Accessory Soleus
| Accessory Soleus | |
|---|---|
| Muscle Type | |
| Origin | Variable: often from anterior distal tibia or fibula, or from soleal line (distinct from main soleus) |
| Insertion | Calcaneus (medial side) or medial Achilles tendon |
| Action | Assists in ankle plantarflexion |
| Synergists | |
| Antagonists | Tibialis Anterior, Extensor Digitorum Longus, Extensor Hallucis Longus |
| Spinal innervation | S1, S2 |
| Peripheral Innervation | Tibial nerve (branch of same segmental origin as soleus) |
| Vasculature | Posterior tibial artery (branches), possibly peroneal artery |
The accessory soleus is an anatomical variant – an additional muscle slip in about 3-5% of individuals. It usually presents as an extra fleshy mass near the inner side of the Achilles tendon, running from the tibia or fibula to the calcaneus (or blending into the Achilles). When present, it can cause a visible bulge above the ankle. The accessory soleus can plantarflex the ankle (like the normal soleus). Often asymptomatic, it occasionally causes exertional pain or compressive symptoms.
Structure
Accessory soleus is an anomalous muscle that appears as an extra muscle in the posterior compartment. It can have a tendinous or fleshy insertion, often separate from the Achilles. It's typically on the medial side of the Achilles, in front of it.
Origin
It commonly arises from the tibia or fibula near the lower leg, or from the deep surface of the soleus muscle or its tendon. MRI studies show origins like the distal medial tibia or fibula or soleal line fascia. Essentially, it's like a duplicated part of soleus arising slightly more distal or anterior than the normal soleus origin.
Insertion
It tends to insert on the calcaneus, often on the medial aspect of the calcaneus separate from the Achilles, or sometimes via a small tendon into the Achilles or into the calcaneal periosteum. There can be multiple insertion slips. Because of insertion location, it might cause a fullness just above the heel.
Action
It would function similarly to soleus – plantarflexing the ankle. Because it's not huge, its contribution is minor. Some think it could help in prolonged plantarflexion tasks or be negligible. If large, it presumably aids calf strength a bit.
Synergists
Works with soleus and gastrocnemius in plantarflexion. It might have independent innervation but from the same tibial nerve, so likely contracts simultaneously with main soleus. It has no unique role aside from extra plantarflexion force or endurance.
Antagonists
Same as soleus – dorsiflexors (tibialis anterior, EDL, EHL). Because it's an anomaly, the usual antagonist interplay remains unchanged: dorsiflexors vs. entire plantarflexor group (with accessory soleus just being part of that group for those who have it).
Innervation
Typically by a branch of the tibial nerve (often the same branch that innervates soleus, or a separate one). Since it's often attached to soleus, it shares nerve supply segments (S1, S2).
Blood Supply
Supplied by branches of the posterior tibial artery or peroneal artery that also supply the region (maybe via the same sural arteries as soleus or a separate small branch).
Clinical Relevance
Usually asymptomatic and found incidentally (e.g., during imaging or surgery for something else) or noted as a fullness. People with accessory soleus might have a soft tissue mass on medial distal leg which can be mistaken for a tumor or cyst. MRI can differentiate it as normal muscle tissue with same intensity as other muscles.
Accessory Soleus Syndrome: Rarely, the muscle can cause pain or swelling with exercise (like a compartment syndrome but of the muscle itself, or tendon pain due to it inserting differently). Patients may present with exertional pain or a mass that becomes tense with activity. Conservative treatment often works (rest, stretching). In persistent cases, surgical excision or fasciotomy of its fascia could relieve pressure.
Differential Diagnosis: A prominent accessory soleus can mimic a soft-tissue tumor (lipoma, sarcoma) in the distal leg. Biopsy is not necessary if imaging clearly identifies it as muscle continuous with known structures. It's benign muscle.
In some reports, accessory soleus has been associated with Tarsal Tunnel Syndrome due to crowding in the medial ankle region (since it's near tibial nerve at ankle). Removing the accessory muscle can alleviate nerve compression if that occurs.
In Achilles tendon surgeries (like lengthening, or for flatfoot corrections where Achilles might be lengthened), noticing an accessory soleus is important because that slip might need to be addressed or it could resist the lengthening. Similarly, when splitting Achilles (for tendon transfer or other procedure), ensure that what might appear as an abnormal medial tendon could be accessory soleus tendon.
Strength Impact: If one has an accessory soleus, they might theoretically have a slightly stronger or more endurance in plantarflexion. However, given most individuals function fine without it, its contribution is not significant in normal circumstances. It might distribute stress differently on the calcaneus (like less load per unit area on Achilles).
Compartment Syndrome of Accessory Soleus: There are rare accounts of isolated compartment syndrome of an accessory soleus, because it might lie outside normal compartments or in its own compartment. Chronic pain in the lower calf with known accessory muscle might prompt a surgical fasciotomy or removal.
The presence of accessory soleus can be hereditary or sporadic and usually unilateral (though can be bilateral in some).
For physical exam: If suspected, asking patient to plantarflex against resistance and feeling for a distinct contraction medial to Achilles might reveal it. It will feel like an extra band of muscle or tendon there.
Typically, no targeted rehab or specific exercise needed for it— one wouldn't isolate it intentionally. If it caused issues, treat like any other muscle strain or consider surgical remedy if severe.
