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

muscle Muscle is a soft tissue, one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Muscle tissue gives skeletal muscles the ability to muscle contra ...

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 cap ...

(the calf). 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 The heel is the prominence at the posterior end of the foot. It is based on the projection of one bone, the calcaneus or heel bone, behind the articulation of the bones of the lower leg. Structure To distribute the compressive forces exerted ...

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 is located superficial to the soleus in the posterior (back) compartment of the leg. It runs from its t ...

, and some anatomists consider this combination to be a single muscle, the triceps surae. Its name is derived from the Latin word "solea", meaning "

sandal Sandals are an open type of shoe, 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 sometim ...

Structure

The soleus is located in the superficial posterior compartment of the leg. The soleus exhibits significant morphological differences across species. It is unipennate in many species. In some animals, such as the rabbit, it is fused for much of its length with the

. The soleus is a complex, multi-pennate muscle in humans, normally having a separate (posterior)

aponeurosis An aponeurosis (; : aponeuroses) is a flattened tendon by which muscle attaches to bone or fascia. Aponeuroses exhibit an ordered arrangement of collagen fibres, thus attaining high tensile strength in a particular direction while being vulnerable ...

from the gastrocnemius muscle. Most 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 (: fibulae or fibulas) 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. ...

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

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

. 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 or

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 calcane ...

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; : calcanei or calcanea) or heel bone is a bone of the Tarsus (skeleton), tarsus of the foot which constitutes the heel. In some other ...

, or heel bone. In contrast to some animals, the human soleus and gastrocnemius muscles are relatively separate, so 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 and a portion of its tendon run between the two muscles. Deep to it (further from the skin) is the transverse intermuscular septum, 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 flexor digitorum longus muscle, and the

flexor hallucis longus muscle The flexor hallucis longus muscle (FHL) attaches to the plantar surface of phalanx of the great toe and is responsible for flexing that toe. The FHL is one of the three deep muscles of the posterior compartment of the leg, the others being the ...

, 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 posteri ...

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 the posterior compartment. The soleus is superficial middle of the tibia.

Function

The action of the calf muscles, including the soleus, is plantar flexion of the foot (that is, they increase the angle between the foot and the leg). They are powerful muscles vital in walking, running, and keeping balance. The soleus 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, ''peripheral heart'' or the ''sural (tricipital) pump''. Soleus muscles have more 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 variable, containing between 60% and 100% slow fibers. The soleus is the most effective muscle for plantar flexion in a bent knee position. 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.

Innervation

The soleus is innervated by the tibial nerve (L4, L5, S1, S2)

Clinical significance

Disease

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

compartment syndrome Compartment syndrome is a serious medical condition in which increased pressure within a Fascial compartment, body compartment compromises blood flow and tissue function, potentially leading to permanent damage if not promptly treated. There are ...

. This pathology relates to tissue inflammation 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