Muscles plantarflexing the ankle joint

Gastrocnemius

Soleus

Plantaris

Peroneus longus

Tibialis posterior

Flexor digitorum longus

Flexor hallucis longus

Gastrocnemius

The shape of the calf is mainly due to the two fleshy bellies of gastrocnemius( figure a), being situated on the back of the leg with its muscle bulk mainly in the upper half. Together with soleus and plantaris, gastrocnemius forms a composite muscle referred to as the triceps surae. The two heads of gastrocnemius form the lower boundaries of the popliteal fossa, which can only really be seen when the knee is flexed. The two heads arise from the medial and lateral condyles of the femur: the medial head, from behind the medial supracondylar ridge and the adductor tubercle on the popliteal surface of the femur, the lateral head from the outer surface of  the lateral condyle of the femur just above the behind the lateral epicondyle. Each head has an additional attachment from the capsule of the knee joint and from the oblique popliteal ligament, below which each head is separated from the capsule by a bursa. The bursa associated with the medial head often communicates with the knee joint: that under the lateral head rarely does. There is often a sesamoid bone, the flabella, in the lateral head as it crosses the lateral condyle of the femur. Less commonly there may be one associated with the medial head.

From each head a fleshy bulk of muscle fibers arise which gradually come together, although not actually blending with each other, to insert into the posterior surface of a broad membranous tendon which fuses with the tendon of soleus to form the upper part of the tendocalcaneus. This broad tendon gradually narrows, becoming more rounded until it reaches about three fingers’ breadth above the calcaneus, where it begins to expand again and continues to do so, until its insertion into the middle part of the posterior surface of the calcaneus. A bursa lies between the tendon and the upper part of the calcaneus while a pad of fat lies between the tendon and the posterior aspect of the ankle joint. Inferior to the insertion is the fat pad of the heel.

Nerve supply

Each head of gastrocnemius is supplied by a branch from the tibial nerve, root value S1, 2. The area of skin covering the muscle has roots L4, 5, S2.

Action

Gastrocnemius, together with soleus, is the chief plantarflexor of the ankle joint. It provides the propelling force for locomotion. As it crosses the knee joint, gastrocnemius is also a powerful flexor of that joint. However, it is not able to exert its full power on both joints simultaneously. For example, if the knee is flexed, gastrocnemius cannot exert maximum power at the ankle joint and vice versa.

Functional activity

In running, walking and jumping gastrocnemius provides a considerable amount of the propulsive force. When one considers the power needed to throw the body into the air, triceps surae must be one of the most powerful muscle groups in the body.

The habitual wearing of shoes with a high heel can cause considerable shortening of the fibres of gastrocnemius, as the two attachments of the muscle fibers are brought closer together. If shortening has occurred, difficulty in walking in flat shoes or bare feet may be experienced due to limited dorsiflexion at the ankle joint.

Soleus

This muscle is situated deep to gastrocnemius, being a broad flat muscle wider in its middle section and narrower below (figure b). It arises from the soleal line on the posterior surface of the tibia, the posterior surface of the upper third of the fibula (including the head) and a fibrous arch between these bony attachments. The fibres pass downwards, forming a belly about half way down the calf to the deep surface of a membranous tendon which faces posteriorly. This tendon glides over a similar one on the deep surface of gastrocnemius, thereby enabling independent movement of the two muscles to occur. Inferiorly the two tendons fuse to form the upper part of the tendocalcaneus, which passes behind the ankle joint to insert into the middle part of the posterior surface of the calcaneus.

Nerve supply

Soleus is supplied by two branches from the tibial nerve, root value S1, 2, one of which arises in the popliteal fossa and enters the superficial surface of the muscle, while the other arises in the calf entering the deep surface. The skin over the region of the muscle is predominantly supplied by root S2.

Action

Soleus is one of two main plantarflexors of the ankle joint. It is so placed to prevent the body falling forwards at the ankle joint during standing, and as such is an important postural muscle. Intermittent contraction of the muscle during standing aids venous return(the soleal pump) due to the communicating vessels joining the deep and superficial venous systems which pass through its substance.

Plantaris

Plantaris(picture c) is a long, slender muscle which is variable in its composition. It may have one muscle belly high up in the calf, or two smaller bellies separated by a tendon. It arises from the lowest part of the lateral supracondylar ridge, the adjacent part of the popliteal surface of the femur and the knee joint capsule. The tendon passes obliquely downwards between gastrocnemius and soleus to emerge on the medial side of the tendocalcaneus. It may insert into the tendocalcaneus or into the medial side of the posterior surface of the calcaneus.

Nerve supply

The supply to plantaris is from the tibial nerve, root value S1, 2.

Action

Plantaris is a weak flexor of the knee and plantarflexor of the ankle joint.

The tendocalcaneus (Achilles tendon)

This is considered to be the thickest and strongest tendon in the body, being the tendon by which the calf muscles exert their force on the posterior part of the foot during the propulsive phase of many activities, for example, walking, running and jumping. It has been suggested that the tendocalcaneus is able to withstand strains of up to 10 tons. As its fibres pass downwards they spiral through some 90°, with the medial fibres passing posteriorly. This unusual arrangement is thought to explain the apparent elastic qualities of the tendon. For example, when jumping the body will land in an upright position with the foot held in plantarflexion by the active triceps surae. The strain is then taken by the tendocalcaneus which produces a recoil effect.

Functional activity of the calf muscles

The action of the calf muscles as a whole is to plantarflex the foot at the ankle joint. Gastrocnemius acts as the propelling force, working mainly on the ankle but also producing flexion of the knee if working strongly enough. The soleus, on the other hand, is better situated to act more as a postural muscle. This is because its lower attachment is the fixed point and prevents the leg from falling forwards under the influence of body weight, because the vertical projection from the centre of gravity of the body falls in front of the ankle joint.

Gastrocnemius is composed of muscle fibres which give it a pale appearance; consequently it is often referred to as “white” muscle, whereas; the soleus has fibres which give it a red appearance and is therefore termed a “red” muscle.

Plantaris takes very little in plantarflexion of the ankle and, in fact, sometimes causes pain and disability when it is torn. This condition is referred to as “tennis leg”, occurring during a game of tennis, when the player believes that he or she has been struck on the back of the calf by a tennis ball. The tendon is often completely ruptured and may have to be surgically removed.

Palpation of the calf muscles

When standing, draw your hand down the back of the knee. The two large muscular bellies of gastrocnemius can be felt on either side of the upper part of the calf. The medial head projects slightly higher and lower than the lateral. Both can be felt joining a board flattened tendon just over half way down the calf. The junction between the muscle fibres and the tendon is very clear and it is along this line that many injuries of the calf occur.

Soleus is not quite so easy to palpate being deep to gastrocnemius, its lateral boundary appearing as a flattened elevation below and lateral to the lateral head of gastrocnemius when the ankle is plantarflexed. When standing on tiptoe, soleus can be seen and felt to bulge either side of gastrocnemius. Passing the hand further down the calf it will encounter the flattened tendocalcaneus, which is felt to narrow and become rounded at the level of the ankle joint. It then expands slightly to its insertion into the middle section of the posterior surface of the calcaneus.

Peroneus longus

Peroneus longus(picture a) is situated on the lateral side of the leg, being a long, thin fusiform muscle with a long belly and an even longer tendon. It is also quite unique in as much as the tendon changes direction three times on its way to its insertion on the medial side of the sole of the foot.

It arises from a small area on the lateral condyle of the tibia( in conjunction with extensor digitorum longus) and the upper two-thirds of the lateral surface of the fibula, its lower half lying behind the upper part of the origin from the lateral side of the head of the fibula, leaving a small area around the neck for the anterior passage of the common peroneal nerve. In front and behind, it attaches to the intermuscular septa and to the fascia surrounding the muscle.

The tendon forms about a hand’s breadth above the lateral malleolus and lies superficial to that of peroneus brevis, sharing the same synovial sheath. It runs in a shallow groove behind the lateral malleolus passing deep to the superior peroneal retinaculum. From here, the tendon passes downwards and slightly forwards to pass below the peroneal tubercle on the calcaneus, being held in position by the inferior band of the peroneal retinaculum. At this point the tendon is enclosed in a separate synovial sheath. As it reaches the inferolateral side of the cuboid, which it grooves, the tendon turns to enter the groove on the inferior aspect of the cuboid. This groove is converted into a tunnel by fibres from the long plantar ligament and tibialis posterior tendon, whilst in this tunnel the tendon is still surrounded by a synovial sheath. The tunnel conveys the tendon forwards and medially across the foot to its final attachment to the plantar and lateral surfaces of the medial cuneiform and base of the first metatarsal.

Nerve supply

Peroneus longus is supplied by the superficial peroneal nerve, root value L5, S1. The skiin covering the muscle is supplied by L5, S1.

Action

The muscle is an obvious evertor of the foot because of the fact that it arises from the lateral side of the leg and passes around the lateral side of the foot. In passing from behind the lateral malleolus to the medial cuneiform and first metatarsal, peroneus longus will produce plantarflexion of the foot, with the medial side of the foot being particularly drawn downwards, as in pronation.

It is worth noting that the insertion of this muscle is to the same two bones as tibialis anterior, although the latter muscle approaches its insertion from the medial side of the foot. This is believed to provide a stirrup for the arches of the foot and help control their height during activity. The attachment of both muscles to the medial cuneiform and base of the first metatarsal certainly emphasizes the importance of control of the medial side of the foot in activity, particularly when dealing with uneven terrain.

Functional activity

In standing, peroneus longus, in company with other surrounding muscles, helps to maintain the erect position. It controls sideways sway by pressing the medial side of the foot on to the ground. This function is better seen and appreciated when standing on one leg when peroneus longus works very hard to maintain the leg over the foot and prevent the body falling to the opposite side. Its main functional activity, however, must be during powerful action of the foot as in running, particularly over rough ground. Here, its control, together with that of tibialis anterior, over the medial side of the foot and the first metatarsal(carrying the great toe), must be vital.

Palpation

When sitting, place the fingers on the lateral side of the knee joint and locate the head of the fibula just below the joint level. The tendon of biceps femoris can be identified coming from the back of the thigh. Run the fingers downwards, keeping the tip of the index finger on the head of the fibula spreading the rest of the finger tips down the lateral side of the fibula. Keeping the fingers in this position, lift up the outer side of the foot. The long vertical belly can be felt contracting. If the fingers are now taken down to the lateral malleolus and placed below and behind it and the same manoeuvre is performed, the two tendons of peroneus longus and brevis can be palpated and can be traced to the peroneal tubercle where they part, the longus passing below and the brevis passing above.

Tibialis posterior

Tibialis posterior is the deepest muscle on the back of the leg. It arises from the upper half of the lateral aspect of the posterior surface of the tibia below the soleal line, the interosseus membrane, the posterior surface of the fibula between the medial crest and interosseus border, and the fascia covering it posteriorly. The tendon, enclosed in its own synovial sheath, passes behind the medial malleolus grooving it, being medial to flexor hallucis longus and flexor digitorum longus. It lies superficial to the deltoid ligament. Lying inferior to the plantar calcaneonavicular ligament, the tendon passes downward to attach principally to the tubercle on the medial side of the navicular and the plantar surface of the medial cuneiform. Tendinous expansions pass to the plantar surfaces of all the tarsal bones except the talus, although a strip passes back to the tip of the sustentaculum tali, and the bases of the middle three metatarsals.

Nerve supply

Tibialis posterior is supplied by a branch of the tibial nerve, root value L4, 5. The skin over the area on the back of the calf is supplied by root S2.

Action

Tibialis posterior is the main invertor of the foot, acting in conjunction with tibialis anterior. By its attachment to the tubercle of the navicular, it pulls upwards and inwards and therefore rotates the forefoot so that the plantar aspect faces medially. It must be noted that inversion and eversion of the foot involve movement at the midtarsal joint, whereby the navicular and cuboid move on the head of the talus and the calcaneus respectively.

The muscle is also a plantarflexor of the foot at the ankle joint, but its contribution is small; gastrocnemius and soleus are better situated and have a more direct line of action. Nevertheless, if the tendocalcaneus is ruptured, then tibialis posterior can produce plantarflexion. Because of its attachments to both the tibia and fibula, contraction of tibialis posterior will tend to bring the two bones closer together. Consequently, during plantarflexion, the malleoli are approximated to maintain their firm grip on the narrower posterior part of the trochlear surface of the talus.

Functional activity

Tibialis posterior will help to maintain the balance of the tibia on the foot, particularly when body weight is tending to move laterally. Being a strong invertor, it controls the forefoot in walking and running by positioning the foot so that the medial arch is not completely flattened. Its many tendinous expansions help to maintain all the various arches of the foot.

Palpation

It is quite impossible to palpate the belly of the muscle due to the other muscles covering it. It is however quite easy to feel the tendon of the muscle as it passes behind the medial malleolus and particularly as it attaches to the tubercle of the navicular. When lying supine, the tendon can be felt and seen behind the medial malleolus when inversion of the plantarflexed foot against resistance is attempted. From just above the flexor retinaculum to its insertion, it is surrounded by a synovial sheath and it is in this area that the tendon can become quite painful if the muscle has been overactive. The pain is sharp and knife-like and is termed tenosynovitis.

Flexor digitorum longus

Flexor digitorum longus is situated on the back of the calf deep to soleus for most of its course. It arises from the medial part of the posterior surface of the tibia below the soleal line, and from the deep transverse fascia surrounding it. The tendon forms about three fingers’ breadth above the medial malleolus, lying next to that of tibialis posterior, which has crossed anterior to it to come to lie on its medial side, and medial to the tendon of extensor hallucis longus. Passing deep to the flexor retinaculum the tendon lies in its own synovial sheath along the medial aspect of the sustentaculum tali, sometimes grooving it, to enter the sole of the foot deep to abductor hallucis. Passing forwards and laterally, it crosses the tendon of flexor hallucis longus (on its plantar aspect) and usually receives a slip from that tendon which passes into the medial two of its four digitations. About half way along the sole, on its lateral side, the tendon is joined by flexor accessories(quadratus plantae) and at this point breaks up into its four individual tendons; one for each of the lateral four toes. Just distal to the attachment of flexor accessorius (quadratus plantae) the lumbrical muscles arise.

Just distal to the metatarsophalangeal joint, the tendons enter their respective fibrous sheaths, together with the appropriate tendon of flexor digitorum brevis which lies superficial to it. The tendon of brevis then splits to enable that of longus to pass through and reach the plantar surface of the base of the distal phalanx where it inserts. Both tendons share a common synovial sheath.

Nerve supply

Flexor digitorum longus is supplied by the tibial nerve, root value L5, S1, 2. The skin covering this area on the medial and posterior aspect of the calf and the sole is supplied by roots L4, 5, S1.

Action

Flexor digitorum longus flexes the lateral four toes. It flexes the distal interphalangeal joints first, then the proximal interphalangeal joints and finally the metatarsophalangeal joints. Its course behind the medial malleolus means that flexor digitorum longus also helps to plantarflex the foot at the ankle joint. With the ankle plantarflexed, its flexing action on the toes is diminished.

Functional activity

In the propulsive phase of running, jumping or walking, flexor digitorum longus pulls the toes firmly downwards towards the ground to get the maximum grip and thrust during the toe-off phase. When the body is in the standing position, the toes tend to grip the ground to improve the balance.

Palpation

This muscle is very difficult to distinguish as its origin is deep to soleus in the calf, while its tendons in the foot, with the lumbricals, lie deeply. However, with care, the tendon can just be identified as it passes alongside the sustentaculum tali.

Flexor hallucis longus

Flexor hallucis longus is a powerful pinnate muscle situated deep to triceps surae below the deep fascia of the calf. It arises from the lower two-thirds of the posterior surface of the fibula and from the adjacent fascia.

The muscle fibers pass to a central tendon which lies on its superficial surface, with those on the lateral side extending lower. The tendon passes downwards, deep to the flexor retinaculum in its own synovial sheath, to cross the posterior aspect of the ankle joint lying lateral to flexor digitorum longus. During its course, it grooves the lower end of the tibia, the back of the talus (between the medial and posterior tubercles) and the inferior surface of the sustentaculum tali, where it is held in position by a synovial-lined fibrous sheath which forms a tunnel for it to run through.

In the sole of the foot, the tendon lies superficial to the plantar calcaneonavicular ligament lying lateral to the tendon of the flexor digitorum longus. As it passes forwards, the tendon of flexor hallucis longus crosses deep to that of flexor digitorum longus, and in doing so usually gives a slip to its medial two tendons. It then enters the fibrous digital sheath of the great toe, passing between the two sesamoid bones situated on either side of the base of the proximal phalanx, to insert into the plantar surface of the base of the distal phalanx.

Nerve supply

Flexor hallucis longus is supplied by a branch of the tibial nerve, root value S1, 2. The skin covering this area is supplied by root S2.

Action

Flexor hallucis longus flexes all of the joints of the great toe. It first acts on the interphalangeal joint and then the metatarsophalangeal joint. As it crosses the ankle joint, it helps to produce plantarflexion of the foot.

Functional activity

Flexor hallucis longus is of great importance in that it produces much of the final thrust from the foot during walking. At this point in the gait cycle, the calf has already produced its maximum power and the flexors of the lateral four toes are just completing their maximum contraction. Flexion of the great toe is thus the final act before the foot is lifted from the ground ready for the next step. It must also be remembered that the muscle is an important factor in maintaining the medial longitudinal arch.

Palpation

Once again, flexor hallucis longus is almost impossible to palpate as it lies deep to the calf muscles, flexor retinaculum, plantar aponeurosis and the muscles in the foot. Its tendon is set both deep within the calf and the plantar aspect of the foot.