The accessory soleus is an anomalous additional soleus muscle belly present in approximately 0.7-5.5% of individuals, running anteromedially in the lower leg to insert near the calcaneus. It presents as a soft tissue mass in the anteromedial ankle region, most prominent in full dorsiflexion when the muscle contracts and fills the pretendinous space medial to the Achilles tendon.
| Origin | Variable: anterior surface of the soleus, fibula, tibia, or flexor digitorum longus |
|---|---|
| Insertion | Superior surface of the calcaneus or the medial calcaneal tuberosity, adjacent to or merging with the Achilles tendon insertion |
| Nerve Supply | Tibial nerve (S1-S2) |
| Blood Supply | Posterior tibial artery or peroneal artery branches |
| Actions | Assists plantarflexion; may contribute to ankle stabilisation |
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The accessory soleus presents clinically as a medial ankle mass in young athletes, appearing or enlarging with exercise. MRI demonstrates the additional muscle belly with signal identical to normal skeletal muscle and confirms the diagnosis, distinguishing it from soft tissue tumour. The mass may cause tarsal tunnel syndrome from space occupation medial to the Achilles tendon. Symptomatic accessory soleus is treated by fasciotomy of the overlying fascia or by surgical excision if tarsal tunnel compression is confirmed.
A young athlete presents with a medial ankle swelling that enlarges with running and disappears at rest; MRI confirms an accessory soleus muscle belly anteromedial to the Achilles tendon with normal muscle signal intensity; symptomatic cases causing exertional pain or tarsal tunnel symptoms are treated by fasciotomy of the accessory compartment.
