The soleus muscle is a large, powerful, deep muscle in the calf, vital for everyday activities such as walking and running. Some people have an extra soleus muscle in the lower calf, called an accessory soleus muscle. Sometimes this accessory muscle causes problems, including unexplained pain in the calf and ankle. So what is an accessory soleus muscle, what issues does it cause, and how do we treat it?
What is an accessory soleus muscle?
The accessory soleus muscle is an additional muscle variant next to the normal soleus. It usually sits deep in the lower part of the soleus and runs close to the inner side of the Achilles tendon, attaching to the heel bone or the Achilles tendon itself.
It’s rare, found in only 1–5% of people. Although it forms during embryonic development as a separate muscle from the main soleus, it becomes more prominent with physical activity and typically appears in the second decade of life. Men are twice as likely as women to have an accessory soleus.
Accessory soleus muscle symptoms
Most people with an accessory soleus muscle have no symptoms at all. But some present with:
- A lump or mass on the inner side of the lower leg, near the ankle. This muscle lump is sometimes mistaken for a cyst or tumour.
- Pain and swelling in the calf during activity. During exercise, the muscle can swell and press on surrounding structures such as blood vessels or nerves.
- Occasionally,y it compresses the tibial nerve in the tarsal tunnel, causing tarsal tunnel syndrome. Studies suggest pain is uncommon and worsens with running and jumping.
- Achilles tendon pain. Sometimes the accessory soleus presses on the Achilles tendon, contributing to Achilles tendonitis.
How do we diagnose an accessory soleus muscle?
We usually suspect the condition based on a lump in the lower inner part of the calf. Since most people have no pain, we reserve imaging for cases of pain or when the diagnosis is unclear.
Both ultrasound and MRI diagnose the condition and exclude other causes of a lower-calf lump. MRI, in particular, provides detailed images that distinguish normal from abnormal muscle tissue and other structures. Other causes of a lump in this area include a ganglion, a blood collection, and muscle tumours, which is why imaging matters.
Management and treatment
In painless cases, reassurance is all that’s needed — no treatment is necessary.
In cases of pain, we start with simple measures: activity modification, physiotherapy, orthotics, and oral NSAIDs. Some doctors have tried Botox injections into the muscle with some success. If conservative measures fail, surgery is considered. There’s debate about the best technique, but most surgeons prefer complete excision over less invasive procedures such as a fasciotomy. Surgery should be seen as a last resort.
Frequently asked questions about the accessory soleus muscle
Is an accessory soleus muscle dangerous?
No. It’s a benign, normal anatomical variant — not a tumour and not harmful in itself. The main issue is that the lump can resemble a tumour or cyst, which understandably causes worry. Imaging quickly confirms it’s just an extra muscle.
Could the lump be cancer or a tumour?
An accessory soleus can look like a soft-tissue mass, so it’s sensible to have a new lower-calf lump checked. Ultrasound and, especially, MRI reliably tell an accessory soleus apart from a ganglion, a blood collection, or a genuine tumour, which is exactly why we image an uncertain or painful lump.
Do I need surgery for an accessory soleus muscle?
Usually not. Most people need no treatment at all, and painful cases often settle with activity modification, physiotherapy, orthotics, and anti-inflammatories. Surgery (usually complete excision) is a last resort for pain that doesn’t respond to these measures.
Can I run or exercise with an accessory soleus muscle?
Yes, if it’s painless, many people never know they have one. If it causes pain when running or jumping, the answer is to modify training and rehab the calf, rather than stop altogether, under the guidance of a sports doctor.
How common is an accessory soleus muscle?
Rare — present in about 1–5% of people, and about twice as common in men as in women. It often becomes noticeable in the teenage years and can grow more prominent with activity.
Final word from Sport Doctor London about the accessory soleus muscle
The accessory soleus muscle is a normal extra muscle in the lower calf. It often shows up as a lump and occasionally causes pain with activity. Because other conditions — a tumour, ganglion, or Achilles tendonitis — can cause similar symptoms, it’s worth seeing a sports medicine doctor experienced in diagnosing it.
If you have a lump or pain in the lower calf, Dr Masci can assess you in London, including an ultrasound in the clinic. Contact the team here or call +44 (0) 203 488 0350.
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