Soleus Anatomy

Soleus is the broad, flat, multipennate muscle lying deep to the gastrocnemius in the superficial posterior compartment of the leg. Together the two form the triceps surae, converging on the tendo-Achilles. The single anatomical fact that governs everything about soleus is that it is monoarticular - it crosses the ankle only, and never the knee. Gastrocnemius crosses both.

It is a postural muscle. Standing quietly, the ground reaction force passes anterior to the ankle axis and creates a continuous dorsiflexion moment; soleus is the muscle that resists it. In walking it is the principal controller of tibial advance over the fixed foot through mid-stance, and it generates the majority of the plantarflexion power at push-off. It is also a pump: the venous sinusoids within its substance are squeezed with every contraction, and this is the dominant mechanism of venous return from the lower limb.

Primary action

• Ankle plantarflexion, and by virtue of its insertion medial to the subtalar axis after the Achilles spiral, a contribution to hindfoot inversion when the hindfoot is neutral or varus. Critically, when the hindfoot is already in valgus the Achilles vector passes lateral to the subtalar axis and the triceps surae becomes an evertor - a deforming force in the collapsing flatfoot.
• It does not act on the knee. Gastrocnemius flexes the knee; soleus does not.

Gastrocnemius versus soleus

Role in gait

• Loading response: relatively quiet; tibialis anterior is controlling the foot.
• Mid-stance: soleus becomes the dominant muscle in the limb. It contracts eccentrically to control the forward advance of the tibia over the fixed foot. Without it the tibia collapses forward into dorsiflexion and the patient adopts a crouch gait with a flexed knee - the exact deformity produced by an over-lengthened Achilles tendon.
• Terminal stance and pre-swing: it contracts concentrically, generating the largest single burst of positive power in the gait cycle and driving push-off.
• Swing: silent.

Force and architecture

• The triceps surae generates by far the largest plantarflexion torque of any muscle group about the ankle. Cadaveric measurement of relative muscle strength below the knee (Silver, de la Garza and Rang, 1985) found the plantarflexors of the ankle to be six times as strong as the dorsiflexors. Soleus contributes the majority of the static and endurance component.
• Its short fibres and huge physiological cross-sectional area mean high force but low excursion - the architecture of a postural strut.
• Type I fibre predominance (commonly quoted around 70 to 90 per cent in soleus, versus roughly half in gastrocnemius) makes it fatigue-resistant and explains its role in quiet standing.

The calf muscle pump

• Each soleus contraction compresses the intramuscular venous sinuses and propels blood proximally; competent valves in the deep and perforating veins prevent reflux.
• Ejection fraction of the calf pump in a normal limb is substantial, and the pump is the principal mechanism opposing the hydrostatic column in the upright human.
• Pump failure - from immobility, muscle weakness, ankle stiffness (which prevents the pump cycling) or valvular incompetence - produces venous hypertension, oedema, lipodermatosclerosis and venous ulceration in the gaiter area.
• Clinical translation: a stiff ankle is a failed calf pump. This is one of the underappreciated arguments for restoring ankle motion after trauma, and the reason ankle pump exercises are prescribed from day one after any lower-limb surgery.

Palpation and inspection

• Soleus is palpable as the broad muscle mass either side of the Achilles in the distal third of the calf, extending further distally than gastrocnemius. Its medial and lateral borders emerge from beneath the gastrocnemius bellies roughly at the junction of the middle and distal thirds of the leg.
• The gastrocnemius bellies are the prominent bulges in the upper calf; the soleus is what you feel below and lateral to them.
• Asymmetry in calf circumference measured 10 cm below the tibial tuberosity is a crude but useful record of triceps surae wasting.

Named tests and signs

Getting the Silfverskiold test right

1. Patient supine, knee fully extended.
2. Lock the subtalar joint in neutral or slight inversion by grasping the heel - this is the step most commonly omitted, and without it the midfoot dorsiflexes and gives a falsely generous reading.
3. Dorsiflex the ankle with a gentle, sustained force applied along the lateral border of the foot, not the forefoot, and record the angle.
4. Flex the knee to 90 degrees and repeat.
5. Interpretation: a marked increase in dorsiflexion with the knee flexed (commonly taken as an increase of 10 degrees or more, or a change from restricted to more than 10 degrees of dorsiflexion) indicates an isolated gastrocnemius contracture. No change means a combined gastrocsoleus contracture or a bony block.

Related pages. The other half of the triceps surae, and the test that separates them: Gastrocnemius Anatomy and Gastrocnemius Equinus Contracture, with Toe Walking and Cerebral Palsy Gait and SEMLS for the population Silfverskiold was actually writing about — and Cavovarus Foot where the equinus must be corrected before any transfer. The tendon: Achilles Tendon Rupture and Achilles Tendinopathy. What the soleal arch traps: Popliteal Artery Entrapment Syndrome, its commonest mimic Chronic Exertional Compartment Syndrome, and Tarsal Tunnel Syndrome for the tibial nerve further down. The flap and its indication: Tibial Shaft Fractures, Open Fracture Management and Compartment Syndrome of the Leg. The venous sinuses: DVT Diagnosis and Treatment and Venous Thromboembolism.

Popliteal artery entrapment syndrome

• Presentation: a young, athletic patient - classically a runner or footballer with well-developed calves - with exercise-induced calf claudication, sometimes paraesthesia or coldness, relieved by rest. It is frequently misdiagnosed as chronic exertional compartment syndrome or medial tibial stress syndrome.
• Anatomical classification (types I to IV, with type V for venous involvement and a separate functional type). The types are conventionally credited to Love and Whelan, whose 1965 paper named the syndrome and described the variants; the numbering, type V and the functional category are later additions by subsequent authors, so do not attribute a specific type number to the 1965 citation.
  • Type I: the popliteal artery courses medial to a normally positioned medial head of gastrocnemius.
  • Type II: the medial head of gastrocnemius attaches abnormally laterally, displacing the artery medially.
  • Type III: an accessory slip of gastrocnemius entraps the artery.
  • Type IV: the artery lies deep to popliteus or a fibrous band.
  • Type V: any of the above with popliteal vein involvement as well.
  • Functional (type VI in some schemes): no anatomical anomaly - entrapment results from muscular hypertrophy, most often of the soleal arch or the medial gastrocnemius, compressing a normally positioned artery during exercise.
• The soleus link: the soleal arch is the specific structure implicated in many functional cases. A hypertrophied, fibrous arch compresses the artery in plantarflexion and in forced dorsiflexion.
• Investigation: duplex ultrasound with provocative manoeuvres, magnetic resonance angiography or CT angiography in neutral and in provoked positions, and ankle-brachial index before and after exercise. Beware the base rate: up to 50 per cent of normal limbs show transient popliteal compression at the extremes of plantarflexion or dorsiflexion (Levien 1999). Provoked compression is therefore close to a normal finding, and the diagnosis rests on provoked compression plus discrete, reproducible, exertional symptoms - never on the image alone.
• Consequences of delay: repeated compression causes intimal damage, post-stenotic aneurysm, thrombosis and distal embolisation - a limb-threatening progression in a young patient.
• Treatment, and it differs by type. For anatomical entrapment (types I to IV), operate at diagnosis rather than observing: in Levien's series 15 of 58 such limbs had already occluded by presentation. Release is through a posterior (S-shaped) or medial approach, dividing the offending muscle slip or the soleal arch. For functional entrapment, decompress only where the symptoms are discrete and typical - given the 50 per cent base rate of provoked compression in normals, a liberal indication here operates on people who do not have the disease. Where the artery is already degenerate or occluded, replace the segment with vein rather than patching it, because the histological damage extends beyond the visibly abnormal wall.

Tibial nerve entrapment at the soleal arch

• A proximal tibial neuropathy, far rarer and far more often missed than distal tarsal tunnel syndrome.
• Symptoms: deep calf pain worse with exercise, plantar paraesthesia, and weakness of the deep posterior compartment - tibialis posterior, flexor digitorum longus and flexor hallucis longus - producing weak inversion and weak toe flexion with a normal ankle jerk pattern and preserved gastrocnemius and soleus function if the branches to them arise proximal to the arch.
• Key discriminator from tarsal tunnel syndrome: in a soleal arch entrapment the Tinel sign is positive high in the proximal calf, not behind the medial malleolus, and the calf pain is prominent.
• Treatment: decompression by division of the soleal arch, taking great care of the popliteal vessels immediately deep to it.

Achilles pathology involving soleus

• Mid-substance Achilles tendinopathy and rupture occur in the hypovascular zone 2 to 6 cm above the insertion, corresponding to the region of the Achilles spiral where soleal and gastrocnemius fibres cross.
• Isolated soleus strain is a recognised but underdiagnosed injury in distance runners, presenting as diffuse deep calf pain that is worse with knee-flexed calf raises (soleus loaded) rather than the more acute, localised, knee-extended pain of a medial gastrocnemius tear (tennis leg).
• Accessory soleus muscle is an anatomical variant presenting as a soft-tissue mass in the posteromedial ankle, sometimes causing exertional pain or even a localised compartment syndrome; it is distinguished from a tumour by its muscle signal on MRI.

Deep vein thrombosis and the soleal sinuses

• The valveless soleal sinusoids are the commonest origin of calf deep vein thrombosis, and this is why calf DVT is the starting point of most propagating lower-limb thrombosis after immobilisation, surgery or long-haul travel.
• Prophylaxis logic: intermittent pneumatic compression, graduated compression stockings, early mobilisation and active ankle pumping all work by re-engaging the calf pump and emptying the sinuses.
• Clinical caveat: calf tenderness and swelling are unreliable; the diagnosis needs duplex ultrasound, guided by a pre-test probability score and D-dimer where appropriate.

Chronic venous insufficiency

• Failure of the calf pump - from valvular incompetence, muscle weakness or a stiff ankle - produces ambulatory venous hypertension, gaiter-area lipodermatosclerosis, haemosiderin staining and venous ulceration.
• Orthopaedic relevance: ankle stiffness after a pilon or plafond fracture, prolonged immobilisation, and post-thrombotic syndrome all degrade the pump. Restoring ankle motion and encouraging active plantarflexion is part of the vascular as well as the musculoskeletal rehabilitation.

Superficial posterior compartment syndrome

• Less common than the anterior compartment, but recognised, and the soleus is the bulk of the compartment. Signs are pain on passive ankle dorsiflexion, a tense calf and sural nerve paraesthesia.
• Do not forget the deep posterior compartment, which sits beneath the soleus behind the deep transverse fascia and is the compartment most frequently missed at fasciotomy. Its late sequel is claw toes with plantar sensory loss.

The soleus flap - the workhorse of the middle third

Gastrocnemius recession versus tendo-Achilles lengthening

• The decision rule is simple and absolute: a positive Silfverskiold test means gastrocnemius recession. A negative test - no improvement with the knee flexed - means the soleus is also tight and a tendo-Achilles lengthening is required. Performing a tendo-Achilles lengthening for an isolated gastrocnemius contracture is the classic error that produces a crouch gait.
• Sural nerve safety in a Strayer recession: the sural nerve and short saphenous vein lie in the posterior midline superficial to the gastrocnemius aponeurosis, passing from between the gastrocnemius heads proximally to the lateral side distally. Use a posteromedial longitudinal incision, identify the plane bluntly, and visualise the lateral edge of the aponeurosis before cutting across it.

Other surgical contexts

• Posteromedial approach to the tibia: the interval passes between the soleus (retracted posteriorly) and the deep flexors. Elevating soleus from the posteromedial tibial border gives access to the posterior tibial cortex for plating or for bone grafting a nonunion.
• Posteromedial fasciotomy: the incision lies 1 to 2 cm posterior to the posteromedial tibial border. The superficial posterior compartment is opened first, then the deep transverse fascia must be deliberately divided along the posterior tibial border to open the deep posterior compartment. Skipping this second step is the classic incomplete fasciotomy.
• Achilles surgery: in chronic rupture with a gap, a gastrocnemius turn-down (Bosworth) or V-Y lengthening uses the proximal aponeurosis; the soleus contributes muscle bulk that can be advanced but is not a graft source.
• Bone grafting and nonunion: the posteromedial soleus can be mobilised to provide a vascularised bed over a grafted middle-third tibial nonunion, and this is often as valuable as the graft itself.

Anatomical variation across populations

• The soleal arch varies from a thin, membranous band to a thick, tough tendinous structure. A thick fibrous arch in a hypertrophied calf is the substrate for functional popliteal artery entrapment, which is why the condition clusters in athletic populations and in military recruits.
• Accessory soleus is a described variant, presenting as a posteromedial ankle mass, occasionally with exertional pain. Reported prevalence in cadaveric and imaging series is low but not negligible, and it is more often recognised now because of the volume of ankle MRI performed.
• Plantaris - the third muscle of the superficial compartment - is absent in a reported 7 to 20 per cent of limbs depending on the series and population. Its long slender tendon is a recognised autograft donor for hand and foot reconstruction where it is present.
• The relative contribution of gastrocnemius and soleus to Achilles fibre orientation varies, and the degree of Achilles spiral is not constant.

Side-by-side guidance

Resource-dependent practice

• Well-resourced settings: microsurgical free tissue transfer is available for any defect, provoked magnetic resonance angiography is used for entrapment, and instrumented gait analysis informs cerebral palsy surgery.
• Limited-resource settings: the soleus flap is one of the most valuable operations in the trauma surgeon repertoire, because it requires no microsurgery, no implants and no special equipment, and delivers definitive vascularised cover to an open tibial fracture. Its anatomical territory and its limitations must be known precisely. The Silfverskiold test costs nothing and prevents the single most damaging error in equinus surgery.
• Popliteal artery entrapment is under-diagnosed globally because the workup requires provoked imaging; a high index of suspicion in the young athlete with exercise-limited calf pain is the practical substitute.

Registry and outcome signals

• National open fracture audits consistently associate early definitive soft tissue cover with lower deep infection rates and better union, supporting the orthoplastic pathway irrespective of whether the cover is local or free.
• Cerebral palsy outcome series consistently report that isolated tendo-Achilles lengthening in ambulant children is associated with progression to crouch gait, and this observation has shifted practice worldwide toward selective gastrocnemius recession and multilevel surgery.