The accessory soleus muscle is an anatomical variant characterized by an additional distinct muscle encountered along a normal soleus muscle. It is uncommon with a prevalence of ~3% (range 0.7-5.5%).

origin: fibula, soleal line of the tibia, or the anterior surface of the soleus muscle
insertion: calcaneus, at either the upper surface or the medial cortex
- into the Achilles tendon distally
- fleshy insertion into the upper surface of the calcaneus
- fleshy insertion into the medial cortex of the calcaneus
- tendinous insertion into the upper calcaneal surface
- tendinous insertion into the medial surface of the calcaneus
arterial supply: posterior tibial artery
innervation: tibial nerve

Gross anatomy

The accessory soleus muscle arises from the deep surface of the soleus or from the fibula and descends anteriorly to the calcaneal (Achilles) tendon. Its insertion may occur into the calcaneal tendon, the upper surface of the calcaneus, or in the medial aspect of the calcaneus.

Clinical presentation

Patients with an accessory soleus muscle can be asymptomatic and thus it will be detected incidentally during imaging performed for an unrelated reason ⁴. If symptomatic, the usual presentation is a soft mass in the posteromedial distal third of leg ⁵. This mass increases in size with activity, particularly plantar flexion ^5-7. Pain is another common symptom (two-thirds of patients) and it worsens with jumping and running ^5-7. These symptoms are likely due to chronic compartment syndrome as the accessory soleus muscle increases in size during physical activity.

Neuropathy from compression of the posterior tibial nerve has also been reported ⁸.

Radiographic features

Plain radiograph

The Kager fat pad is obliterated by a well-defined area of increased soft-tissue attenuation.

MRI

MRI is considered the most specific and sensitive test for diagnosis. The accessory soleus muscle appears as an oval or fusiform, well-defined image with its own fascia ^7-9. It additionally has normal muscle signal intensity on both T1 and T2 ^7-9.