Quadratus Plantae Anatomy

Quadratus plantae, also called flexor accessorius, is a flat quadrilateral muscle of the second plantar layer. That is the layer of the long flexor tendons, flexor digitorum longus and flexor hallucis longus, and of the muscles that hang off them: quadratus plantae behind and the lumbricals in front.

Its unique insertion. It arises by two heads from the plantar surface of the calcaneus and inserts into the tendon of another muscle, the posterolateral margin of flexor digitorum longus. No other muscle in the body inserts solely into another muscle's tendon in this way.

What it is for. It has no bony distal attachment, no attachment to any toe and no independent action on any joint. It exists to redirect the line of pull of flexor digitorum longus and to give the long flexor a second, ankle-independent motor, an anatomical correction for a design compromise in the way the long flexor enters the foot. How far that account has actually been measured is taken up under Action and Biomechanics.

Why the Muscle Exists

This is the concept the whole page hangs on, and the one an examiner will ask.

The oblique pull. Flexor digitorum longus arises in the deep posterior compartment of the leg and enters the foot behind the medial malleolus, curving forward around the sustentaculum tali to reach the sole. It therefore approaches the lesser toes from postero-medial, running obliquely across the sole from medial-posterior to lateral-anterior. Unaided, its contraction would flex the lesser toes with a component of medial deviation, curling them toward the hallux rather than flexing them straight down.

The correction. Quadratus plantae arises from the calcaneus laterally and posteriorly and inserts on the lateral margin of the long flexor tendon, so its pull is posterolateral. Contracting synchronously with the long flexor, it rotates the resultant vector laterally into alignment with the axes of the toes, its principal and defining action. Flexor digitorum longus supplies the force; quadratus plantae supplies the direction. Without it, toe flexion becomes toe curling with adduction.

Ankle independence. Flexor digitorum longus is a two-joint, in fact multi-joint, muscle crossing the ankle. As the ankle plantarflexes in terminal stance its muscle-tendon unit shortens passively, moving down its length-tension curve and losing active force, at precisely the point in the gait cycle where toe purchase matters most. Quadratus plantae arises from the calcaneus, distal to the ankle, so it is unaffected by ankle position and can continue to load the long flexor tendon.

Hence flexor accessorius. The old name describes exactly that: an accessory motor for the long flexor tendon, allowing effective toe purchase in terminal stance while the ankle is plantarflexing. It is the answer to give when asked why a muscle would insert into another muscle's tendon, because it converts a leg muscle into a foot muscle for the part of the cycle when the leg muscle is least effective.

During the windlass. A subtler role: as the metatarsophalangeal joints dorsiflex in terminal stance and the plantar aponeurosis winds around the metatarsal heads, quadratus plantae helps keep the long flexor tendon aligned in its distal course rather than bowstringing medially.

At the joints. It has no direct action on any joint. It assists flexion of the lesser toes at the metatarsophalangeal, proximal interphalangeal and distal interphalangeal joints only through the flexor digitorum longus tendon, and it contributes indirectly to the intrinsic sling supporting the longitudinal arch by keeping the long flexor efficient throughout terminal stance.

A rope-tensioner. The hand solves the oblique-pull problem for flexor digitorum profundus largely through the fibrous digital sheaths and the lumbricals. The foot adds a dedicated proximal vector-correcting muscle, its answer to the lumbricals of the hand plus a rerouting pulley. Where a synergist would work in parallel on the same bone, quadratus plantae is a rope-tensioner working on the rope itself.

How settled this is. The vector-correction account is derived from the muscle's geometry and from its old name; it has never been measured directly. Sooriakumaran's review asks whether the muscle is simply an accessory flexor that brings the line of pull into the long axis, as its name would suggest, and answers that the electromyographic evidence points elsewhere: quadratus plantae behaves as a primary toe flexor, preferentially recruited ahead of flexor digitorum longus in voluntary flexion, and it appears to resist extension of the toes during stance, which stabilises the foot.

Two anatomical facts cut the same way. The lateral head is absent in about a third of feet (31 of 100 in Pretterklieber's series) and the whole muscle is single-headed in 34 per cent, which is hard to reconcile with an indispensable vector-correcting role. In vivo, the two heads act on different toes, the medial on the second toe alone and the lateral on all four. Teach the vector explanation as the anatomical rationale it is, and say plainly that the recorded behaviour is that of an accessory flexor and a stance-phase stabiliser.

When It Fails

Lesser Toe Balance and Why the Foot Claws

The lesser toe is governed by a four-way balance, and quadratus plantae is the vector regulator within it.

• Extensor digitorum longus extends the metatarsophalangeal joint through the extensor sling.
• Flexor digitorum longus flexes the distal interphalangeal joint and, with quadratus plantae correcting its direction, contributes to axial flexion at all three joints.
• Flexor digitorum brevis flexes the proximal interphalangeal joint.
• Interossei and lumbricals flex the metatarsophalangeal joint, and are the only intrinsic counterweight to the long extensor, because in the foot, unlike the hand, they have no extensor expansion into the dorsal hood.

How the balance fails. Loss of the intrinsics gives a claw toe: metatarsophalangeal hyperextension from unopposed extensor digitorum longus, with flexion at both interphalangeal joints from the unopposed long and short flexors. Loss of the vector correction superimposes medial deviation and curling, which is part of why the neuropathic foot shows adducted, overriding lesser toes rather than simple sagittal clawing.

Why the foot ulcerates. Once the metatarsophalangeal joint hyperextends, the plantar fat pad migrates distally and the metatarsal head becomes prominent plantarward. The neuropathic foot then ulcerates under the metatarsal head and at the toe tip.

Synergists and Antagonists

Not palpable. Quadratus plantae lies in the calcaneal compartment, deep to flexor digitorum brevis, the plantar aponeurosis and the heel pad, and cannot be felt. What can be localised is the interval it forms with abductor hallucis. Deep tenderness there, with the ankle in neutral and in the absence of tenderness on the tuberosity itself, is the physical sign of Baxter nerve entrapment.

Beside the medial head. The medial head origin lies immediately plantar to the sustentaculum tali, with the flexor hallucis longus tendon in its groove above it. Pain and crepitus on hallux motion behind the medial malleolus localise to that tendon rather than to the muscle.

There is no isolated test. With no bony insertion and no independent joint action, no manoeuvre can isolate the muscle: nothing changes at any joint when quadratus plantae alone contracts, only the direction in which the long flexor moves the toes. Saying that clearly is a better answer than inventing a test. What can be assessed is the consequence of losing it, the direction of lesser toe flexion, and the conditions in which it is implicated.

Imaging

• Radiographs are of limited direct use for the muscle, but are needed for the calcaneus, hindfoot alignment and cavus assessment.
• MRI is the investigation that shows the muscle. Look for fatty atrophy of quadratus plantae in chronic proximal tibial or lateral plantar nerve lesions, denervation oedema in acute lesions, and selective fatty atrophy of abductor digiti minimi in Baxter neuropathy. It also shows accessory muscles occupying the calcaneal compartment and any space-occupying lesion in the tarsal tunnel.
• Ultrasound can demonstrate the interval between abductor hallucis and quadratus plantae and guide a diagnostic injection into it, a useful confirmatory step in suspected Baxter neuropathy.
• Electrodiagnostics may support a proximal tibial or lateral plantar nerve lesion, but they are technically difficult for short distal motor branches and a negative study does not exclude the diagnosis.

Related pages. The tendon it inserts into, and its neighbours in the sole: Flexor Digitorum Longus Anatomy — the muscle that supplies the force this one is said to redirect — with Flexor Digitorum Brevis Anatomy, Flexor Hallucis Longus Anatomy, Lumbrical Muscles, Plantar Interossei Anatomy and Dorsal Interossei. The nerve that crosses its surface: Baxter Neuropathy — the first branch of the lateral plantar nerve runs between abductor hallucis and quadratus plantae — plus Tarsal Tunnel Syndrome, Tibial Nerve Anatomy and Plantar Fasciitis Management, the diagnosis Baxter's patients had usually been given first. The compartment it occupies: Foot Compartment Syndrome and Calcaneal Fractures — late clawing may be the signature of a missed calcaneal compartment. The deformities attributed to intrinsic failure: Lesser Toe Deformities, Claw Toes, Hammer Toes, Curly Toes — the only randomised evidence on this page is about that last one — and Metatarsalgia. Where the intrinsics are lost: Diabetic Foot and Diabetic Foot Ulcers, Charcot-Marie-Tooth Disease, Cavovarus Foot and Nerve Injury and Regeneration. The hand comparison the analogy rests on: Intrinsic-Minus and Intrinsic-Plus Hand.

Baxter Neuropathy: Quadratus Plantae as the Compressing Wall

The first compression point. The first branch of the lateral plantar nerve (inferior calcaneal nerve, Baxter nerve) arises from the lateral plantar nerve within about 1 cm of the tibial nerve bifurcation. Its descending, vertical segment runs between the deep fascia of abductor hallucis medially and the medial caudal margin of quadratus plantae laterally, the first and commonest compression point.

The second compression point. The nerve then turns laterally, running superficial to quadratus plantae and along the deep surface of flexor digitorum brevis, immediately anterior to the medial calcaneal tuberosity, to reach abductor digiti minimi. Here a heel spur or a thickened plantar fascial origin can compress it.

What it supplies. It supplies the periosteum of the medial calcaneal tuberosity, the long plantar ligament, twigs to flexor digitorum brevis, and motor supply to abductor digiti minimi. Its cutaneous contribution is negligible, which is why the neuropathy produces no sensory loss.

Presentation. Chronic plantar heel pain, maximal just distal and medial to the tuberosity rather than on it, is often burning and sometimes radiates laterally across the heel; there is sometimes wasting or weakness of abductor digiti minimi. It accounts for a meaningful minority of chronic recalcitrant plantar heel pain, commonly quoted as up to around a fifth.

The surgical point. A decompression that releases only the abductor hallucis deep fascia leaves the lateral wall of the tunnel, the quadratus plantae fascial margin, intact and the nerve still tethered. Both walls must be released.

Lesser Toe Deformity and Flexor Surgery

Quadratus plantae is not usually the cause of a lesser toe deformity, but it is central to understanding the tendon balance and to executing the operations.

Curly toe. In childhood this is a flexion, adduction and varus rotation deformity of a lesser toe, most often the third, fourth or fifth, attributed to a tight or abnormally inserting long and short flexor complex. It illustrates the adducting effect of an uncorrected oblique long-flexor pull. The recognised treatment is open flexor tenotomy of both the long and short flexors; Hamer's double-blind randomised trial is conventionally cited as showing that it performs as well as the more extensive flexor-to-extensor transfer, a result quoted from convention rather than from an indexed abstract.

Flexible claw and hammer toe. These are treated by a flexor-to-extensor (Girdlestone-Taylor) transfer of flexor digitorum longus. The technique, and the one point in it that involves quadratus plantae, are under Surgical Relevance.

Diabetic claw toe with an apical ulcer. A simple percutaneous flexor tenotomy treats it very effectively, offloading the toe tip and letting the ulcer heal without any bony surgery.

Fixed hammer or claw toe. This needs a proximal interphalangeal resection arthroplasty or arthrodesis, with metatarsophalangeal release and sometimes a shortening metatarsal osteotomy.

The Intrinsic-Minus Foot

Charcot-Marie-Tooth disease. Hereditary motor and sensory neuropathy is the classic setting: intrinsic wasting, claw toes, a plantarflexed first ray, forefoot-driven cavus and hindfoot varus. Quadratus plantae is denervated with the other lateral plantar nerve intrinsics, and the medial deviation of the lesser toes reflects loss of vector correction as well as clawing.

Diabetic neuropathy. Distal symmetrical polyneuropathy produces the same intrinsic-minus pattern. Fatty atrophy of the intrinsic foot muscles on MRI is one of the earliest structural markers of diabetic neuropathy, appearing before deformity becomes clinically obvious. The harm is mechanical, through the hyperextension and fat pad migration set out under Action and Biomechanics.

Management is protective. It comprises neuropathy screening, footwear and total-contact insoles, callus care, flexor tenotomy for apical ulcers, and reconstruction for fixed deformity.

Calcaneal Compartment Syndrome

The compartment. The calcaneal compartment contains quadratus plantae, together with the lateral plantar neurovascular bundle, and communicates with the deep posterior compartment of the leg. Its relatively poor collateral supply and tight fascial envelope contribute to its vulnerability in compartment syndrome.

The compartments of the foot. The common teaching model groups them as follows.

• Medial: abductor hallucis, flexor hallucis brevis
• Central: flexor digitorum brevis, the flexor digitorum longus tendons, the lumbricals, adductor hallucis
• Lateral: abductor digiti minimi, flexor digiti minimi brevis
• Interosseous: the interossei, in four separate spaces
• Calcaneal: quadratus plantae

Manoli and Weber's cadaveric study described nine compartments: medial, superficial, lateral, adductor, four separate interossei and calcaneal.

When to suspect it. After a calcaneal fracture, a Lisfranc or forefoot crush, or another high-energy foot injury, and after prolonged pressure or reperfusion.

Diagnosis. It is clinical: a tense, swollen sole, pain out of proportion, and pain on passive toe extension, with pulses and capillary refill preserved until very late. Compartment pressures confirm it, and the calcaneal compartment must be measured specifically, as it can be elevated when others are not.

Treatment. Urgent decompression through the medial (Henry) approach, which decompresses the medial, central (superficial and deep) and calcaneal compartments through one incision, with dorsal incisions for the interosseous compartments if required. The technique is under Surgical Relevance.

If it is missed. The sequel is fibrosis of quadratus plantae and the deep intrinsics with fixed claw toes, a rigid contracted sole, chronic neuropathic pain and an insensate plantar surface. It is one of the most disabling outcomes in foot trauma and there is no good reconstruction.

Other Conditions in the Region

• Tarsal tunnel syndrome. The branch to quadratus plantae and the whole lateral plantar nerve are in the territory; look on MRI for a space-occupying lesion (ganglion, varicosity, accessory muscle, tumour).
• Accessory muscles in the calcaneal compartment, most notably an accessory flexor digitorum longus, can compress the tarsal tunnel contents or the Baxter nerve and present as heel or medial ankle pain with a soft mass.
• Plantar space infection tracks along the plane superficial to quadratus plantae and can reach the calcaneal compartment and the tarsal tunnel. Drainage requires a formal medial approach with awareness of the lateral plantar bundle.
• Flexor hallucis longus tenosynovitis occurs in the groove beneath the sustentaculum, immediately above the medial head origin, and is a differential for deep medial hindfoot pain.

Anatomical Variation Across Populations

• Absence of the lateral head is the commonest variation, described across dissection series in a meaningful minority of feet, with the medial head essentially always present.
• Complete absence of the muscle is reported but rare. Where it occurs, the digital slips of flexor digitorum longus tend to run more directly and accessory interconnections at the knot of Henry are more prominent.
• Accessory slips to the fifth toe tendon, and fusion of the origin with flexor digitorum brevis, are recognised.
• Accessory muscles in the same compartment, particularly an accessory flexor digitorum longus, are found in a small percentage of feet and are a recognised cause of tarsal tunnel or Baxter nerve compression and of a soft mass in the medial hindfoot.
• The origin of the first branch of the lateral plantar nerve varies - from the lateral plantar nerve, directly from the tibial nerve, or as multiple branches - in a meaningful proportion of feet. This variability is exactly why a decompression should release the whole interval rather than seek a single named nerve.
• Prevalence of the conditions that make this muscle relevant is driven by diabetes, hereditary neuropathy and high-energy trauma, not by population anatomy.

Side-by-Side Guidance and Practice Differences

Areas of Genuine Uncertainty

• Compartment pressure thresholds in the foot are less well established than in the leg, and the number and boundaries of the compartments are still debated between anatomical models. The clinical diagnosis therefore carries more weight than the number.
• Whether prophylactic fasciotomy is warranted in high-energy calcaneal or Lisfranc crush injuries with borderline findings.
• Reported prevalence of Baxter neuropathy among recalcitrant heel pain varies widely because it depends on how the diagnosis is made, and there is no accepted diagnostic gold standard.
• Whether quadratus plantae denervation contributes measurably to lesser toe deformity independently of global intrinsic loss has not been isolated experimentally in humans.
• Optimal treatment of established post-compartment-syndrome intrinsic contracture - the balance between soft-tissue release, tenotomy, arthrodesis and accommodation - has no evidence base beyond small series.

High- versus Limited-Resource Practice

• Well-resourced settings: MRI to demonstrate intrinsic and abductor digiti minimi atrophy and to exclude tarsal tunnel lesions, ultrasound-guided diagnostic injection into the abductor hallucis-quadratus plantae interval, compartment pressure monitoring, and access to negative pressure dressings after fasciotomy.
• Limited-resource settings: the two things that matter most cost nothing. Foot compartment syndrome is diagnosed by examination, and a medial Henry fasciotomy needs only a scalpel and knowledge of the anatomy. Likewise, percutaneous flexor tenotomy for a diabetic apical toe ulcer is a local-anaesthetic, minimal-equipment procedure with a high yield, and protective footwear plus callus care prevents far more disability than any operation.
• Everywhere: in the neuropathic foot, the absence of pain removes the warning sign for both compartment syndrome and ulceration, so structured examination and offloading substitute for symptoms.