery that carries blood to the posterior compartment of the leg and plantar surface of the foot. It branches from the popliteal artery via the tibial-fibular trunk. Structure The poster ...

and

posterior tibial vein The posterior tibial veins are veins of the leg in humans. They drain the posterior compartment of the leg and the plantar surface of the foot to the popliteal vein. Structure The posterior tibial veins receive blood from the medial and lat ...

and the

tibial nerve The tibial nerve is a branch of the sciatic nerve. The tibial nerve passes through the popliteal fossa to pass below the arch of soleus. Structure Popliteal fossa The tibial nerve is the larger terminal branch of the sciatic nerve with root val ...

. Since the anterior compartment of the leg is lateral to the tibia, the bulge of muscle medial to the tibia on the anterior side is actually the posterior compartment. The soleus is superficial middle of the tibia.

Function

The action of the calf muscles, including the soleus, is

plantarflexion Motion, the process of movement, is described using specific anatomical terms. Motion includes movement of organs, joints, limbs, and specific sections of the body. The terminology used describes this motion according to its direction relativ ...

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 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 The skeletal-muscle pump is a collection of skeletal muscles that aid the heart in the circulation of blood. It is especially important in increasing venous return to the heart, but may also play a role in arterial blood flow. General The skele ...

, ''peripheral heart'' or the ''sural (tricipital) pump''. Soleus muscles have a higher proportion of slow muscle fibers than many other muscles. In some animals, such as the guinea pig and cat, soleus consists of 100% slow muscle fibers. Human soleus fiber composition is quite variable, containing between 60 and 100% slow fibers. The soleus is the most effective muscle for plantarflexion 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 plantarflexion due to the slowtwitch fibers resisting fatigue.

Innervation

The Soleus is Innervated by the Tibial Nerve (L4, L5, S1, S2)

Clinical significance

Disease

Due to the thick fascia covering the muscles of the leg, they are prone to

compartment syndrome Compartment syndrome is a condition in which increased pressure within one of the body's anatomical compartments results in insufficient blood supply to tissue within that space. There are two main types: acute and chronic. Compartments of the ...

. This pathology relates to the inflammation of tissue affecting blood flow and compressing nerves. If left untreated compartment syndrome can lead to atrophy of muscles, blood clots, and neuropathy.

Additional images

References

* Gray, Henry. Pick, T. Pickering, & Howden, Robert (Eds.) (1995). ''Gray's Anatomy'' (15th ed.). New York: Barnes & Noble Books. * Saladin, Kenneth S. Anatomy & Physiology: The Unity of Form and Function. 6th ed. New York, NY: McGraw-Hill, 2007. Print.

External links

* * A. Agur. Architecture of the human soleus muscle, three-dimensional computer modelling of cadaveric muscle and ultrasonographic documentation in vivo. University of Toronto (PhD Thesis). ((https://tspace.library.utoronto.ca/bitstream/1807/16553/1/NQ59030.pdf))
Soleus Muscle - Your Second Heart
{{Authority control Calf muscles Muscles of the lower limb