Leg

Tibialis anterior is a superficial muscle and is therefore readily palpable lateral to the tibia. It arises from the lateral condyle and proximal half to two-thirds of the lateral surface of the tibial shaft; the adjoining anterior surface of the interosseous membrane; the deep surface of the deep fascia; and the intermuscular septum between itself and extensor digitorum longus. The muscle descends vertically and ends in a tendon on its anterior surface in the lower third of the leg. The tendon passes through the medial compartments of the superior and inferior retinacula, inclines medially, and is inserted into the medial and inferior surfaces of the medial cuneiform and the adjoining part of the base of the first metatarsal bone.

Attachments to the talus, first metatarsal head, base of the proximal phalanx of the hallux, and extensor retinaculum have been recorded.

Tibialis anterior overlaps the anterior tibial vessels and deep fibular nerve in the upper part of the leg. It lies on the tibia and interosseous membrane. Extensor digitorum longus and extensor hallucis longus lie laterally.

The main body of tibialis anterior is supplied by a series of medial and anterior branches of the anterior tibial artery: the branches may occur in two columns. There is a proximal accessory supply from the anterior tibial recurrent artery. The tendon is supplied by the anterior medial malleolar artery and network, dorsalis pedis artery, medial tarsal arteries, and by the medial malleolar and calcaneal branches of the posterior tibial artery.

Tibialis anterior is innervated by the deep fibular nerve, L4 and 5.

Tibialis anterior is a dorsiflexor of the ankle joint and invertor of the foot. It is most active when both movements are combined, as in walking. Its tendon can be seen through the skin lateral to the anterior border of the tibia and can be traced downwards and medially across the front of the ankle to the medial side of the foot. Tibialis anterior elevates the first metatarsal base and medial cuneiform and rotates their dorsal aspects laterally.

The muscle is usually quiescent in a standing subject, since the weight of the body acts through a vertical line that passes anterior to the ankle joints. Acting from below, it helps to counteract any tendency to overbalance backwards by flexing the leg forwards at the ankle. It has a role in supporting the medial longitudinal arch of the foot: although electromyographically detectable activity is minimal during standing, it is manifest during any movement that increases the arch, such as toe-off in walking and running.

Tibialis anterior can be seen to act when the foot is dorsiflexed against resistance.

Extensor hallucis longus

Extensor hallucis longus lies between, and partly overlapped by, tibialis anterior and extensor digitorum longus. It arises from the middle two-fourths of the medial surface of the fibula, medial to extensor digitorum longus, and from the adjacent anterior surface of the interosseous membrane. Its fibres run distally and end in a tendon that forms on the anterior border of the muscle. The tendon passes deep to the superior extensor retinaculum and through the inferior extensor retinaculum, crosses anterior to the anterior tibial vessels to lie on their medial side near the ankle, and is inserted into the dorsal aspect of the base of the distal phalanx of the hallux. At the metatarsophalangeal joint a thin prolongation from each side of the tendon covers the dorsal surface of the joint. An expansion from the medial side of the tendon to the base of the proximal phalanx is usually present.

Extensor hallucis longus is sometimes united with extensor digitorum longus and may send a slip into the second toe.

The anterior tibial vessels and deep fibular nerve lie between extensor hallucis longus and tibialis anterior. Extensor hallucis longus lies lateral to the artery proximally, crosses it in the lower third of the leg, and is medial to it on the foot.

Extensor hallucis longus is supplied by the anterior tibial artery via obliquely running branches, with a variable contribution from the perforating branch of the fibular artery. More distally, the tendon is supplied via the anterior medial malleolar artery and network, the dorsalis pedis artery, and the plantar arteries of the first ray via perforating branches.

Extensor hallucis longus is innervated by the deep fibular nerve, L5.

Extensor hallucis longus extends the phalanges of the hallux and dorsiflexes the foot. When the hallux is actively extended, relatively little external force is required to overcome the extension of the distal phalanx, whereas considerable force is needed to overcome the extension of the proximal phalanx.

When the hallux is extended (dorsiflexed) against resistance, the tendon of extensor hallucis longus can be seen and felt on the lateral side of the tendon of tibialis anterior. This is a useful and very specific test of the integrity of the L5 spinal nerve.

Extensor digitorum longus

The tendons to the second and fifth toes are sometimes duplicated, and accessory slips may be attached to metatarsals or to the hallux.

Extensor digitorum longus arises from the inferior surface of the lateral condyle of the tibia, proximal three-quarters of the medial surface of the fibula, adjacent anterior surface of the interosseous membrane, deep surface of the deep fascia, anterior crural intermuscular septum, and from the fascial septum between itself and tibialis anterior. These origins form the walls of an osseo-aponeurotic tunnel. Extensor digitorum longus becomes tendinous at about the same level as tibialis anterior, and the tendon passes deep to the superior extensor retinaculum and within a loop of the inferior extensor retinaculum with fibularis tertius (see Figs 83.7, 84.2). It divides into four slips which run forward on the dorsum of the foot and are attached in the same way as the tendons of extensor digitorum in the hand. At the metatarsophalangeal joints the tendons to the second, third and fourth toes are each joined on the lateral side by a tendon of extensor digitorum brevis. The dorsal digital expansions thus formed on the dorsal aspects of the proximal phalanges, as in the fingers, receive contributions from the appropriate lumbrical and interosseous muscles. The expansion narrows as it approaches a proximal interphalangeal joint, and divides into three slips. These are a central (axial) slip, attached to the base of the middle phalanx, and two collateral (coaxial) slips, which reunite on the dorsum of the middle phalanx and are attached to the base of the distal phalanx.

Extensor digitorum longus lies on the lateral tibial condyle, fibula, lower end of the tibia, ankle joint and extensor digitorum brevis. Tibialis anterior and extensor hallucis longus lie medially in the leg, and the fibular muscles lie laterally. In the upper part of the leg the anterior tibial vessels and deep fibular nerve lie between extensor digitorum longus and tibialis anterior; the nerve runs obliquely and medially beneath its upper part.

The main blood supply to extensor digitorum longus is derived from anteriorly and laterally placed branches of the anterior tibial artery, supplemented distally from the perforating branch of the fibular artery. Proximally there may also be a supply from the lateral inferior genicular, popliteal or anterior tibial recurrent arteries. At the ankle and in the foot, the tendons are supplied by the anterior lateral malleolar artery and malleolar network, and by lateral tarsal, metatarsal, plantar and digital arteries.

Extensor digitorum longus is innervated by the deep fibular nerve, L5, S1.

Extensor digitorum longus extends the lateral four toes. Acting synergistically with tibialis anterior, extensor hallucis longus and fibularis tertius, it is a dorsiflexor of the ankle. Acting with extensor hallucis longus it helps tighten the plantar aponeurosis.

The tendons of extensor digitorum longus can be seen when the toes are extended (dorsiflexed) against resistance.

Fibularis tertius

Fibularis tertius (peroneus tertius) is a muscle unique to humans. It often appears to be part of extensor digitorum longus, and might be described as its ‘fifth tendon’. The muscle fibres of fibularis tertius arise from the distal third or more of the medial surface of the fibula, the adjoining anterior surface of the interosseous membrane, and the anterior crural intermuscular septum. The tendon passes deep to the superior extensor retinaculum and within the loop of the inferior extensor retinaculum alongside the extensor digitorum longus (see Figs 83.7, 84.2). It is inserted into the medial part of the dorsal surface of the base of the fifth metatarsal bone; a thin expansion usually extends forwards along the medial border of the shaft of the bone.

Fibularis tertius lies lateral to extensor digitorum longus.

Fibularis tertius is supplied by the same vessels as extensor digitorum longus. In the foot it receives an additional supply from the termination of the arcuate artery and the fourth dorsal metatarsal artery.

Fibularis tertius is innervated by the deep fibular nerve, L5, S1.

Electromyographic studies show that during the swing phase of gait (see Fig. 84.26), fibularis tertius acts with extensor digitorum longus and tibialis anterior to produce dorsiflexion of the foot, and with fibularis longus and fibularis brevis to effect eversion of the foot (Jungers et al 1993). This levels the foot and helps the toes to clear the ground, an action that improves the efficiency and enhances the economy of bipedal locomotion. Fibularis tertius is not active during the stance phase, a finding that is at variance with the suggestion that the muscle acts primarily to support the lateral longitudinal arch and to transfer the centre of pressure of the foot medially.

Fibularis tertius cannot be tested in isolation, but its tendon can sometimes be seen and felt when the foot is dorsiflexed against resistance.

LATERAL (FIBULAR OR PERONEAL) COMPARTMENT

The lateral compartment contains fibularis (peroneus) longus and fibularis (peroneus) brevis (Figs 83.7, 83.8). Both muscles evert the foot and are plantar flexors of the ankle, and both probably play a part in balancing the leg on the foot in standing and walking.

Fibularis longus

Fibularis longus is the more superficial of the two muscles of the lateral compartment. It arises from the head and proximal two-thirds of the lateral surface of the fibula, the deep surface of the deep fascia, the anterior and posterior crural intermuscular septa, and occasionally by a few fibres from the lateral condyle of the tibia. The muscle belly ends in a long tendon that runs distally behind the lateral malleolus in a groove it shares with the tendon of fibularis brevis. The groove is converted into a canal by the superior fibular retinaculum, so that the tendon of fibularis longus, and that of fibularis brevis which lies in front of the longus tendon, are contained in a common synovial sheath. If the fibular retinaculum is ruptured by injury and fails to heal, the tendons can dislocate from the groove. The fibularis longus tendon runs obliquely forwards on the lateral side of the calcaneus, below the fibular trochlea and the tendon of fibularis brevis, and deep to the inferior fibular retinaculum. It crosses the lateral side of the cuboid and then runs under the cuboid in a groove that is converted into a canal by the long plantar ligament (see Fig. 84.19). It crosses the sole of the foot obliquely and is attached by two slips, one to the lateral side of the base of the first metatarsal bone and one to the lateral aspect of the medial cuneiform; occasionally a third slip is attached to the base of the second metatarsal bone. The tendon changes direction below the lateral malleolus and on the cuboid bone. At both sites it is thickened and at the second a sesamoid fibrocartilage (sometimes a bone, the os peroneum) is usually present. A second synovial sheath invests the tendon as it crosses the sole of the foot.

Proximally fibularis longus lies posterior to extensor digitorum longus and anterior to soleus and flexor hallucis longus. Distally in the leg it lies posterior to fibularis brevis. Between its attachments to the head and shaft of the fibula there is a gap through which the common fibular nerve passes.

Vascular supply of lateral compartment

Usually the predominant supply of the lateral compartment muscles is derived from superior and inferior branches of the anterior tibial artery; the superior is commonly the larger. There is also a lesser, variable, contribution from the fibular artery in the distal part of the leg. A fibular branch may replace the inferior branch of the anterior tibial artery; less commonly the fibular artery provides the main supply to the whole compartment. The upper part of fibularis longus is also supplied by the circumflex fibular artery, which is usually a branch of the anterior tibial artery. The companion artery to the common fibular nerve, a branch of the popliteal artery, provides a minor contribution proximally. Distally the tendons are supplied by the fibular perforating, anterior lateral malleolar, lateral calcaneal, lateral tarsal, arcuate, lateral and medial plantar arteries (see Fig. 84.9).

Fibularis longus is innervated by the superficial fibular nerve, L5, S1.

There is little doubt that fibularis longus can evert the foot and plantar flex the ankle, and possibly act on the leg from its distal attachments. The oblique direction of its tendon across the sole would also enable it to support the longitudinal and transverse arches of the foot. With the foot off the ground, eversion is visually and palpably associated with increased prominence of both tendon and muscle. It is not clear to what extent this helps to maintain plantigrade contact of the foot in normal standing, but electromyographic records show little or no fibular activity under these conditions. Fibular activity is necessary to stand plantigrade with the legs crossed. Fibularis longus and brevis come strongly into action to maintain the concavity of the foot during toe-off and tip-toeing. If the subject deliberately sways to one side, the fibulares contract on that side, but their involvement in influencing postural interactions between the foot and leg remains uncertain.

Fibularis longus and brevis are tested together by eversion of the foot against resistance: the tendons can be identified laterally at the ankle and in the foot.

Fibularis brevis

Fibularis brevis arises from the distal two-thirds of the lateral surface of the fibula, anterior to fibularis longus, and from the anterior and posterior crural intermuscular septa. It passes vertically downwards and ends in a tendon that passes behind the lateral malleolus together with, and anterior to, that of fibularis longus. The two tendons run deep to the superior fibular retinaculum in a common synovial sheath. The tendon of fibularis brevis then runs forwards on the lateral side of the calcaneus above the fibular trochlea and the tendon of fibularis longus, and is inserted into a tubercle on the lateral side of the base of the fifth metatarsal bone.

Anteriorly lie extensor digitorum longus and fibularis tertius. Fibularis longus and flexor hallucis longus are posterior. On the lateral surface of the calcaneus the tendons of fibularis longus and fibularis brevis occupy separate osteofascial canals formed by the calcaneus and the inferior fibular retinaculum; each tendon is enveloped in a separate distal prolongation of the common synovial sheath (see Fig. 84.2).

See under fibularis longus.

Fibularis brevis is innervated by the superficial fibular nerve, L5, S1.

Fibularis brevis may limit inversion of the foot and so relieve strain on the ligaments that are tightened by this movement (lateral part of interosseous talocalcaneal, lateral talocalcaneal and calcaneofibular ligaments). It participates in eversion of the foot and may help to steady the leg on the foot. See also under fibularis longus.

See under fibularis longus.

Variants of fibular muscles

Tendinous slips from fibularis longus may extend to the base of the third, fourth or fifth metatarsal bone, or to adductor hallucis. Fusion of fibularis longus and brevis has been reported, but is rare. Fibularis tertius is highly variable in its form and bulk but is seldom completely absent; its distal attachment may be to the fourth metatarsal rather than the fifth. Two other variant fibular muscles, arising from the fibula between fibularis longus and fibularis brevis, have been described. These are fibularis accessorius, whose tendon joins that of fibularis longus in the sole, and fibularis quartus, which arises posteriorly and inserts into the calcaneus or into the cuboid.

POSTERIOR (FLEXOR) COMPARTMENT

The muscles in the posterior compartment of the lower leg form superficial and deep groups, separated by the deep transverse fascia.

Superficial flexor group

The superficial flexors gastrocnemius, plantaris and soleus (Figs 83.8, 83.10; see Figs 82.2, 80.25) form the bulk of the calf. Gastrocnemius and soleus, collectively known as the triceps surae, constitute a powerful muscular mass whose main function is plantar flexion of the foot, although soleus in particular has an important postural role (see below). Their large size is a defining human characteristic, and is related to the upright stance and bipedal locomotion of the human. Gastrocnemius and plantaris act both on the knee and ankle joints; soleus on the latter alone.

Fig. 83.10 Muscles of the left leg, posterior aspect. Gastrocnemius partially removed.

(From Sobotta 2006.)

Gastrocnemius

Gastrocnemius is the most superficial muscle of the group and forms the ‘belly’ of the calf. It arises by two heads, connected to the condyles of the femur by strong, flat tendons. The medial, larger, head is attached to a depression at the upper and posterior part of the medial condyle behind the adductor tubercle, and to a slightly raised area on the popliteal surface of the femur just above the medial condyle. The lateral head is attached to a recognizable area on the lateral surface of the lateral condyle and to the lower part of the corresponding supracondylar line. Both heads also arise from subjacent areas of the capsule of the knee joint. The tendinous attachments expand to cover the posterior surface of each head with an aponeurosis, from the anterior surface of which the muscle fibres arise. The fleshy part of the muscle extends to about midcalf. The muscle fibres of the larger medial head extend lower than those of the lateral head. As the muscle descends, the muscle fibres begin to insert into a broad aponeurosis that develops on its anterior surface; up to this point the muscular masses of the two heads remain separate. The aponeurosis gradually narrows and receives the tendon of soleus on its deep surface to form the calcaneal tendon (for a detailed description of the calcaneal tendon see Ch. 84).

On occasion the lateral head, or the whole muscle, is absent. A third head, arising from the popliteal surface of the femur, is sometimes present.

Proximally, the two heads of gastrocnemius form the lower boundaries of the popliteal fossa. The lateral head is partially overlaid by the tendon of biceps femoris, the medial head by semimembranosus. Over the rest of its length the muscle is superficial, and the two heads can easily be seen in the living subject. The superficial surface of the muscle is separated by the deep fascia from the short saphenous vein and the fibular communicating and sural nerves. The common fibular nerve crosses behind the lateral head of the muscle, partly under cover of biceps femoris. The deep surface lies posterior to the oblique popliteal ligament, popliteus, soleus, plantaris, popliteal vessels and the tibial nerve. A bursa, which communicates with the knee joint, is located anterior to the tendon of the medial head; if the bursa expands into the popliteal fossa it does so in the plane between the medial head of gastrocnemius and semimembranosus. The tendon of the lateral head frequently contains a fibrocartilaginous or bony sesamoid, the fabella, where it moves over the lateral femoral condyle. A sesamoid may occasionally occur in the tendon of the medial head. These can be seen on radiographs and may be mistaken for loose bodies in the knee joint.

Each head of gastrocnemius is supplied by its own sural artery. These arteries are branches of the popliteal artery, and arise at variable levels, usually at the level of the tibiofemoral joint line. The medial sural artery almost always arises more proximally than the lateral: the medial may arise proximal to the joint line, the lateral sometimes distal to the line. Each sural artery enters the corresponding muscle head with its nerve of supply, the pedicle entering the muscle near its axial border at the level of the middle of the popliteal fossa. Medial or lateral gastrocnemius musculocutaneous flaps may be raised, each based on its neurovascular pedicle. Minor accessory sural arteries arise from the popliteal or from the superior genicular vessels.

The blood supply to the calcaneal tendon is described in Chapter 84.

Gastrocnemius is innervated by the tibial nerve, S1 and 2.

The action of gastrocnemius is considered with soleus.

Gastrocnemius is tested by plantar flexion of the foot against resistance, in the supine position and with the knee extended.

Plantaris

Plantaris arises from the lower part of the lateral supracondylar line and the oblique popliteal ligament. Its small fusiform belly is 7–10 cm long and ends in a long slender tendon which crosses obliquely, in an inferomedial direction, between gastrocnemius and soleus, then runs distally along the medial border of the calcaneal tendon and inserts on to the calcaneus just medial to the calcaneal tendon. Occasionally it ends by fusing with the tendon.

The muscle is sometimes double, and is absent in approximately 10% of cases. Occasionally, its tendon merges with the flexor retinaculum or with the fascia of the leg.

Plantaris is supplied superficially by the lateral sural and popliteal arteries, and deeply by the lateral superior genicular artery. The distal tendon shares a blood supply with the calcaneal tendon.

Plantaris is innervated by the tibial nerve, often from the ramus that supplies the lateral head of gastrocnemius, S1 and 2.

Plantaris is the lower limb equivalent of palmaris longus, and in many mammals is well developed and inserts directly or indirectly into the plantar aponeurosis. In man the muscle is almost vestigial and is normally inserted well short of the plantar aponeurosis, usually into the calcaneus. It is therefore presumed to act with gastrocnemius.

Soleus

Soleus is a broad flat muscle situated immediately deep (anterior) to gastrocnemius. It arises from the posterior surface of the head and proximal quarter of the shaft of the fibula; the soleal line and the middle third of the medial border of the tibia; and from a fibrous band between the tibia and fibula (the soleal arch) that arches over the popliteal vessels and tibial nerve. This origin is aponeurotic: most of the muscular fibres arise from its posterior surface and pass obliquely to the tendon of insertion on the posterior surface of the muscle. Other muscle fibres arise from the anterior surface of the aponeurosis. They are short, oblique and bipennate in arrangement, and converge on a narrow, central intramuscular tendon that merges distally with the principal tendon. The latter gradually becomes thicker and narrower, and joins the tendon of gastrocnemius to form the calcaneal tendon. The muscle is covered proximally by gastrocnemius, but below midcalf it is broader than the tendon of gastrocnemius and is readily accessible on either side of the tendon.

An accessory part of the muscle is sometimes present distally and medially. It may be inserted into the calcaneal tendon, the calcaneus or the flexor retinaculum.

The superficial surface of soleus is in contact with gastrocnemius and plantaris. Its deep surface is related to flexor digitorum longus, flexor hallucis longus, tibialis posterior and the posterior tibial vessels and tibial nerve, from all of which it is separated by the deep transverse fascia of the leg.

Soleus is supplied by two main arteries: the superior arises from the popliteal artery at about the level of the soleal arch, and the inferior from the proximal part of the fibular artery or sometimes from the posterior tibial artery. A secondary supply is derived from the lateral sural, fibular or posterior tibial vessels.

There is a venous plexus within the muscle belly that is important physiologically as part of the muscle pump complex (see below). Pathologically, it is a common site of deep vein thrombosis.

Soleus is innervated by two branches from the tibial nerve, S1 and 2.

See under triceps surae.

Soleus is tested by plantar flexion of the foot against resistance in the supine position, with hip and knee flexed: the muscle belly can be palpated separately from those of gastrocnemius.

Actions of triceps surae

Gastrocnemius and soleus are the chief plantar flexors of the foot; gastrocnemius is also a flexor of the knee. The muscles are usually large and correspondingly powerful. Gastrocnemius provides force for propulsion in walking, running and leaping. Soleus, acting from below, is said to be more concerned with steadying the leg on the foot in standing. This postural role is also suggested by its high content of slow, fatigue-resistant (type 1) muscle fibres. In many adult mammals the proportion of this type of fibre in soleus approaches 100%. However, such a rigid separation of functional roles seems unlikely in man: soleus probably participates in locomotion, and gastrocnemius in posture. Nevertheless, the ankle joint is loose-packed in the erect posture, and since the weight of the body acts through a vertical line that passes anterior to the joint, a strong brace is required behind the joint to maintain stability. Electromyography shows that these forces are supplied mainly by soleus: during symmetrical standing, soleus is continuously active, whereas gastrocnemius is recruited only intermittently. The relative contributions of soleus and gastrocnemius to phasic activity of the triceps surae in walking have yet to be satisfactorily analysed.

Deep flexor group

The deep flexor group (Fig. 83.8, Fig. 83.11), lies beneath (anterior to) the deep transverse fascia, and consists of popliteus, which acts on the knee joint, and flexor digitorum longus, flexor hallucis longus and tibialis posterior, which all produce plantar flexion at the ankle in addition to their specific actions on joints of the foot and digits.

Fig. 83.11 Muscles of the left leg, posterior aspect. A, Superficial muscles extensively removed. B, Superficial muscles extensively removed, popliteus sectioned and the tendon of flexor digitorum longus removed as it crosses the tendon of tibialis posterior.

(From Sobotta 2006.)

Popliteus

Popliteus is described in Chapter 82.

Flexor digitorum longus

A supplementary head of the muscle, flexor accessorius longus, with its own tendon, may arise from the fibula, tibia or deep fascia and insert into the main tendon or into flexor accessorius in the foot. It may send communicating slips to tibialis anterior or to flexor hallucis longus.

Flexor digitorum longus is thin and tapered proximally, but widens gradually as it descends. It arises from the posterior surface of the tibia medial to tibialis posterior from just below the soleal line to within 7 or 8 cm of the distal end of the bone; it also arises from the fascia covering tibialis posterior. The muscle ends in a tendon that extends along almost the whole of its posterior surface. The tendon gradually crosses tibialis posterior on its superficial aspect and passes behind the medial malleolus where it shares a groove with tibialis posterior, from which it is separated by a fibrous septum, i.e. each tendon occupies its own compartment lined by a synovial sheath. The tendon of flexor digitorum longus then curves obliquely forwards and laterally, in contact with the medial side of the sustentaculum tali, passes deep to the flexor retinaculum, and enters the sole of the foot on the medial side of the tendon of flexor hallucis longus (see Fig. 84.18). It crosses superficial to that tendon and receives a strong slip from it (and may also send a slip to it). The tendon of flexor digitorum longus then passes forward as four separate tendons, one each for the second to fifth toes, deep to the tendons of flexor digitorum brevis. After giving off the lumbricals, it passes through the fibrous sheaths of the lateral four toes. The tendons of flexor accessorius insert into the long flexor tendons of the second, third and fourth digits; flexor hallucis longus makes a variable contribution through the connecting slip mentioned above. The long flexor tendons of the lateral four digits are attached to the plantar surfaces of the bases of their distal phalanges; each passes between the slips of the corresponding tendon of flexor digitorum brevis at the base of the proximal phalanx.

Flexor digitorum longus lies medial to flexor hallucis longus. In the leg its superficial surface is in contact with the deep transverse fascia, which separates it from soleus, and distally from the posterior tibial vessels and tibial nerve. Its deep surface is related to the tibia and to tibialis posterior. In the foot it is covered by abductor hallucis and flexor digitorum brevis and it crosses superficial to flexor hallucis longus.

A series of transversely running branches of the posterior tibial artery enters the lateral border of flexor digitorum longus. There may be a secondary supply from fibular branches to flexor hallucis longus. The tendons are supplied by the vessels of the ankle and sole.

Flexor digitorum longus is innervated by branches of the tibial nerve, L5, S1 and S2.

See under flexor hallucis longus.

Flexor digitorum longus is tested by flexing the toes against resistance. Aberrant function of flexor digitorum longus is implicated in toe deformities such as hammer toe, claw toe and mallet toe. It may be used in tendon transfer procedures for the surgical treatment of tibialis posterior tendon dysfunction.

Flexor hallucis longus

The distal extent of the muscle belly is a distinctive characteristic: in the posteromedial surgical approach to the ankle, flexor hallucis longus is readily identifiable by the fact that muscle fibres are evident almost to calcaneal level. In athletes the muscle fibres may be present so far inferiorly into the tendon as to be susceptible to impingement when pulled into the tunnel at the talus.

Flexor hallucis longus arises from the distal two-thirds of the posterior surface of the fibula (except for the lowest 2.5 cm of this surface); the adjacent interosseous membrane and the posterior crural intermuscular septum; and from the fascia covering tibialis posterior, which it overlaps to a considerable extent. Its fibres pass obliquely down to a tendon that occupies nearly the whole length of the posterior aspect of the muscle. This tendon grooves the posterior surface of the lower end of the tibia, then, successively, the posterior surface of the talus and the inferior surface of the sustentaculum tali of the calcaneus (see Fig. 84.2B). Fibrous bands convert the grooves on the talus and calcaneus into a canal lined by a synovial sheath. In dancers, overuse causes thickening of the tendon in this region, and pain and even ‘triggering’ can occur (hallux saltans). In the sole of the foot, the tendon of flexor hallucis longus passes forward in the second layer (Ch. 84) like a bowstring. It crosses the tendon of flexor digitorum longus from lateral to medial, curving obliquely superior to it. At the crossing point (knot of Henry) it gives off two strong slips to the medial two divisions of the tendons of flexor digitorum longus and then crosses the lateral part of flexor hallucis brevis to reach the interval between the sesamoid bones under the head of the first metatarsal. It continues on the plantar aspect of the hallux, and runs in an osseo-aponeurotic tunnel to be attached to the plantar aspect of the base of the distal phalanx. The tendon is retained in position over the lateral part of flexor hallucis brevis by the diverging stems of the distal band of the medial intermuscular septum.

The connecting slip to flexor digitorum longus varies in size: it usually continues into the tendons for the second and third toes but is sometimes restricted to the second and occasionally extends to the fourth.

Soleus and the calcaneal tendon lie superficial (i.e. posterior) to flexor hallucis longus, separated by the deep transverse fascia. Deeply situated are the fibula, tibialis posterior, fibular vessels, distal part of the interosseous membrane and the talocrural joint. Laterally lie fibularis longus and fibularis brevis; medially are tibialis posterior, posterior tibial vessels and the tibial nerve. Flexor hallucis longus is an important surgical landmark at the ankle: staying lateral to it prevents injury to the neurovascular bundle.

Flexor hallucis longus is supplied by numerous branches of the fibular artery. The tendon is supplied by arteries of the ankle and foot.

Flexor hallucis longus is innervated by branches of the tibial nerve, L5, S1 and 2 (mainly S1).

Flexor hallucis longus is tested by flexion of the hallux against resistance.

Actions of deep digital flexors

Both flexor hallucis longus and flexor digitorum longus can act as plantar flexors but this action is weak compared with that of gastrocnemius and soleus. When the foot is off the ground, both muscles flex the phalanges of the toes, acting primarily on the distal phalanges. When the foot is on the ground and under load, they act synergistically with the small muscles of the foot and, especially in the case of flexor digitorum longus, with the lumbricals and interossei to maintain the pads of the toes in firm contact with the ground, enlarging the weightbearing area and helping to stabilize the heads of the metatarsal bones, which form the fulcrum on which the body is propelled forwards. Activity in the long digital flexors is minimal during quiet standing, so they apparently contribute little to the static maintenance of the longitudinal arch, but they become very active during toe-off and tip-toe movements.

Tibialis posterior

Tibialis posterior is the most deeply placed muscle of the flexor group. At its origin it lies between flexor hallucis longus and flexor digitorum longus, and is overlapped by both, but especially by the former. Its proximal attachment consists of two tapered processes, separated by an angular interval that is traversed by the anterior tibial vessels. The medial process arises from the posterior surface of the interosseous membrane, except at its most distal part, and from a lateral area on the posterior surface of the tibia between the soleal line above and the junction of the middle and lower thirds of the shaft below. The lateral part arises from a medial strip of the posterior fibular surface in its upper two-thirds. The muscle also arises from the deep transverse fascia, and from the intermuscular septa that separate it from adjacent muscles. In the distal quarter of the leg its tendon passes deep to that of flexor digitorum longus, with which it shares a groove behind the medial malleolus, each enclosed in a separate synovial sheath. It then passes deep to the flexor retinaculum and superficial to the deltoid ligament to enter the foot. In the foot it is at first inferior to the plantar calcaneonavicular ligament, where it contains a sesamoid fibrocartilage. The tendon then divides into two. The more superficial and larger division, which is a direct continuation of the tendon, is attached to the tuberosity of the navicular, from which fibres continue to the inferior surface of the medial cuneiform. A tendinous band also passes laterally and a little proximally to the tip and distal margin of the sustentaculum tali. The deeper lateral division gives rise to the tendon of origin of the medial limb of flexor hallucis brevis, and then continues between this muscle and the navicular and medial cuneiform to end on the intermediate cuneiform and the bases of the second, third and fourth metatarsals; the slip to the fourth metatarsal is the strongest.

The slips to the metatarsals vary in number. Slips to the cuboid and lateral cuneiform may also occur. An additional muscle, the tibialis secundus, has been described running from the back of the tibia to the capsule of the ankle joint.

The superficial surface of tibialis posterior is separated from soleus by the deep transverse fascia, and is related to flexor digitorum longus, flexor hallucis longus, the posterior tibial vessels, the tibial nerve and the fibular vessels. The deep surface is in contact with the interosseous membrane, tibia, fibula and talocrural joint.

Tibialis posterior is supplied by numerous branches of small calibre arising from the posterior tibial and fibular arteries. The tendon is supplied by arteries of the medial malleolar network and by the medial plantar artery.

Tibialis posterior is innervated by the tibial nerve, L4 and 5.

Tibialis posterior is a powerful muscle which, on contraction, describes an excursion of only 1–2 cm. It is the principal invertor of the foot and also initiates elevation of the heel: it is responsible for the hindfoot varus that occurs during a single heel raise. By reason of its insertions into the cuneiform bones and the bases of the metatarsals, it has long been thought to assist in elevating the longitudinal arch of the foot, although electromyography shows that it is actually quiescent in standing. It is phasically active in walking, during which it probably acts with the intrinsic foot musculature and the lateral calf muscles to control the degree of pronation of the foot and the distribution of weight through the metatarsal heads. It is said that when the body is supported on one leg, the invertor action of tibialis posterior, exerted from below, helps to maintain balance by resisting any tendency to sway laterally.

Tibialis posterior is also important in the maintenance of the medial longitudinal arch. In overweight individuals with pes planus, unaccustomed activity can result in inflammation and degeneration of the terminal portion of the tendon, which leads to elongation of the tendon, attenuation of the spring ligament and progressive collapse of the medial longitudinal arch. As the excursion is so short, the muscle cannot compensate for the lengthening of its tendon, a failure that results in tibialis posterior tendon dysfunction.