Congenital variant in deep posterior compartment | Posteromedial mass that hardens on plantarflexion | MRI is diagnostic | Excise only if symptomatic and refractory
- Not a tumour: it is normal skeletal muscle on MRI (iso-intense to soleus on all sequences) with its own tendon - biopsy is almost never indicated and risks unnecessary morbidity
- Becomes prominent on contraction: the mass or fullness posteromedial to the Achilles appears or hardens when the patient performs a heel rise or resisted plantarflexion - this is pathognomonic on examination
- MRI first for the mass: any unexplained posteromedial ankle "swelling" in a young adult gets an MRI before any invasive step; it maps insertion, excludes sarcoma or ganglion, and shows whether the tibial nerve is compressed
- Most are incidental: the majority of accessory soleus muscles are asymptomatic and require only reassurance; surgery is reserved for clear mechanical pain, documented nerve entrapment, or failure of conservative care in active patients
- Simple excision is effective: when indicated, complete excision from origin to insertion via a posteromedial approach, protecting the tibial neurovascular bundle, gives reliable pain relief and return to sport with low recurrence
- “Always ask the patient to stand on tiptoes or perform resisted plantarflexion during examination - the accessory soleus will visibly and palpably contract and become firmer while a lipoma or cyst will not
- “Tarsal tunnel symptoms (medial heel/plantar paraesthesia worse at night or with activity) in a young athletic patient without trauma should prompt a search for space-occupying lesions including accessory soleus on MRI
- “If you see an incidental "mass" in the deep posterior compartment on an MRI done for other reasons, describe it fully (origin, insertion, relation to neurovascular structures) and reassure the patient and referrer - it is a variant, not a sarcoma
- “Document the presence of an accessory soleus in the operative note even if incidental during Achilles or ankle surgery - future surgeons will thank you and it explains any unusual anatomy
Clinical Imaging
MRI is the key investigation; radiographs are usually normal or show only soft-tissue fullness
On MRI the accessory soleus has identical signal intensity to normal skeletal muscle on T1, T2 and PD sequences, with a visible tendon inserting distal to the normal soleus. A mass that follows muscle signal exactly in a young patient is the accessory soleus until proven otherwise - biopsy is almost never required and can be avoided with a good history and MRI description.
The posteromedial fullness or mass hardens and may become more prominent when the patient stands on the toes or performs resisted plantarflexion. A lipoma, ganglion or sarcoma will not change with contraction. This single manoeuvre during physical examination is highly suggestive and should be documented.
The accessory soleus can occupy space in the tarsal tunnel or compress the tibial nerve or its medial calcaneal branch against the flexor retinaculum or medial calcaneus. In a young athletic patient with unilateral tarsal tunnel symptoms and no trauma, image for space-occupying lesions including this variant before attributing to idiopathic tarsal tunnel syndrome.
Many accessory soleus muscles are found incidentally on MRI performed for Achilles tendinopathy, ankle instability or other reasons. If there is no corresponding clinical mass, pain on contraction, or nerve compression, simple reassurance and description in the report is sufficient - no treatment or follow-up imaging is needed.
Memory aids
TYPESInsertion Patterns
Hook:Remember the insertion TYPES - Achilles, superior calcaneus, plantar, medial calcaneus (the painful one), and soft-tissue endings. The medial calcaneal insertion is the one examiners love because it explains tarsal tunnel in a young adult.
PAINSWhen the Mass Matters
Hook:PAINS - the five reasons an accessory soleus moves from incidental finding to surgical problem: Pain, Asymmetric contracting mass, Imaging (diagnostic), Nerve signs, and Symptoms that persist despite conservative treatment.
RESECTSafe Excision Principles
Hook:RESECT safely: protect the nerve, remove the whole thing, send histology, mobilise early, use the MRI to plan, and counsel that this is normal muscle.
Overview
The accessory soleus is a congenital anatomic variant consisting of an extra muscle belly in the deep posterior compartment of the leg. It is present in roughly 1 to 6 percent of individuals depending on the series (imaging or cadaveric) and is bilateral in a minority of cases. Most people with an accessory soleus live their entire lives without symptoms and the finding is incidental on MRI performed for unrelated foot or ankle pathology.
When symptomatic, the accessory soleus produces a characteristic posteromedial ankle mass or pain that is activity-related and often first noticed in adolescence or young adulthood during sports or military training. The key clinical clue is that the mass becomes firmer and sometimes visibly larger when the patient actively plantarflexes the ankle. Because the muscle follows normal skeletal muscle signal on every MRI sequence and has its own tendon, it is readily diagnosed without biopsy once the examiner and radiologist are familiar with the entity.
For the exam, the topic links surface anatomy, compartment anatomy, tarsal tunnel contents, MRI interpretation, and the simple principle that a symptomatic accessory muscle in a young active patient can be cured by complete excision when conservative measures have failed.
Pathophysiology
The accessory soleus arises from the anterior (deep) surface of the soleus muscle, the fibula, or the soleal line on the tibia, usually distal to the normal soleus origin. It lies anterior or anteromedial to the Achilles tendon within the deep posterior compartment and remains muscular well into the distal leg or retromalleolar region, where it tapers to a tendon that inserts separately from the Achilles.
Insertion patterns matter clinically. The tendon may join the Achilles, insert on the superior, medial, or inferior surfaces of the calcaneus, reach the plantar fascia, or end freely in the fat or on the flexor retinaculum. The medial calcaneal insertion is the pattern most often responsible for focal pain and compression of the tibial nerve or its branches within or just proximal to the tarsal tunnel.
Innervation is via a branch of the tibial nerve (separate from the branch to the soleus in most descriptions). Blood supply comes from muscular branches of the posterior tibial artery. Because the muscle is enclosed by the deep posterior compartment fascia and the flexor retinaculum distally, any increase in its volume (hypertrophy with training) or simple presence of extra bulk in a tight space produces pressure effects during activity.
Pathophysiology of symptoms:
- During plantarflexion the accessory soleus contracts and increases in cross-sectional area, raising pressure against the Achilles, the neurovascular bundle, or the calcaneal insertions.
- In the retromalleolar region or tarsal tunnel it acts as a space-occupying lesion, producing local pain, swelling sensation, or true compression neuropathy of the tibial nerve (medial heel/plantar paraesthesia, Tinel's sign, night symptoms).
- Exertional compartment-like pain can occur because the extra muscle is working inside a non-compliant fascial envelope.
- Visible or palpable mass effect is simply the contracting muscle belly itself; there is no true sheath or bursa in most cases.
The variant is entirely benign. It does not degenerate into sarcoma and does not represent a neoplastic process. Misinterpretation as a soft-tissue tumour is the most common serious error and is avoided by recognising the normal muscle signal and the contraction sign on examination.
Classification
There is no single universally used classification, but the practical scheme for surgeons and examiners is based on the distal insertion of the accessory tendon. This determines both the location of symptoms and the extent of surgical dissection required.
| Insertion Site | Anatomic Location | Typical Symptoms / Notes |
|---|---|---|
| Achilles tendon | Accessory belly merges with or inserts onto the Achilles proper | Visible bulk on contraction; less likely to cause focal nerve compression or bony pain |
| Superior calcaneus | Inserts on upper posterior calcaneus, deep to Achilles insertion (Kager's fat pad) | Posteromedial fullness and pain with running; occupies space that is normally fat only |
| Medial calcaneus (most symptomatic) | Medial calcaneal tuberosity, sustentaculum tali region or flexor retinaculum area | Posteromedial pain, tarsal tunnel syndrome, positive Tinel's; the pattern examiners associate with nerve compression |
| Inferior / plantar calcaneus or fascia | Extends to lower calcaneus or plantar aponeurosis | Arch pain, heel pain, or a mass that can be mistaken for plantar fibromatosis |
| Free / retinacular (no bony insertion) | Ends in Kager's fat or on the flexor retinaculum | Mobile, less fixed mass; can still produce pressure or be mistaken for a ganglion or lipoma |
When describing an accessory soleus on MRI or in the operating note, always state the origin, the course relative to the Achilles and neurovascular structures, and the precise insertion. This information guides both prognosis and the surgical plan if excision is required.
Clinical Presentation
Demographics and history
Patients are typically adolescents or young adults (15-40 years), often involved in running, jumping sports, or military training. The history is usually of gradual onset posteromedial ankle or distal leg "swelling" or pain noticed during or after activity. Pain is described as aching, pressure, or cramping and is worse with prolonged standing, running, or wearing tight boots. Some patients report plantar foot tingling or burning that is worse at night or after exercise - a tarsal tunnel picture. There is no history of trauma in the classic case.
Examination - look, feel, move, special tests
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Look: With the patient standing, inspect from behind for asymmetric fullness or a visible mass posteromedial to the Achilles tendon, just above or at the level of the medial malleolus. The contralateral side is usually flat in that location. The skin is normal (no erythema or sinus).
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Feel: Palpate a firm, non-fluctuant mass that is distinct from the Achilles tendon (a groove or plane may be felt between them). The mass is usually non-tender or only mildly tender. Ask the patient to perform a single-leg heel rise or resisted plantarflexion while you palpate - the mass becomes visibly larger and rock-hard. This contraction sign is the single most useful physical finding.
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Move: Ankle and subtalar range of motion are usually full. Pain may be reproduced at the extreme of passive dorsiflexion (stretch) or with resisted plantarflexion. Strength is normal unless chronic disuse or nerve compression has caused weakness.
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Special tests: Tinel's percussion over the tarsal tunnel (posterior to the medial malleolus and along the course of the tibial nerve) may elicit plantar paraesthesia if the accessory soleus is compressing the nerve. The dorsiflexion-eversion test for tarsal tunnel may be positive. Compare pulses and sensation; vascular or neurologic deficit is rare but should be documented if present.
Differential diagnosis
Lipoma (soft, does not contract), ganglion cyst (fluctuant, transilluminates, no muscle signal on MRI), Achilles tendinopathy or paratendinitis (no separate mass), sarcoma or other neoplasm (progressive night pain, heterogeneous on MRI, rapid growth), chronic exertional compartment syndrome (bilateral, no mass), and idiopathic tarsal tunnel syndrome (no space-occupying lesion on imaging).
Investigations
Plain radiographs
Usually normal. May show a non-specific soft-tissue shadow posterior to the Achilles or a small enthesophyte at the calcaneal insertion site of the accessory tendon. Not diagnostic but useful to exclude bony tumour or fracture in the work-up of ankle pain.
Ultrasound
Useful first-line or bedside tool in clinics with US available. Demonstrates a structure with normal skeletal muscle echotexture (hypoechoic with hyperechoic septa) that contracts dynamically on plantarflexion. Can map the tendon insertion and assess for compression of the tibial nerve. Operator-dependent; negative US does not exclude the diagnosis when clinical suspicion is high.
MRI (gold standard)
MRI is the investigation of choice. The accessory soleus appears as a fusiform or pennate muscle belly of identical signal intensity to the soleus and gastrocnemius on T1-weighted, T2-weighted, and proton-density sequences. There is no fat suppression dropout or enhancement after gadolinium (unlike many tumours). A tendon is visible extending from the muscle belly to its insertion, separate from the Achilles. Axial and sagittal planes best show the relationship to the neurovascular bundle and the exact insertion. MRI also excludes lipoma (high T1 fat signal), ganglion (fluid signal, thin wall), and sarcoma (heterogeneous, invasion, perilesional oedema).
Other
CT is rarely helpful except when bony detail of the insertion is needed for surgical planning. Nerve conduction studies and EMG are indicated when there is clinical or imaging evidence of tibial nerve compression; they confirm the level and severity but are not required for the diagnosis of the accessory muscle itself. Biopsy is contraindicated in the typical imaging picture - it risks sampling error, unnecessary surgery, and patient anxiety.
Management
The majority of symptomatic accessory soleus cases settle with conservative treatment. Activity modification (reduce running volume, avoid boots that compress the area), a heel-raise insert or orthosis to off-load the posterior structures, physiotherapy focused on gastroc-soleus flexibility and eccentric control, and a short course of NSAIDs are reasonable. Many young athletes can be kept active with load management while the tissues adapt. A 3- to 6-month trial is appropriate before considering surgery in most cases.
Even when surgery is ultimately required, a documented trial of conservative care strengthens the indication and sets patient expectations. Many patients adapt and return to full activity without operation.
Complications
Of the condition itself
- Chronic exertional pain limiting sport or duty.
- Missed or delayed diagnosis leading to inappropriate interventions (tarsal tunnel release alone, biopsy, or oncology referral).
- Tarsal tunnel syndrome with permanent sensory change or weakness if long-standing compression is ignored.
Of surgical excision
- Incomplete excision with recurrent mass effect or pain (prevent by removing the entire unit to the insertion).
- Iatrogenic injury to the tibial nerve or its calcaneal branches (prevent by identifying and protecting the nerve throughout).
- Wound healing problems or scar sensitivity in the posteromedial ankle (thin soft-tissue envelope).
- Deep vein thrombosis or infection (standard prophylaxis and sterile technique).
- Perceived weakness (rare; the normal soleus and gastrocnemius compensate for loss of the small accessory belly).
| Complication | Mechanism | Prevention / Management |
|---|---|---|
| Incomplete excision / recurrence | Leaving proximal origin or distal tendon attached | Preoperative MRI mapping of entire muscle-tendon unit; excise from origin to insertion |
| Tibial nerve injury | Failure to identify and protect the nerve lying adjacent to the muscle | Wide exposure, nerve stimulation if available, gentle retraction only |
| Wound breakdown or scar pain | Posteromedial skin is thin and under tension | Meticulous layered closure, avoid excessive undermining, early motion once healed |
| Misdiagnosis as neoplasm | Operating without MRI or misreading normal muscle signal as tumour | MRI before any invasive step; describe normal muscle signal and separate tendon explicitly in the report |
Clinical Relevance
The accessory soleus is a high-yield exam topic because it is common enough to appear in clinic and on imaging lists, yet unfamiliar enough that candidates who do not know it will misdiagnose the mass as a tumour or will recommend unnecessary biopsy. It tests the ability to link surface anatomy (posteromedial fullness that contracts), deep compartment anatomy (deep posterior compartment and tarsal tunnel contents), imaging interpretation (normal muscle signal on MRI), and surgical decision-making (reassure the incidental finding; excise the symptomatic refractory one completely).
In practice it appears in three common scenarios: the young athlete or recruit with a "swelling" that hurts on running, the patient with unexplained unilateral tarsal tunnel symptoms, and the incidental finding on an MRI already performed for Achilles or ankle pathology. In each setting the examiner wants to hear: recognise the contraction sign, order MRI rather than biopsy, describe the insertion, and reserve excision for clear refractory cases after conservative treatment has been tried.
Evidence
Clinical Presentation of the Accessory Soleus Muscle: A Quantitative Systematic Review
- Systematic review pooling data from 68 studies and 1,072 feet with accessory soleus
- Overall prevalence 4.4% in imaging studies and 3.8% in cadaveric series; bilateral in approximately 13%
- Most common insertion was to the medial calcaneus (approximately 40% of symptomatic cases); posteromedial pain and swelling were the presenting complaint in the majority
- Tarsal tunnel syndrome or tibial nerve compression signs were reported in roughly 10% of symptomatic patients; MRI was the diagnostic modality in over 80% of recent cases
The Accessory Soleus Muscle: A Narrative Review of the Literature
- Narrative review of 62 articles covering embryology, classification, clinical presentation and treatment
- Five main insertion patterns are described; the classification guides both the location of symptoms and the surgical approach
- Conservative treatment (activity modification, orthotics, physiotherapy) is successful in the majority of mildly symptomatic cases
- Surgical excision via a posteromedial approach, with complete removal of the muscle-tendon unit, produced good or excellent results in more than 90% of reported operated cases with low complication rates
Extrinsic Compression Neuropathy of the Tibial Nerve Secondary to Accessory Soleus Muscle in a Young Teenager
- Case report and literature summary of a 14-year-old with posteromedial ankle pain and plantar paraesthesia caused by an accessory soleus compressing the tibial nerve
- MRI clearly demonstrated the accessory muscle and its relationship to the neurovascular bundle; nerve conduction confirmed compression at the tarsal tunnel level
- Complete surgical excision of the accessory soleus produced rapid resolution of pain and sensory symptoms with no recurrence at follow-up
- The case illustrates that nerve compression can occur in adolescents and that excision is curative when the accessory muscle is the documented compressing structure
Symptomatic Accessory Soleus Muscle: A Cause for Exertional Compartment Syndrome in a Young Soldier
- Case report of a young soldier with exertional posteromedial leg pain and swelling caused by an accessory soleus within the deep posterior compartment
- MRI demonstrated the accessory muscle and ruled out other causes of exertional compartment syndrome; conservative measures failed
- Surgical excision of the accessory soleus produced complete resolution of exertional symptoms and return to full military duty
- Highlights that accessory soleus can produce a compartment-syndrome-like picture in high-demand individuals even without classic tarsal tunnel findings
Exam Viva Scenarios
Practise clinical reasoning and management decisions out loud
“A 22-year-old military recruit presents with a 4-month history of posteromedial right ankle 'swelling' and aching pain when running in boots. On examination there is a firm mass posteromedial to the Achilles that becomes visibly larger and rock-hard when he performs a heel rise. How do you approach this?”
Recognition: The history and the contraction sign (mass that hardens and enlarges on active plantarflexion) are classic for a symptomatic accessory soleus muscle. This is a congenital variant in the deep posterior compartment, not a neoplasm.
Investigation: I would obtain an MRI of the ankle (with the foot in neutral and if possible a view in plantarflexion). The accessory soleus will show identical signal intensity to normal skeletal muscle on all sequences and will have a tendon inserting separately from the Achilles, usually on the medial calcaneus. Radiographs are usually normal or show only soft-tissue fullness.
Management: Most cases settle with activity modification, a heel raise, physiotherapy and load management. I would give a 3- to 6-month trial of conservative care. If symptoms remain functionally limiting for military training or sport and the MRI confirms the diagnosis with no other pathology, I would offer complete surgical excision via a posteromedial approach, protecting the tibial nerve, removing the entire muscle-tendon unit, and sending the specimen for histology (which will show normal skeletal muscle).
“A 19-year-old footballer has 6 months of burning pain and tingling in the medial heel and plantar foot that is worse after training and at night. There is a subtle posteromedial fullness that hardens on resisted plantarflexion. Tinel's sign is positive over the tarsal tunnel. MRI shows an accessory soleus inserting on the medial calcaneus with the tibial nerve flattened against it. How do you manage?”
Diagnosis: This is tarsal tunnel syndrome secondary to extrinsic compression by an accessory soleus. The contraction sign, the age, the unilateral nature, and the MRI finding of a space-occupying muscle with normal signal make the cause clear.
Why not simple tarsal tunnel release: Releasing the flexor retinaculum alone will not remove the compressing structure. The accessory soleus must be excised completely for durable relief.
Management: I would confirm with nerve conduction studies if not already done (to document the level and severity and to have a baseline). After discussion, I would proceed to surgical excision via an extended posteromedial approach. Identify and protect the tibial nerve and its branches throughout, excise the entire accessory muscle from its origin on the soleus/fibula down to its calcaneal insertion, and send the specimen. Postoperatively, early motion and progressive loading; most patients regain full sport by 3-4 months.
Outcome expectation: In reported series, complete excision for documented compression produces reliable resolution of neuropathic symptoms with low recurrence when the whole unit is removed.
“You are reviewing the MRI of a 35-year-old runner with mid-portion Achilles tendinopathy. The report mentions an 'accessory muscle in the deep posterior compartment consistent with accessory soleus'. The patient has no posteromedial mass or pain on the opposite side of the Achilles on examination. What do you tell the patient and the referrer?”
Interpretation: This is an incidental finding of a common congenital variant. The accessory soleus is present in 1-6% of people and is usually asymptomatic. Because the patient has no corresponding clinical mass, no pain on contraction, and the MRI was performed for Achilles pathology, the variant is unrelated to the current complaint.
Communication: I would document the finding clearly in the clinic letter, describe it to the patient in plain language ("you have an extra slip of normal muscle that many people have; it is not a tumour and does not need treatment or surveillance"), and reassure both patient and referrer that no further imaging or biopsy is required. I would treat the Achilles tendinopathy on its own merits.
When it would matter: If the patient later develops posteromedial pain that is separate from the Achilles, or signs of tarsal tunnel, I would revisit the MRI and consider the accessory soleus as a possible contributor.
Imaging Atlas
Key Facts
- Congenital variant, 0.7-5.5% population, often incidental
- Posteromedial mass that hardens on resisted plantarflexion or heel rise
- MRI: normal skeletal muscle signal + separate tendon = diagnostic
- Most need only reassurance; operate for refractory pain or nerve compression
Classification (Insertion)
- To Achilles tendon - bulk without focal compression
- Superior calcaneus (Kager's) - space occupation deep to Achilles
- Medial calcaneus - commonest symptomatic, tarsal tunnel risk
- Inferior/plantar or free-ending - less common patterns
Investigations
- X-ray: normal or non-specific soft-tissue shadow
- US: dynamic confirmation of muscle that contracts
- MRI gold standard - maps origin, course, insertion, nerve relation
- NCS/EMG only if clinical nerve compression suspected
Management
- Conservative first: activity mod, heel raise, physio, 3-6 months
- Surgery: complete excision (origin to insertion) via posteromedial approach
- Protect tibial nerve and branches throughout
- Send specimen (normal skeletal muscle); early motion postop
Red Flags / Traps
- Biopsy or oncology referral before MRI (the mass follows muscle signal)
- Partial excision (recurrence of symptoms)
- Isolated tarsal tunnel release when the muscle is the compressor
- Missing the contraction sign on examination
Guidelines, Registries & Global Practice
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Epidemiology is consistent worldwide: cadaveric and imaging series from multiple continents report prevalence between 0.7% and 5.5%, with no strong evidence of major geographic or ethnic variation. Bilateral cases occur in approximately 10-15% of individuals with the variant.
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No dedicated society guidelines: because the accessory soleus is a rare cause of symptoms rather than a primary disease, major organisations (AAOS, BOA, EFORT, AOFAS) have not issued specific clinical practice guidelines. Management is guided by level IV and V evidence (case series, small comparative studies, and two recent systematic/narrative reviews).
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Diagnostic standard is MRI: across published series the shift from routine biopsy or exploration in the 1980s-1990s to MRI-first diagnosis has been universal where MRI is available. In resource-limited settings, ultrasound plus plain films plus a clear contraction sign on examination can be sufficient to avoid invasive steps.
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Surgical outcomes are reported from athletic and military cohorts: good-to-excellent pain relief and return to pre-symptom activity levels are described in greater than 85-90% of operated cases when complete excision is performed. These results come from series in Europe, North America, the Middle East and Asia; the principles (protect the tibial nerve, remove the entire unit, confirm normal muscle on histology) are the same everywhere.
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Registries: not applicable. The accessory soleus is not an implant or arthroplasty-related condition; no joint registries capture it.
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Global practice variation: the main variation is access to MRI for definitive non-invasive diagnosis and the threshold for surgery in borderline symptomatic cases. In high-performance military or elite sport settings, earlier imaging and lower threshold for excision are common; in general orthopaedic practice a longer trial of conservative care is usual. The core teaching - recognise the variant, image before you cut, excise completely when operating - remains constant.