Soleus
Musculus soleus
Also known as: triceps surae, peripheral heart, second heart
A broad, flat, powerful plantarflexor lying deep to gastrocnemius. It is mostly slow (type I) fibres, keeps us upright against forward sway, and is the main pump of venous return from the leg.
Overview
Soleus (solea, sole, from its flat fish-like shape) arises from the back of the head and upper quarter of the fibula, the soleal line and medial border of the tibia, and a tendinous arch between them. The popliteal vessels and tibial nerve pass under this arch into the deep compartment. Its multipennate fibres insert, via the calcaneal tendon, on the calcaneus. It crosses only the ankle, so it plantarflexes regardless of knee position.
Soleus contains a very high proportion of type I fibres (≈70–80%). It is continuously active in standing, preventing the body from tipping forward over the ankles. It is the most important part of the calf muscle pump. The large, thin-walled soleal venous sinuses within it are emptied by contraction, and the deep vein valves direct blood upward. This is why soleus is called the "peripheral heart". It is also why prolonged immobility allows stasis and deep vein thrombosis to begin there.
Anatomy
- Origin
Posterior aspect of the head and upper quarter of the fibula, the soleal line and medial border of the tibia, and the tendinous arch between them.
- Insertion
Posterior surface of the calcaneus via the calcaneal tendon.
- Action
Plantarflexion of the ankle; steadies the leg on the foot in standing.
- Innervation
Tibial nerve.
- Nerve roots
S1–S2
- Arterial supply
Posterior tibial, fibular and sural arteries.
- Relations
Deep to gastrocnemius and plantaris. Its tendinous arch transmits the popliteal (then posterior tibial) vessels and the tibial nerve. The deep transverse fascia separates it from the deep compartment.
Function
- Plantarflexes the ankle (independent of knee position)
- Maintains upright posture (anti-gravity muscle)
- Pumps venous blood out of the leg (calf muscle pump)
Clinical significance
Deep vein thrombosis
Stasis in the soleal sinuses during immobility (surgery, long flights) is a key factor in calf DVT. Early mobilisation, calf exercises and compression stockings use the soleal pump to prevent it.
Accessory soleus
An anatomical variant that presents as a soft-tissue mass medial to the calcaneal tendon, sometimes with exertional pain. It can be mistaken for a tumour.
Compiled from standard anatomy and physiology references — see the Muscular System sources. For education only; not medical advice.