Lower Limb Muscles: Origins, Insertions, Actions, and Innervation by Compartment

Fascial septa divide the thigh and the leg into compartments, and each compartment has one main nerve and one main job. Learn the compartment and you already know most of what an exam will ask about the muscles inside it. The thigh has three. The anterior compartment extends the knee and is supplied by the femoral nerve. The medial compartment adducts the hip and is supplied by the obturator nerve. The posterior compartment flexes the knee and extends the hip, and is supplied by the sciatic nerve. The leg has three more. The anterior compartment dorsiflexes the ankle (deep fibular nerve), the lateral compartment everts the foot (superficial fibular nerve), and the posterior compartment plantarflexes the ankle and flexes the toes (tibial nerve). The gluteal region sits above them with its own two nerves: the superior gluteal nerve for the abductors that hold the pelvis level, and the inferior gluteal nerve for gluteus maximus. Two habits make the details stick. First, ask which joints a muscle crosses: a muscle that crosses both the hip and the knee, such as rectus femoris or the hamstrings, acts on both. Second, learn the exceptions, because that is where questions come from: the short head of biceps femoris, the hamstring part of adductor magnus, and pectineus all break the one-nerve rule.

Gluteus maximus: origin = posterior ilium behind the posterior gluteal line, the dorsal sacrum and coccyx, and the sacrotuberous ligament. Insertion = mostly the iliotibial tract, with deeper fibers on the gluteal tuberosity of the femur. Action = extends the hip and laterally rotates it. Nerve = inferior gluteal nerve (L5-S2). It does its hardest work when the hip extends from a flexed position, as when you rise from a chair or climb stairs, and contributes little to walking on level ground. Gluteus medius: origin = outer surface of the ilium between the anterior and posterior gluteal lines. Insertion = lateral surface of the greater trochanter. Gluteus minimus: origin = outer ilium between the anterior and inferior gluteal lines. Insertion = anterior surface of the greater trochanter. Both abduct and medially rotate the hip, and both are supplied by the superior gluteal nerve (L4-S1). Their most tested job is keeping the pelvis level when the opposite foot is off the ground. That job explains the Trendelenburg sign. When the superior gluteal nerve or the abductors themselves are weak, standing on the affected leg lets the pelvis drop on the opposite, unsupported side. Walking then produces a lurching gluteal gait as the trunk leans over the weak side to compensate. Tensor fasciae latae: origin = anterior superior iliac spine and the front of the iliac crest. Insertion = iliotibial tract, which continues to the lateral condyle of the tibia. Action = abducts, medially rotates and helps flex the hip, and steadies the extended knee through the tract. Nerve = superior gluteal nerve. Beneath gluteus maximus lie the small lateral rotators: piriformis, obturator internus with the two gemelli, and quadratus femoris. Piriformis is the landmark of the region. It runs from the anterior sacrum through the greater sciatic foramen to the top of the greater trochanter, and the sciatic nerve usually emerges just below it.

Iliopsoas is the strongest hip flexor. Psoas major arises from the bodies, discs and transverse processes of T12 to L5, iliacus from the iliac fossa, and the two insert together on the lesser trochanter. Psoas major is supplied directly by the anterior rami of L1-L3; iliacus is supplied by the femoral nerve. Sartorius: origin = anterior superior iliac spine. Insertion = upper medial surface of the tibia. Action = flexes, abducts and laterally rotates the hip and flexes the knee, which together produce the cross-legged sitting position. Nerve = femoral nerve. It is the longest muscle in the body and the roof of the adductor canal. Quadriceps femoris has four heads that share one insertion: the base of the patella, then the tibial tuberosity through the patellar ligament. Rectus femoris arises from the anterior inferior iliac spine and the ilium just above the acetabulum. It is the only head that crosses the hip, so it flexes the hip as well as extending the knee. Vastus lateralis arises from the greater trochanter and the lateral lip of the linea aspera, vastus medialis from the intertrochanteric line and the medial lip of the linea aspera, and vastus intermedius from the front and lateral side of the femoral shaft. All four extend the knee, and all four are supplied by the femoral nerve (L2-L4). The femoral nerve enters the thigh lateral to the femoral artery under the inguinal ligament. Injury weakens knee extension, so the knee tends to buckle, reduces the patellar reflex (mainly L4), and numbs the front of the thigh and, through the saphenous nerve, the medial side of the leg.

Adductor longus arises from the body of the pubis below the pubic crest and inserts on the middle third of the linea aspera. Adductor brevis lies deep to it, from the body and inferior ramus of the pubis to the upper linea aspera. Both adduct the hip and are supplied by the obturator nerve (L2-L4). Adductor magnus is two muscles in one, which makes it a favorite exam question. Its adductor part arises from the inferior pubic ramus and the ischial ramus, inserts along the gluteal tuberosity and linea aspera, and is supplied by the obturator nerve. Its hamstring part arises from the ischial tuberosity, inserts on the adductor tubercle of the femur, extends the hip like a hamstring, and is supplied by the tibial division of the sciatic nerve. The gap between its two insertions is the adductor hiatus, where the femoral vessels pass to the back of the knee and become the popliteal vessels. Gracilis is the long, thin, most superficial adductor: origin = body and inferior ramus of the pubis; insertion = upper medial surface of the tibia. Because it crosses the knee, it also flexes the knee and medially rotates the leg. Nerve = obturator nerve. Pectineus sits at the top of the group and is the classic dual-supply muscle: the femoral nerve, sometimes with a branch of the obturator nerve. Obturator externus, deep in the group, is supplied by the obturator nerve and laterally rotates the hip. Three tendons meet on the upper medial tibia to form the pes anserinus: sartorius, gracilis and semitendinosus. From front to back, the mnemonic is Say Grace before Tea. Each comes from a different compartment and a different nerve (femoral, obturator and tibial), which is exactly why examiners like it.

Three true hamstrings arise from the ischial tuberosity and cross both the hip and the knee: semitendinosus, semimembranosus and the long head of biceps femoris. All three extend the hip and flex the knee. Semitendinosus inserts on the upper medial surface of the tibia as part of the pes anserinus. Semimembranosus inserts on the back of the medial tibial condyle. With the knee bent, both medially rotate the leg. Biceps femoris inserts on the head of the fibula and laterally rotates the bent knee. The short head of biceps femoris is the exception. It arises from the linea aspera and the lateral supracondylar line, crosses only the knee, and flexes it without acting on the hip. The other hamstrings are supplied by the tibial division of the sciatic nerve; the short head is the only muscle of the thigh supplied by the common fibular division. The sciatic nerve (L4-S3) leaves the pelvis through the greater sciatic foramen, usually below piriformis, runs down between the greater trochanter and the ischial tuberosity under gluteus maximus, and descends deep to the hamstrings. It usually divides into the tibial and common fibular nerves near the top of the popliteal fossa, although the split can happen anywhere higher up.

Anterior compartment (deep fibular nerve, L4-L5). Tibialis anterior: origin = lateral condyle and upper lateral surface of the tibia and the interosseous membrane; insertion = medial cuneiform and the base of the first metatarsal; action = dorsiflexes the ankle and inverts the foot. Extensor hallucis longus extends the great toe, extensor digitorum longus extends the lateral four toes, and fibularis tertius dorsiflexes and helps evert. All of them help lift the foot clear of the ground during the swing phase of walking. Lateral compartment (superficial fibular nerve, L5-S2). Fibularis longus arises from the head and upper lateral fibula, runs behind the lateral malleolus, crosses the sole and inserts on the base of the first metatarsal and the medial cuneiform. Fibularis brevis arises from the lower lateral fibula and inserts on the tuberosity at the base of the fifth metatarsal. Both evert the foot. Posterior compartment, superficial layer (tibial nerve). Gastrocnemius arises by two heads from the lower end of the femur, at and just above the condyles, so it crosses the knee and flexes it as well as plantarflexing the ankle. Soleus arises below the knee, from the upper fibula and the soleal line of the tibia, so it plantarflexes whatever the knee is doing. Both insert on the calcaneus through the calcaneal tendon, the tendon tapped in the ankle reflex (S1). Soleus also squeezes the deep veins during walking, which is why it is called the muscle pump of the leg. Posterior compartment, deep layer (tibial nerve). Popliteus unlocks the extended knee by rotating the femur laterally on the fixed tibia. Tibialis posterior plantarflexes and inverts the foot and supports the medial arch. Flexor digitorum longus and flexor hallucis longus flex the toes. Behind the medial malleolus, from front to back, run tibialis posterior, flexor digitorum longus, the posterior tibial artery and veins, the tibial nerve and flexor hallucis longus: Tom, Dick, And Very Nervous Harry. Two pairings are worth memorizing on their own. Inversion is tibialis anterior plus tibialis posterior. Eversion is fibularis longus plus fibularis brevis.

Common fibular nerve: it winds around the neck of the fibula just under the skin, which makes it the most frequently injured nerve of the lower limb (a fibular neck fracture, a tight cast, prolonged leg crossing). Losing both of its branches causes foot drop, with weak dorsiflexion and eversion, a high-stepping gait and numbness over the lateral leg and the dorsum of the foot. Deep fibular nerve alone: weak dorsiflexion and toe extension with numbness limited to the web space between the first and second toes. Superficial fibular nerve alone: weak eversion with numbness over most of the dorsum of the foot. Tibial nerve: weak plantarflexion and toe flexion, a reduced ankle reflex and numbness of the sole. Femoral nerve: weak knee extension, a reduced patellar reflex, and numbness of the anterior thigh and medial leg. Obturator nerve: weak adduction and a patch of numbness on the medial thigh. Superior gluteal nerve: a positive Trendelenburg sign. Root levels tie these patterns to the spine: L4 for knee extension and the patellar reflex, L5 for great toe extension and hip abduction, and S1 for plantarflexion, eversion and the ankle reflex. Pair this page with the myotome and dermatome maps and lower limb localization questions become a matter of matching three columns: which muscles are weak, which reflex is reduced, and where the skin is numb.