In humans and some other mammals, the soleus is a powerful muscle in the back
part of the lower leg. 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".
Structure The soleus is located in the superficial
posterior compartment of the leg. Not all mammals have a soleus muscle; one
familiar species that lacks the soleus is the dog. Soleus is vestigial in the
horse. 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 gastrocnemius muscle. In the human, soleus is a complex
multi-pennate muscle, usually having a separate aponeurosis 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 surfaces of the head of the fibula and
its upper quarter, as well as the middle third of the medial border of the tibia.
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 and inserts onto the posterior
surface of the calcaneus, 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.
= Relations= The gastrocnemius muscle is superficial
to the soleus, which lies below the gastrocnemius.
The plantaris muscle and a portion of its tendon run between the two muscles.
Deep to it 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, along with the posterior tibial artery and posterior
tibial vein and the tibial nerve. 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
of the foot. They are powerful muscles and are vital in walking, running, and
dancing. 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, peripheral heart or the sural
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. This is because the
gastrocnemius originates at on the femur, so bending the leg limits its
effective tension. During regular movement the soleus is the primary
muscle utilized for plantarflexion due to the slowtwitch fibers resisting
fatigue. Clinical significance
= Disease= Due to the thick fascia covering the
muscles of the leg, they are prone to compartmentalization. This pathology
relates to the inflammation of tissue effecting blood flow and compressing
nerves. If left untreated compartmentalization can lead to atrophy
of muscles, blood clots, and neuropathy. Additional images
References Gray, Henry. Pick, T. Pickering, &
Howden, Robert. Gray's Anatomy. 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 Anatomy photo:15:st-0414 at the SUNY
Downstate Medical Center A. Agur. Architecture of the human
soleus muscle, three-dimensional computer modelling of cadaveric muscle
and ultrasonographic documentation in vivo. University of Toronto.)