The muscles of the leg are important for movements of the ankle, and for adjustments to changing surfaces we are walking on. There are three distinct groups i.e. an anterior group, a posterior group, and a lateral group. The function of these compartments helps us in activities such as walking, kicking, running and dancing. It is important to consider muscle groups as functional units. The individual muscles in each compartment may have additional functions, but the primary function of each compartment is the most important thing to remember. In this article, we will discuss the gross anatomy, functional anatomy and clinical relevance of the leg muscles.
Anatomy of the leg
Anatomically the leg is defined as the region of the lower limb below the knee. It consists of the posterior, anterior and lateral compartment. There is no medial compartment of the leg (unlike the thigh). The leg is innervated by branches of the sciatic nerve, and receives its blood supply from branches of the popliteal artery. Drainage also occurs from the popliteal vein, and superficial drainage from the small and great saphenous veins.
Anterior (Dorsiflexor) compartment
This compartment contains the tibialis anterior, extensor digitorum longus, extensor hallucis longus and fibularis tertius.
- Tibialis anterior muscle originates from the superior two-thirds of the lateral tibial condyle, and inserts onto the medial cuneiform and first metatarsal of the foot. Its action is inversion of the subtalar joint.
- Extensor digitorum longus muscle originates from the anterior tibial shaft, the superior three-quarters of the interosseus membrane, and the anterior surface of the lateral tibial condyle. It inserts onto the dorsal surface of the middle and distal phalanges of the lateral four toes. Its action is to extend the 2nd to 5th toes.
- Extensor hallucis longus muscle arises from the central portion of the fibula, the interosseus membrane and inserts onto the dorsal surface of the distal phalanx of the hallux. Its action is to extend the hallux.
- Fibularis tertius muscle arises from the lower aspect of the fibular shaft and interosseus membrane and inserts onto the dorsal surface of the fifth metatarsal. Its action is to dorsiflex the ankle.
The motor innervation to the anterior compartment comes from the deep branch of the common fibular nerve (ventral rami of L4-S2).
The blood supply of the anterior compartment is from the anterior tibial artery, a branch of the popliteal artery.
Posterior (Plantarflexor) compartment
Superficial muscle group
This compartment is divided into a deep and superficial group. The superficial group contains the gastrocnemius and soleus muscles. They are also known as the ‘calf muscle pump’, which assists venous return to the heart as the veins of the leg run through the leg.
- Gastrocnemius muscle is a large and strong plantar flexor of the ankle and flexor of the knee. It originates from the dorsal surfaces of the femoral condyles, above their articulating portion.
- Soleus muscle is deep to the gastrocnemius and is another plantar flexor. It arises from the fibula and soleal line on the dorsal surface of the tibia.
Both the gastrocnemius and soleus muscles converge together to form the calcaneal tendon. This tendon then inserts onto the calcaneus tarsal bone, i.e. the heel bone.
Deep muscle group
The deep group consists of the popliteus, flexor digitorum longus, flexor hallucis longus and the tibialis posterior.
- Popliteus muscle arises from the lateral femoral condyle and inserts onto the posterior shaft of the tibia above the soleal line. It causes lateral rotation of the tibia with respect to the femur (unlocks the knee), as the tibia undergoes medial rotation in terminal extension and the anterior cruciate ligament and extrinsic ligaments of the knee (medial collateral, lateral collateral) become taut.
- Flexor digitorum longus muscle arises from the proximal shaft of the tibia and inserts onto the plantar surface of the distal phalanges of the lateral four toes.
- Flexor hallucis longus muscle arises from the dorsal surface of the middle third of the fibula and inserts onto the plantar surface of the base of the distal phalanx of the hallux. The long flexor muscles flex their respective digits.
- Tibialis posterior muscle arises from adjacent parts of the tibia and fibula and medial cuneiform and navicular bone. It plantarflexes the ankle and inverts the subtalar joint.
The motor innervation to the posterior compartment comes from the tibial branch of the sciatic nerve (ventral rami of L4-S3). The muscles of this compartment cause plantar flexion. The tibialis posterior also acts to invert the subtalar joint.
The blood supply to the posterior compartment is from the posterior tibial artery, a branch of the popliteal artery.
Lateral (Fibular) compartment
This compartment consists of the fibularis longus and fibularis brevis muscles.
- Fibularis longus muscle arises from superior lateral shaft of the fibula. Its tendon runs around the lateral aspect of the cuboid bone, runs in a tunnel formed by the long plantar ligament against the inferior surface of the cuboid bone, and eventually inserts onto the first metatarsal and medial cuneiform. It maintains the lateral arch of the foot in this regard.
- Fibularis brevis muscle arises from lower two thirds on the lateral surface of the fibula and inserts onto the 5th metatarsal.
Both these muscles evert the subtalar joint, and plantar flex the ankle (talocrural joint). Inversion and eversion of the ankle is important for our adjustment when walking on uneven surfaces.
The motor innervation to the lateral compartment comes from the superficial branch of the common fibular nerve (ventral rami of L4-S2).
The blood supply to the lateral compartment is from the fibular artery, a branch of the posterior tibial artery.
Acute compartment syndrome: This is a condition in which the pressure in a closed compartment increases to such extent that the microcirculation of the tissues in that compartment is diminished. The resulting inadequate perfusion and oxygenation leads to soft tissue ischemia, anoxia and cell death, especially in skeletal muscle. The most common site of compartment syndrome is the leg. Risk factors include the following:
- trauma, especially the type and site
- revascularization after acute arterial injury or obstruction
- incorrect limb positioning during prolonged surgical procedures
- excessive exercise
Shin Splints/Medial tibial stress syndrome: If the anterior compartment of the leg is repetitively used, the anterior shin adjacent to the tibia becomes painful. This condition is known as shin splints, and commonly affects athletes who engage in running or jumping. It usually affects the middle or lower third of the medial side of the tibia.
Common Fibular Nerve palsy: The common fibular nerve (L4-S2) winds around the neck of the fibula, and divides into the deep and superficial fibular nerves. The nerve runs close to the skin as it winds around the neck of the fibula, and is hence vulnerable to compression from tight clothing or casts on the leg. Symptoms of this nerve palsy include:
- foot drop
- loss of sensation over the anterior and lateral surface of the leg, and the dorsum of the foot.