Soleus: Origin, Insertion, Nerve, Action & The "Second Heart"

The Soleus is a broad, flat muscle located deep to the Gastrocnemius in the posterior leg. Named for its resemblance to a flatfish (Latin solea), it is the primary endurance muscle of the calf, critical for standing posture and pumping blood back to the heart.

[Image of Plantaris muscle anatomy]

Quick Anatomy Snapshot

Origin (Proximal)	Posterior head and upper 1/4 of the Fibula. Soleal Line and middle 1/3 of the medial border of the Tibia. Tendinous arch between the tibia and fibula.
Insertion (Distal)	Posterior surface of the Calcaneus (Heel bone) via the Achilles Tendon (shared with Gastrocnemius).
Nerve Supply	Tibial Nerve (S1, S2).
Blood Supply	Posterior Tibial Artery, Peroneal Artery, and Sural Arteries.
Primary Actions	Plantarflexion: Points the foot down (Primary mover when knee is bent). Postural Support: Prevents the body from falling forward at the ankle during standing. Venous Pump: Pumps blood against gravity back to the heart.

Deep Dive: The Endurance Workhorse

While the Gastrocnemius is the "sprinter" (Fast Twitch), the Soleus is the "marathoner."

1. Muscle Fiber Type

The Soleus has one of the highest concentrations of Type I (Slow Twitch) muscle fibers in the human body (up to 80%). This allows it to contract for hours without fatigue, which is essential for maintaining upright posture against gravity.

2. "The Second Heart"

The Soleus contains large venous sinuses (reservoirs of blood) within the muscle belly. When the muscle contracts during walking, it squeezes these veins, forcefully pumping blood upward back to the heart. This mechanism is crucial for preventing Deep Vein Thrombosis (DVT) and fainting.

Physio Corner: Clinical Relevance

💪 Functional Fact: When you walk with your knees slightly bent (crouch gait), or when seated, the Gastrocnemius is slack. In these positions, the Soleus becomes the sole (pun intended) generator of plantarflexion force.

Palpation

The Soleus is wider than the Gastrocnemius. You can palpate it on the medial and lateral sides of the lower calf, extending out from under the Gastrocnemius borders. Ask the patient to bend their knee to 90° and plantarflex against resistance to feel it bulge.

⚠️ Clinical Pathology: Shin Splints
Medial Tibial Stress Syndrome (Shin Splints) often involves the medial origin of the Soleus (Soleal Line) pulling on the periosteum of the tibia. This is common in runners who overpronate.

Manual Muscle Testing (MMT)

To isolate the Soleus, you must mechanically disadvantage the Gastrocnemius.

Testing Tip: Flex the knee to at least 90 degrees. This puts the Gastrocnemius in "Active Insufficiency" (too slack to work), leaving the Soleus to do the lifting.

Step-by-Step Procedure (Oxford Scale)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Gravity)	Position: Prone (face down) with knee flexed to 90°. Action: Patient points the toes up toward the ceiling (Plantarflexion). Resistance: Applied to the sole of the foot (metatarsal heads) pushing down into dorsiflexion. Grade 3: Complete range. Grade 4/5: Holds against strong pressure. (Note: For a Grade 5 functional test, the patient performs a single-leg heel raise while standing with the knee bent).
Grade 2 (Gravity Eliminated)	Position: Side-lying with knee flexed. Action: Patient points toes. Result: Full range of motion.
Grade 0, 1 (Palpation)	Action: Palpate just medial or lateral to the Achilles tendon, deep to the Gastroc. Cue: "Push your toes down."

Frequently Asked Questions

Does Soleus cross the knee?

No. It originates on the Tibia and Fibula below the knee joint. Therefore, it has no action on the knee, unlike the Gastrocnemius and Plantaris.

How do I stretch the Soleus?

You must bend your knee while dorsiflexing the ankle (e.g., putting your toe against a wall and bending the knee toward the wall). If the knee is straight, you are primarily stretching the Gastrocnemius.

What is the Triceps Surae?

It is the collective name for the three muscles that insert into the Achilles Tendon: the two heads of the Gastrocnemius + the Soleus (and sometimes Plantaris).

Test Your Knowledge: Soleus Quiz

1. Which nerve supplies the Soleus?

2. To isolate the Soleus during testing, the knee must be:

3. The Soleus is composed primarily of which muscle fiber type?

4. Where does the Soleus insert?

5. Does the Soleus cross the knee joint?

6. The "Second Heart" refers to the Soleus's role in:

7. Which bony landmark on the tibia serves as an origin for the Soleus?

8. The Soleus lies deep to which muscle?

9. What is the primary action of the Soleus?

10. Which condition involves inflammation of the medial tibial origin of the Soleus?

References

• Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2018). Clinically Oriented Anatomy. 8th ed. Philadelphia: Wolters Kluwer.
• Standring, S. (2016). Gray's Anatomy: The Anatomical Basis of Clinical Practice. 41st ed. Elsevier.
• Brukner, P., & Khan, K. (2017). Clinical Sports Medicine. 5th ed. McGraw-Hill Education.