Soleus Muscle: Anatomy, Origin, Insertion, Nerve and Blood Supply, Function, MRI & CT Imaging

Topic

Soleus muscle

The soleus muscle is a broad, flat muscle of the posterior compartment of the leg. Along with the gastrocnemius and plantaris, it forms the triceps surae group. Unlike the gastrocnemius, which crosses both the knee and ankle joints, the soleus acts solely on the ankle joint, producing powerful plantarflexion. It is composed predominantly of slow-twitch (type I) fibers, making it a highly fatigue-resistant postural muscle.

The soleus is essential for standing, walking, and running by maintaining upright posture and propelling the body forward. Its deep venous sinuses also contribute to the calf muscle pump, assisting venous return from the lower limb.

Synonyms

Calf soleus
Flat calf muscle
Soleus of triceps surae

Origin and Insertion

Origin:
- Posterior aspect of the head and shaft of the fibula
- Soleal line and middle third of the tibia
- Tendinous arch between tibia and fibula
Course:
- Muscle fibers run vertically downward, forming a broad belly
- Contributes to the Achilles tendon together with gastrocnemius
Insertion:
- Posterior surface of the calcaneus via the Achilles (calcaneal) tendon

Nerve Supply

Tibial nerve (S1–S2)

Arterial Supply

Posterior tibial artery
Peroneal (fibular) artery branches
Popliteal artery (muscular branches)

Venous Drainage

Posterior tibial vein
Fibular vein
Popliteal vein
Communicates with soleal venous sinuses (important for calf pump mechanism)

Relations

Anteriorly: Tibia, fibula, tibialis posterior, flexor digitorum longus, posterior tibial vessels and tibial nerve
Posteriorly: Gastrocnemius (superficial), deep fascia of leg
Medially: Flexor digitorum longus and posterior tibial neurovascular bundle
Laterally: Fibula, peroneal artery and vein, flexor hallucis longus
Inferiorly: Joins with gastrocnemius to form Achilles tendon

Function

Powerful plantarflexion of the ankle joint
Stabilizes the tibia on the calcaneus during standing
Provides postural control due to predominance of slow-twitch fibers
Assists venous return through the soleal venous sinuses (calf pump)

Clinical Significance

Common site of muscle strain in athletes
May be affected in Achilles tendon injuries or posterior compartment syndrome
Soleus venous sinuses are a frequent site of deep vein thrombosis (DVT)
Important donor muscle for flap surgery in reconstructive procedures

MRI Appearance

T1-weighted images:

Muscle shows low-to-intermediate signal intensity
Fat planes and septa appear bright
Chronic atrophy or fatty infiltration shows increased signal intensity

T2-weighted images:

Normal muscle shows low-to-intermediate signal
Strain, edema, or inflammation shows focal or diffuse bright hyperintensity

STIR (Short Tau Inversion Recovery):

Normal muscle appears low-to-intermediate signal
Pathology (strain, myositis, hematoma) appears bright hyperintense

Proton Density Fat-Saturated (PD FS):

Normal muscle shows low-to-intermediate signal
Acute strain, tears, or tendinopathy appear bright after fat suppression

T1 Fat-Sat Post-Contrast:

Normal muscle enhances mildly and homogeneously
Inflammation or neoplasm shows heterogeneous enhancement
Abscess or fluid collection shows rim enhancement with non-enhancing center

CT Appearance

Non-Contrast CT:

Muscle shows homogeneous soft tissue density
Fat planes between soleus and gastrocnemius are visible
Acute hematoma appears hyperdense, while chronic fatty change appears as low attenuation

Post-Contrast CT: