The soleus muscle is a large and powerful deep muscle in the calf. It plays a vital role in everyday activities such as walking and running. Some people have an extra soleus muscle in the lower part of the calf called an accessory soleus muscle. Sometimes, this accessory muscle causes problems and may cause unexplained pain in the calf and ankle. So, what is an accessory soleus muscle, what issues are associated with this rare muscle, and how do we treat it? 

What is an accessory soleus muscle? 

The accessory soleus muscle is an additional muscle variant adjacent to the normal soleus. Generally, it is situated deep to the lower part of the soleus and runs close to the inside part of the Achilles tendon to attach to the heel bone or the Achilles itself. This accessory muscle is rare, found in only 1-5% of the population. Although it forms during embryogenesis as a separate muscle from the bigger soleus, it gets more prominent with physical activity and appears during the second decade of life. Men are twice as likely to have an accessory soleus than women. 

Common symptoms 

young man with accessory soleus muscle causing

Common symptoms 

Generally, most people with an accessory soleus muscle don’t have any symptoms. However, in some cases, people present to doctors with the following problems: 

  • A Lump or mass on the inside of the lower leg close to the ankle. Sometimes, this muscle lump can be mistaken for a cyst or tumour. 
  • Pain and swelling in the calf during physical activity. During exercise, this muscle can swell and put pressure on surrounding structures such as blood vessels or nerves.  Sometimes, the muscle can compress the tibial nerve in the tarsal tunnel, causing tarsal tunnel syndrome.  Studies suggest that pain occurs rarely and worsens with running and jumping. 
  • Sometimes, having an accessory soleus leads to pressure on the Achilles tendon, causing Achilles tendonitis. 

How do we diagnose accessory soleus muscle?

Generally, we suspect this condition is based on a lump on the lower inside part of the calf.  Most people with an accessory soleus don’t have pain. However, in painful cases, we perform imaging to confirm the diagnosis. 

Both ultrasound and MRI are used to diagnose and exclude other causes of a lump in the lower calf, such as Achilles tendonitis or a muscle tumour. MRI, in particular, provides detailed images that can distinguish between normal and abnormal muscle tissue and other structures. Other causes of lumps in this area include ganglions, blood collection, and muscle tumours. 

 
accessory soleus muscle on MRI

Management and Treatment 

In cases that are not painful, reassurance is all that is needed, and no treatment is necessary.

In painful cases, we start with simple treatment, such as activity modification, physiotherapy, orthotics, and oral NSAIDs. Some doctors have experimented with Botox injections in the muscle with some success.  If conservative measures fail, we often consider surgery. Although there is some debate as to the preferred surgical technique, most surgeons prefer complete excision to other less invasive procedures, such as fasciotomy. Generally, surgery should be seen as a last resort. 

Final Word from Sportdoctorlondon about Accessory Soleus Muscle 

The accessory soleus muscle is a normal extra muscle in the lower calf. It often causes a lump on the lower calf, although it sometimes causes pain with physical activity. If you think you have an accessory soleus muscle, you should see a sports medicine doctor with experience diagnosing this condition. Other conditions, such as a tumour, ganglion, and Achilles tendonitis, can cause similar symptoms. 

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