What are the lower extremities?

Structure and Functions

The lower extremities consist of the thighs, lower legs, and feet. Each extremity attaches to the pelvis (innominate bone) at the hip joint. The lower extremity is made mostly of bones and muscles, but it also contains blood vessels, lymphatics, nerves, skin, toenails, and other structures. Important directional terms for the lower extremity include proximal (closer to the base or attached end), distal (further from the base or attached end), medial (on the same side as the tibia and big toe), and lateral(on the same side as the fibula and little toe). Along the foot, the lower surface is called plantar; the upper surface is called dorsal. The lower extremity is clothed in skin (or integument). The sole or plantar surface of the foot is unusual, along with the palm of the hand, in being completely hairless;
it also contains the thickest outer skin layer (the stratum corneum) of any part of the body. Each toe has a hardened toenail on its dorsal surface.

The pelvic girdle that supports the lower extremity develops as three separate bones: the ilium, ischium, and pubis. All three help form the acetabulum, a socket into which the femur fits. Below the acetabulum, the ischium and pubis surround a large opening called the obturator foramen. The right and left pubis meet to form a pubic symphysis. The bones of the lower extremity include the femur, tibia, fibula, tarsals, metatarsals, and phalanges. The femur (thigh bone) is the largest bone in the body. Its rounded upper end, or head, fits into the acetabulum and is attached by a short neck. A rough-surfaced greater trochanter lies just beyond this neck and serves for the attachment of many muscles. The lesser trochanter, also for muscle attachments, lies just below the neck. The knee joint is covered and protected by the kneecap, or patella, the largest of the sesamoid bones formed within tendons at points of stress. The lower leg, from the knee to the ankle, contains two bones: the tibia on the medial side and the more slender fibula on the lateral side. The tarsus, or ankle, includes the talus, calcaneus, and five smaller bones. The talus (or astragalus)
has a pulleylike facet for the tibia and other curved surfaces for articulation with the calcaneus and navicular. The calcaneus, or heel bone, is vertically enlarged in humans; the Achilles tendon attaches to its roughened lower tuberosity. Smaller tarsal bones include the navicular, the medial (or inner) cuneiform, the intermediate cuneiform, the lateral (or outer) cuneiform, and the cuboid. Beyond the tarsal bones, the foot is supported by five
metatarsal bones. The big toe, or hallux, contains two phalanges; each of the remaining toes contains three phalanges.

The muscles of the lower extremity include extensors, which straighten joints, and flexors, which bend joints. Abductor muscles move the limbs sideways, away from the midline, while adductors pull the limbs back, toward the midline. The muscles of the iliac region attach the lower extremity to the body. The psoas major runs from the lumbar vertebrae to the lesser trochanter of the femur. The iliacus runs from the ilium and part of the sacrum to the femur, including the lesser trochanter. The anterior muscles of the thigh include the sartorius, the quadriceps femoris, and the articularis genus. The sartorius, the longest muscle in the body, flexes both hip and knee joints. It runs obliquely from the anterior border of the ilium across the front of the thigh to insert onto the medial side of the knee at the upper end of the tibia. The quadriceps femoris consists of the rectus femoris and the three vastus muscles; all four are strong extensors of the knee. The rectus femoris originates from the region surrounding the acetabulum. The vastus lateralis, vastus medialis, and vastus intermedius muscles all originate along the shaft of the femur. All four quadriceps muscles insert onto a common tendon that runs over the knee and inserts onto the top of the tibia. The patella is a sesamoid bone enclosed within this tendon where it runs over the front of the knee. The smaller articularis genus muscle originates on the anterior side of the shaft of the femur; it inserts onto the kneecap.

The extensor muscles of the hip and thigh help to maintain upright posture. The gluteus maximus, the largest of these muscles, originates from the posterior portion of the ilium and inserts high on the femur, especially onto the greater trochanter. The gluteus medius and gluteus minimis both originate from the outer surface of the ilium and insert onto the greater trochanter. The tensor fasciae latae originates along the iliac crest; it inserts onto a broad, sheetlike tendon (the fascia lata) which covers much of the lateral surface of the thigh. The piriformis runs from the sacrum to the greater trochanter of the femur. The obturator internus runs from the inner surface of the pelvis through the obturator foramen to the greater trochanter of the femur. The gemellus superior and the gemellus inferior originate from the rear margin of the ischium; they both insert onto the greater trochanter. The quadratus femoris originates from the lateral surface of the ischium; it inserts between the greater and lesser trochanters of the femur. The obturator externus originates along the outer surface of the pelvis below the obturator foramen and inserts near the greater
trochanter.

The muscles on the medial (or inner) side of the thigh are all abductors of the thigh. The gracilis is a long, thin muscle that originates from the pubis, runs along the medial side of the thigh, and inserts high on the tibia. The pectineus originates anteriorly on the pubis and inserts onto the shaft of the femur below the lesser trochanter. The adductor longus originates from the pubis and inserts onto the posterior edge of the femur. The adductor brevis originates from the pubis and inserts onto the posterior edge of the femur. The adductor magnus is a large, triangular muscle that originates from the lower portion of the ischium and pubis; it expands to a long, thin insertion along the posterior edge of the femur.

The hamstring muscles run along the posterior side of the femur; they flex the knee and extend the hip joint. The biceps femoris originates from the posterior portion (the tuberosity) of the ischium and separately from the posterior edge of the femur. Both portions converge onto a common tendon that inserts primarily onto the top of the fibula. The semitendinosus originates from the posterior end of the ischium; it inserts by a long tendon onto the medial side of the tibia. The semimembranosus runs from the ischium to the posterior surface of the tibia.

The muscles on the front (anterior) side of the lower leg raise the foot by flexing it dorsally. At the ankle, their tendons are all held in place by two transverse bands, the extensor retinacula. The tibialis anterior originates along the anterior edge of the tibia; it inserts by a tendon onto the medial cuneiform and the base of the first metatarsal. The extensor hallucis longus originates from the anterior surface of the fibula; its tendon passes beneath the extensor retinacula to insert onto the distal phalanx of the big toe. The extensor digitorum longus originates near the top of the tibia and along the anterior side of the fibula. Its tendon passes beneath the extensor retinacula and splits into four tendons, inserted onto the second and third phalanges of the second through fifth digits. The peroneus tertius originates along the anterior edge of the fibula and runs alongside the extensor digitorum longus. It inserts onto the base of the fifth metatarsal bone.

The muscles on the posterior surface of the lower leg are mostly extensors of the foot; some also flex the knee. The gastrocnemius originates in two heads from opposite sides of the femur. It inserts onto the Achilles tendon, which attaches to the calcaneus. The soleus originates from the posterior surface of the fibula; it inserts onto the Achilles tendon. The plantaris originates from the posterior surface of the femur and inserts onto the posterior portion of the calcaneus. The popliteus runs from the lateral side of the femur across the back of the knee to insert onto the tibia. The flexor hallucis longus originates along the posterior surface of the fibula; its tendon runs around to the medial side of the ankle and inserts onto the base of the big toe. The flexor digitorum longus originates from the posterior surface of the tibia; its tendon crosses the sole of the foot obliquely and divides into four tendons that insert onto the distal phalanges of the second through fifth toes. The tibialis posterior originates from the posterior surfaces of the tibia, the fibula, and the interosseous membrane that joins them;
its tendon passes around to insert onto the navicular bone. The peroneus longus originates along the lateral surface of the fibula; its tendon runs along a groove on the bottom of the cuboid to insert obliquely onto the base of the first metatarsal. The peroneus brevis originates along the lateral margin of the fibula; its tendon inserts onto the fifth metatarsal. The extensor digitorum brevis originates from the calcaneus and runs obliquely across the dorsal side of the foot, dividing into four tendons. One tendon inserts onto the base of the big toe; the remaining tendons insert onto the tendons of the extensor digitorum longus.

Several flexor muscles of the foot are attached to the plantar aponeurosis, a flat ligament that runs from the calcaneus along the sole of the foot to the bases of the toes and to several flexor tendons. The abductor hallucis originates from the calcaneus and the plantar aponeurosis; it inserts onto the base of the big toe. The flexor digitorum brevis originates from the plantar aponeurosis and the calcaneus; it divides into four portions, each of which gives rise to a tendon. These tendons run into the second through fifth toes, each splitting in half to insert onto opposite sides of the second phalanx, separated by the tendons of the flexor digitorum longus, which emerge between them. The abductor digiti quinti originates from the calcaneus and the plantar aponeurosis; it inserts onto the base of the fifth toe. The quadratus plantae originates from the calcaneus and inserts onto the tendons of the flexor digitorum longus. The four small lumbricals run from the tendons of the flexor digitorum longus to the corresponding tendons of the extensor digitorum longus. The flexor hallucis brevis originates from the cuboid and lateral cuneiform bones; its two portions insert onto the big toe from opposite sides. The adductor hallucis originates from the second through fourth metatarsals and also
from the bases of the third through fifth toes. Its tendon inserts onto the base of the big toe. The flexor digiti quinti originates from the base of the fifth metatarsal and inserts onto the base of the fifth toe. The four dorsal interossei originate from the bases of the metatarsal bones; they insert onto the bases of the second through fourth toes. The three plantar interossei originate from the third through fifth metatarsals and run beneath these bones to insert onto the bases of the corresponding toes.

Blood vessels of the lower extremity include both arteries and veins. The common iliac arteries arise from the dorsal aorta; each divides into an internal and an external iliac. The internal iliac artery supplies many muscles of the thigh region and pelvis. The external iliac artery branches into an inferior epigastric artery and a deep iliac circumflex artery; it then continues along the femur as the femoral artery. The femoral artery gives rise to a deep femoral artery running to the medial and posterior regions of the thigh; the base of this artery also gives rise to two circumflex arteries that send branches upward into many thigh muscles. Near the knee, the femoral artery branches into a descending geniculate artery to the knee, then continues as the popliteal artery, forming several branches to the thigh muscles and other small branches to the knee before splitting into anterior and posterior tibial arteries.

The anterior tibial artery descends along the front of the tibia, forming several small branches. It then continues into the foot as the dorsalis pedis artery, giving rise to a lateral tarsal artery and an arcuate artery, both of which form arches by joining with branches of the peroneal artery. The deep plantar artery and hallucis dorsalis artery also branch from the dorsalis pedis artery, while individual arteries to the second through fourth metatarsals arise from the arcuate artery. Arterial branches to all the toes arise from the individual metatarsal arteries, including the hallucis dorsalis, forming a system of collateral
circulation in which multiple alternate routes permit blood flow even if one of the routes is temporarily blocked.

The posterior tibial artery gives rise to a peroneal artery; the two arteries then run down the posterior side of the lower leg, forming small branches to the muscles of the lower leg and nutrient arteries to the tibia and fibula. The posterior tibial artery branches to the calcaneus before it splits into a medial plantar artery, which runs along the medial margin of the foot into the big toe, and a much larger lateral plantar artery. The lateral plantar artery runs across the foot obliquely to the lateral side, then turns and runs obliquely in the other direction to the base of the big toe, where it runs into the deep plantar artery to form a loop. From this loop arise a series of plantar metatarsal arteries to all five toes. Blood can reach each toe from either side, and the arch that supplies this blood can receive its blood either by way of the posterior tibial and lateral plantar arteries or by way of the anterior tibial and deep plantar arteries, providing another example of collateral circulation.

There are several important
veins draining the lower extremity. The deep veins originate from a series of plantar digital veins draining the individual toes into a deep plantar venous arch. This arch is drained to either direction by a lateral plantar vein and a medial plantar vein, which later unite to form a posterior tibial vein; this vein and the peroneal vein run parallel to the corresponding arteries along the posterior side of the lower leg. An anterior tibial vein drains the anterior side of the lower leg and the dorsal side of the foot. Near the knee, the peroneal vein and the anterior and posterior tibial veins unite to form the popliteal vein, which continues into the thigh as the femoral vein. The femoral vein receives the deep femoral vein as a tributary, then the saphenous vein. The femoral vein then continues as the external iliac vein.

The lower extremity is also covered with a network of superficial veins that lie just beneath the skin. The vessels of this network are drained along the medial side of the lower leg and thigh by the great saphenous vein, which runs into the femoral vein just below the groin. The lateral side of the foot and the posterior surface of the lower leg are drained by the small saphenous vein, which drains into the popliteal vein.

The nerves to the lower extremity arise from two series of complex branchings, the lumbar plexus and sacral plexus. The largest nerve formed from the lumbar plexus is the femoral nerve, supplying muscles on the anterior side of the thigh and part of the lower leg. Other branches to the muscles include the obturator nerve to the adductor muscles and separate muscular branches to the psoas and iliacus muscles. Cutaneous sensory nerves to the skin include the lateral femoral cutaneous nerve to the lateral side of the thigh, the anterior cutaneous branches of the femoral nerve to the medial side of the thigh, and the saphenous nerve, a branch of the femoral nerve to the medial side of the lower leg.

The sacral plexus gives rise to the very large sciatic nerve and to several smaller nerves, including the superior gluteal and inferior gluteal nerves to the gluteal muscles, and separate muscular branches to the piriformis, quadratus femoris, obturator internus, and gemelli. Cutaneous branches such as the posterior femoral cutaneous nerve supply sensory fibers to the skin on the posterior surface of the thigh. The sciatic nerve, the largest nerve in the body, branches off to the hamstring muscles before splitting into tibial and peroneal nerves. The tibial nerve supplies the muscles on the posterior side of the lower leg and then runs onto the sole of the foot, where it splits into the medial and lateral plantar nerves, which together supply both cutaneous sensation and muscular innervation to the sole of the foot. The peroneal nerve divides into deep and superficial portions. The deep peroneal nerve supplies the muscles on the anterior side of the lower leg and the dorsal surface of the foot. The superficial peroneal nerve supplies cutaneous sensation to the lateral surface of the lower leg and the dorsal surface of the foot.

Disorders and Diseases

Many medical conditions and disorders affect the lower extremity; these include animal bites (including snakebites), injuries, fungus infections such as athlete’s foot, contact dermatitis (including poison ivy), and an assortment of neuromuscular disorders, including nerve paralyses, muscular atrophies, and muscular dystrophies. Nerve paralyses of the lower extremities usually arise from traumatic injury.

Muscular atrophies are diseases in which muscle tissues become progressively weaker and smaller, usually beginning after the age of forty. Spastic movements sometimes occur. The small muscles of the hands and feet are usually affected sooner and more severely in comparison to the larger muscles of the legs and thighs. Amyotrophic lateral sclerosis (ALS), commonly known as Lou Gehrig’s disease, is one such disease that usually begins with weakness and deterioration of the distal muscles. The disease proceeds to affect the rest of the extremities, then other parts of the body; it is usually fatal within three to five years after onset. A more rare type of atrophy, myelopathic muscular atrophy (or Aran-Duchenne atrophy), affects both upper and lower extremities and eventually spreads to the trunk. A degenerative lesion of the gray matter in the cervical region of the spinal cord is usually responsible.

Muscular dystrophy is a series of inherited diseases that begin in early childhood, affecting males more often than females. The most common type,
Duchenne muscular dystrophy, is caused by a sex-linked recessive trait that impairs the body’s ability to synthesize a large protein called dystrophin. Muscular dystrophy primarily affects the large muscles of the thigh and lower leg, impairing the ability to stand unassisted or to walk. The affected muscles become very weak but remain approximately normal in size and may even increase as muscle tissue is replaced by fatty and fibrous tissue. Progressive weakening makes walking and similar motor functions impossible, but, with proper care, patients can live for decades.

Sports injuries often occur in the lower extremities and are generally treated by orthopedic specialists. Fractured bones are generally set in casts and kept immobile until they heal. Injured or ruptured ligaments often require surgical treatment. Snakebites and other animal bites occur more often to the lower extremities than to other parts of the body. The bites of poisonous snakes must be treated quickly, before the venom reaches the heart. The patient must be kept calm and quiet, and experienced medical attention should be sought as soon as possible.

Agur, Anne M. R., and Arthur F. Dalley. Grant’s Atlas of Anatomy. 13th ed. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins, 2013.

Brummett, Chad M., and Steven P. Cohen. Managing Pain: Essentials of Diagnosis and Treatment. New York: Oxford University Press, 2013.

Crouch, James E. Functional Human Anatomy. 4th ed. Philadelphia: Lea & Febiger, 1985.

Currey, John D. Bones: Structures and Mechanics. 2d ed. Princeton, N.J.: Princeton University Press, 2006.

Iyer, K. Mohan. Orthopedics of the Upper and Lower Limb. New York: Springer, 2013.

Kim, Daniel H., Alan R. Hudson, and David G. Kline. Atlas of Peripheral Nerve Surgery. Philadelphia: Elsevier/Saunders, 2013.

Marieb, Elaine N. Essentials of Human Anatomy and Physiology. 10th ed. San Francisco: Pearson/Benjamin Cummings, 2012.

Rosse, Cornelius, and Penelope Gaddum-Rosse. Hollinshead’s Textbook of Anatomy. 5th ed. Philadelphia: Lippincott-Raven, 1997.

Standring, Susan, et al., eds. Gray’s Anatomy. 40th ed. New York: Churchill Livingstone/Elsevier, 2008.

Van De Graaff, Kent M. Human Anatomy. 6th ed. New York: McGraw-Hill, 2002.