The soleus muscle is a wide flat leg muscle found on the posterior leg. It runs from just below the knee to the heel and lays immediately deep to the gastrocnemius. These two muscles, along with the plantaris muscle, belong to the group of superficial posterior compartment calf muscles. Soleus’ contraction results in strong plantar flexion. It also allows us to maintain an upright posture due to its important role as an antigravity muscle.
Together with the gastrocnemius, they form the three-headed group of muscles referred to as the triceps surae. They both insert on the calcaneus via the calcaneal tendon and act in many basic activities, such as walking, running, and leaping. The size and shape of the triceps surae muscle bellies determine the interindividual differences of human calves’ appearance - from slim to rather robust. This article will outline the morphology of the soleus muscle, as well as its functional and clinical anatomy.
|Origin||Soleal line, medial border of tibia, head of fibula, posterior border of fibula|
|Insertion||Posterior surface of calcaneus (via calcaneal tendon)|
|Innervation||Tibial nerve (S1, S2)|
|Vascularization||Posterior tibial artery and vein|
|Function||Talocrural joint: Foot plantar flexion|
|Clinical relations||Soleus tear, soleus pain|
Stand on your Soles. Gas explodes!
(refers to the functions of posterior leg muscles; Soleis is for posture, while Gastrocnemius is for explosive movements)
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 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.
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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. Blood supply is provided by two main branches. The superior branch arises from the popliteal artery while the inferior branch arises from peroneal artery (fibular artery) or the posterior tibial artery. The peroneal and posterior tibial arteries are direct branches of the popliteal artery and arise in the popliteal fossa. Minor accessory arteries may also branch off the peroneal, posterior tibial and lateral sural arteries.
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.
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The soleus’ function is closely related to that of the gastrocnemius muscle. Together, they constitute a chief plantar flexor - their contraction results in the plantar flexion of the upper ankle joint, enabling the lifting of the heel against gravity when walking or jumping. The soleus muscle belongs to the group of so-called antigravity muscles (along with the extensors of the leg, gluteus maximus and back muscles), which maintain the upright posture in humans.
As the body's center of gravity is anterior to the ankle joint, the body has a natural tendency to lean forward. This is counteracted by a continuous state of plantar flexion produced mainly by the soleus when standing. For this reason, the soleus consists mostly of type 1 slow, fatigue-resistant fibers. The gastrocnemius muscle is more involved in locomotion where it contributes slightly to the flexion of the knee as it crosses multiple joints. When the knee is flexed, plantar flexion attributes of the gastrocnemius are very limited, making the soleus the main muscle responsible for plantar flexion.
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Torn calf muscles are common injuries seen in primary care and sports medicine clinics. A soleus tear often results from overuse. Injuries occur typically in runners due to fatigue and overtraining. Athletes specialized in long-distance and endurance running or walking, which require constant soleus action, are at higher risk of injury. The symptoms include:
- Lateral calf pain on either side of the soleus,
- Weakness in the calf and weak plantar flexion, especially with flexed knee
- Exercise-related dull calf pain and pain related to foot dorsiflexion or calcaneal tendon pressure
Treatment includes the RICE method: Rest, Icing, Compression, and Elevation. A soleus tear needs to be differentiated from gastrocnemius tear, which is usually a sudden injury caused by quick movements, such as jumping and sprinting. Thanks to its etiology, it is commonly called the tennis leg.
Soleus pain might be a symptom of a soleus tear or overuse. Affected patients typically report pain onset in the beginning of the exercise, which decreases with time. Once the training is over, the pain usually returns. The muscle pain typically develops in three common areas: calcaneal tendon and muscle belly transition, lateral midsection area, or top of the muscle, which may radiate the pain all the way up into the sacroiliac area. Establishing the diagnosis of a soleus injury might be challenging. Along with clinical symptoms and physical examination, MRI or ultrasound are usually used to determine pathologies within the muscle.
Sudden calf pain may also indicate deep vein thrombosis (DVT), which is a potentially life-threatening medical emergency due to the risk of pulmonary embolism (PE). Besides calf pain, swelling and redness in the calf area are usual symptoms of DVT. Treatment involves anticoagulation with pharmacotherapeutic agents such as heparin, warfarin, and factor Xa inhibitors. Compression stockings are used as a preventative strategy.