Accessory Muscles of the Ankle

What Works

Tier 1 — Strong Evidence: nothing qualifies NO DATA

There are zero randomised trials, zero controlled comparisons and zero natural-history studies of an untreated symptomatic accessory muscle anywhere in this literature. This slot is left empty rather than filled by promoting a case series into it.

Checked rather than assumed: across the four named variants, 194 papers exist and exactly one carries a randomised-trial label. That one is a prevalence study of 100 pain-free ankles, mislabelled in the database.

Tier 2 — Correct attribution, before any treatment MODERATE

The highest-graded action here is a decision, not an intervention: confirm that the variant's anatomical territory actually matches where the symptoms are. If it does not, it is incidental and the diagnosis is still open.

Four independent lines of pooled data converge on this. A peroneus quartus sits in 17% of pain-free volunteer ankles, a flexor digitorum accessorius longus in 6% of pain-free scans, some variant in 12.5% of dissected limbs, and peroneal tendon problems are not associated with having an accessory peroneal muscle at all.

What would change this: a study recording variants blind to symptoms and symptoms blind to the scan, in the same 500 people.

Tier 3 — Emerging: what people actually try, and how thin the evidence is

Activity modification and load management. EMERGING No study prescribes or measures it. Symptoms are exertional in 60% of cases, so reducing the provoking load reduces the provocation. That is reasoning, not proof.

Footwear and external pressure review. EMERGING A superficial lump under a boot line or strap is a pressure problem with an obvious remedy, untested.

Conservative management with imaging follow-up. EMERGING, n=1 One 21-year-old soldier, managed conservatively, whose swelling around the muscle and whose symptoms resolved together on repeat scanning. It is the only documented case of this settling without surgery, which makes one patient disproportionately important.

Botulinum toxin injection. EMERGING, n=5 Five patients, all of whom returned to sport, with no side effects. No control group, no blinding, outcomes judged by the treating team. This is the strongest treatment evidence in the entire topic.

Surgery: decompression, excision, fasciotomy or tendon release. EMERGING Case series only. One published patient had symptoms return two years after a lesser procedure, settling only once the muscle itself was removed.

What Doesn't Work

Treating the variant because it is the most interesting thing on the report. The base rate makes this near-inevitable, and it is an error of reasoning rather than technique.
Accepting a normal static scan as exclusion. The mechanism is positional, so a scan taken with the foot in neutral tests the wrong condition.
Reading the case literature as a treatment ranking. Every published operation was a success. So is every published operation in any topic assembled from case reports.
Any exercise programme aimed at the extra muscle. No evidence, and no mechanism either.
Biopsy before scanning. Documented to have removed normal muscle from two people.

Cinematic study of ankle tendon and muscle anatomy under dramatic lighting

Exercise Prescription

There is no exercise programme for this, and that is a finding rather than an omission

Not one study in the reviewed evidence prescribes, measures or tests any exercise, stretch, mobility drill or hands-on treatment for an extra ankle muscle. Any list of exercises attached to this condition would be invented.

There is no mechanism for one either. An extra muscle is a space problem, and no exercise makes a muscle take up less room.

What the evidence does support is measuring and monitoring. That is what replaces the exercise table.

Your monitoring plan

What to record	How	How often
Size of the lump	Measure across its widest point, same spot each time, and write the number down	Monthly, or sooner if it feels different
Whether it softens	Feel it with the foot relaxed, then point the toes hard and feel it again	Each time you measure
What brings the pain on	Note the activity, and how long into it the pain starts	Whenever it happens
Numbness or pins and needles	Note where in the foot, and whether it is spreading	Whenever it happens

Take the written record to appointments. "It feels about the same" is not answerable. A number from three months ago is.

Red Flags

An ankle lump is a lump until proven otherwise, and in this topic the mistake runs in both directions.

A lump that is hard when your foot is relaxed and stays hard when you point your toes. Muscle contracts and relaxes. Something that does not is not behaving like muscle.
A lump that is getting bigger. No extra muscle appears or grows in adulthood.
Pain at night, constant pain, or pain that no longer settles with rest.
Numbness, pins and needles or weakness that is spreading or getting worse.
Feeling generally unwell, or losing weight without trying.
Anyone proposing to cut into it before it has been scanned. Two people in the published literature had a completely normal muscle removed because the scan came after the operation instead of before it.

Refer to: orthopaedics or a sarcoma service for any hard, enlarging or non-positional lump, with imaging first and before biopsy. Vascular assessment if the foot is pale, cold or pulseless.

Cinematic anatomical study of the medial ankle region

Return to Training

Symptom-free at the previous training load for 2 consecutive weeks.
No numbness, pins and needles or burning in the foot, at rest or under load.
Recorded lump measurement unchanged from baseline.
Full pain-free ankle range of motion, including end-range toe-pointing.
Normal strength on resisted testing, symmetrical with the other side.
Red flag screen negative at reassessment.

Across the caseload: sedentary people rarely become symptomatic at all, because the exertional mechanism needs exertion, so for them the presentation is a lump and the question is what it is. Runners and lifters are the group most likely to reach a surgical decision, because the provoking load is the thing they are least willing to reduce.

Feel the lump with your foot relaxed, then point your toes hard and feel it again. If it goes firm, that is muscle. If it stays hard, book an appointment this week.

A lump that changes consistency when a muscle contracts is behaving like a muscle. Nothing else on the differential list does that.

Takes less than a minute. No equipment needed.

The Verdict

An extra ankle muscle is usually harmless. Get a lump checked if it stays hard when you relax.

Think of the inside of your ankle as a cable duct running behind the ankle bone, carrying nerves and tendons through a fixed opening with a strap across it. Some people are born with an extra muscle threaded into that duct, and for most of them nothing ever happens, because the duct still has room. It only becomes a problem when the muscle swells during exercise, the way any muscle does, and the duct cannot widen to match. That is why it hurts when you run and not when you sit, and why nothing you did in training caused it.

Here is what is really happening: about one ankle in eight has an extra muscle in it, and almost nobody with one ever finds out.
What most people get wrong: finding one on a scan is not the same as finding the cause, because 17% of people with completely pain-free ankles have one too.
Start here: feel whether the lump softens when your foot is relaxed, and write down how big it is, so that next time someone can answer whether it has changed.

Best for

People who have been told a scan found an extra muscle, and want to know whether it actually explains their symptoms.

Skip if

Your lump is hard at rest, growing, or painful at night. That needs assessment now, not reading.

Want the full evidence? Keep scrolling

Conviction

LOW overall LOW, scored per claim, because the evidence quality is wildly uneven across this topic.

Claim	Conviction
Accessory ankle muscles are present in roughly 12.5% of ankles	MODERATE-HIGH
The overwhelming majority are without symptoms and incidental	MODERATE-HIGH
They can cause tarsal tunnel syndrome, most often one specific variant	MODERATE
Peroneal tendon problems are not associated with an accessory peroneal muscle	MODERATE
A lump that hardens on toe-pointing indicates a symptomatic accessory soleus	LOW
Any treatment outperforms any other, or outperforms doing nothing	NO DATA
Any exercise, loading or hands-on therapy helps	NO DATA

What would change my mind: the attribution claim

A prospective study of at least 500 consecutive adults having an ankle scan for any reason, in which every extra muscle is recorded blind to the clinical picture and the symptoms are recorded blind to the scan. That would give, for the first time, the proportion of carriers who are symptomatic in their own variant's territory. If that proportion came out above roughly 15% for the flexor digitorum accessorius longus, the "usually incidental" framing here would need softening for that variant specifically.

What would change my mind: the treatment claim

A randomised trial of at least 60 people with a symptomatic, scan-confirmed accessory soleus, comparing botulinum toxin against a saline injection, with pain-free treadmill running time at 12 weeks as the primary outcome. A null result would be as valuable as a positive one, because it would put the first real number on a topic that currently reports 100% success for everything anyone has tried.

Go Deeper

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The Full Picture — Anatomy, Diagnosis & Evidence

What's Actually Going On

An accessory muscle is an extra muscle belly you are born with, sitting in a compartment that was not sized for it. You have it or you do not. It does not develop, and training does not create it.

Space occupation. The tarsal tunnel, behind the inner ankle bone, is roofed by a fixed band and cannot expand. One variant matters more than the others here because it travels alongside the nerve bundle rather than near it, so its resting position is already against the nerve.

Dynamic compression. The muscle contracts. A belly that is tolerable at rest is not tolerable at end range. In one published case the nerve was only visibly compressed once the foot was pointed, and that finding determined the operation.

Exertional swelling. Muscle swells under load. That is why symptoms are exertional in 60% of cases, and why an examination at rest can be completely normal.

Cinematic cross-sectional study of the tarsal tunnel region

How to Identify It

The one characteristic sign is a lump that is soft at rest and firms on active toe-pointing, reported in 44% of symptomatic accessory soleus cases. Sn: NO DATA | Sp: NO DATA That 44% is a frequency among people who already had the diagnosis, not a measure of how good the sign is at finding it.

A tap over the nerve behind the inner ankle bone reproduces symptoms in 44.6% of cases where the variant has caused nerve compression. Sn: NO DATA | Sp: NO DATA

No physical examination test for this condition has a published sensitivity or specificity anywhere in the retrieved literature. Any number offered elsewhere for these tests has been invented. That is why both rows above read NO DATA rather than carrying a plausible-looking figure.

The decisive step is not a test at all. Does the variant's anatomical territory match where the symptoms are? Numbness in the sole maps to one nerve, the top of the foot to another, the outer ankle to the tendons. If the territory and the symptom do not overlap, the variant is incidental.

Cinematic study of the posteromedial ankle in examination position

The Debate

Does an accessory peroneal muscle cause lateral ankle problems?

Clinical convention

An accessory peroneal muscle on imaging contributes to peroneal tendon problems and lateral ankle symptoms.

vs

Yammine 2015, meta-analysis, 46 studies, 3,928 legs

Peroneal tendon pathology is not associated with the presence of an accessory peroneal muscle. It appears in 5.5% of peroneal tendon surgeries against a 10.2% population prevalence.

Follow the pooled data. The variant is found less often in operated patients than in the general population, and 17% of pain-free volunteer ankles have one.

Does surgery work?

Published case literature, 128 treated cases

"Regardless of the treatment modality, all surgical patients were satisfied."

vs

What that number is made of

The corpus is 41 case reports. Nobody writes up an operation that failed, so there is no count of the patients who did not improve.

Neither figure is an efficacy claim. A 100% success rate assembled from case reports measures publication practice, not the operation.

Is peroneus tertius an accessory muscle?

Radiology convention, 2023

"The most frequently observed accessory muscle around the ankle is the peroneus tertius muscle."

vs

Yammine 2017, meta-analysis, 7,601 legs

It is present in 93.2% of adult limbs, and absent in about 7%. A structure present in nine of ten people is normal, and its absence is the variant.

A collision of vocabulary rather than of fact. Both usages are internally consistent, and neither supports treating peroneus tertius as a compression cause. It contributes nothing to this literature at all.

Honest Limitations

1. The symptomatic literature has no denominator, by design

The main review explicitly excluded incidental findings. That is correct methodology for describing how a condition presents, and it makes the review structurally incapable of answering the only question a clinician has, which is what proportion of people carrying the variant ever become symptomatic. The prevalence research and the clinical research are two bodies of work that never touch, and the ratio between them has to be assembled by hand from both.

2. Every treatment number comes from patients who were written up

There is no registry, no cohort and no trial. Publication bias here is not a partial distortion of the dataset. It is the entire dataset.

3. The positional finding does not survive the standard imaging pathway

Routine ankle scans are taken static and in neutral, which tests the wrong condition for a problem whose whole mechanism is what happens at end range. In the published cases, plain X-ray was the most-used investigation at 62.7%, and X-ray cannot image muscle at all.

The Nuance

Surgery versus conservative management: nobody knows, because the comparison has never been made.

Conservative success rate: NO DATA. Not low, not "unknown but probably poor". There is no cohort and no trial. The evidence contains exactly one conservatively managed patient with imaging follow-up, and he resolved. Conservative management was used in 36.7% of published treated cases, and no outcome was reported for that group.

Surgical success rate: reported as 100%, and that figure should not be used.

What the evidence does support is narrower and more useful than either number. Most people who have one of these muscles never know, and finding one on a scan is not the same as finding the answer. The bigger clinical risk in this topic is not under-treating a compression. It is confidently attributing a symptom to an anatomical variant that ten percent of the pain-free population also has, and stopping the search there. The second risk is the mirror image of the first, and it put two people with entirely normal muscles on an operating table before anyone thought to scan them.

Cinematic anatomical study contrasting soft tissue structures of the ankle

Sources

Yammine K, et al. (2022). Surgical and Radiologic Anatomy. PMID 35353216. Systematic review, 25 studies, 39 patients, 47 ankles. The entire published literature of accessory-muscle tarsal tunnel syndrome.
Yammine K, et al. (2025). Surgical and Radiologic Anatomy. PMID 41350425. Systematic review, 134 muscles in 122 patients. Presentation proportions and treatment distribution.
Yammine K (2015). Surgical and Radiologic Anatomy. PMID 25638531. Meta-analysis, 46 studies, 3,928 legs. Peroneal tendon pathology not associated with an accessory peroneal muscle.
Yammine K, Erić M (2017). BioMed Research International. PMID 28596965. Meta-analysis, 35 studies, 7,601 legs. Fibularis tertius present in 93.2% of adult cadaveric limbs.
Hohenberger GM, et al. (2026). Annals of Anatomy. PMID 42066867. Cadaveric study, 200 lower extremities. Accessory muscles present in 12.5%.
Saupe N, et al. (2007). Radiology. PMID 17255421. Prospective MRI, 65 asymptomatic volunteers. Peroneus quartus in 17% of pain-free ankles.
Cheung YY, et al. (1999). Skeletal Radiology. PMID 10231910. Prevalence series, 100 asymptomatic individuals. Flexor digitorum accessorius longus in 6%.
Hur MS, et al. (2014). Clinical Anatomy. PMID 24535960. Cadaveric dissection, 80 limbs. Three-type classification, all able to compress the nerve.
Isner-Horobeti ME, et al. (2016). Scandinavian Journal of Medicine & Science in Sports. PMID 26627136. Uncontrolled case series, N=5. Strongest treatment evidence in the topic.
Doda N, et al. (2006). British Journal of Radiology. PMID 16980668. Case report. The only conservatively managed patient with imaging-confirmed resolution.
Paul MA, et al. (1991). Acta Orthopaedica Scandinavica. PMID 1767661. Case report, 2 patients biopsied before the diagnosis was considered.
Tomažin T, et al. (2023). Seminars in Musculoskeletal Radiology. PMID 37011621. Imaging review of anatomical variants around the ankle.