Thigh
The thigh is enclosed by the fascia lata, a strong deep fascial stocking, thickened laterally as the iliotibial tract.
- Three intermuscular septa (medial, lateral, and a posterior septum) extend from the fascia lata to the linea aspera of the femur, dividing the thigh into anterior, medial, and posterior compartments.
- Anterior compartment: extensors of the knee/flexors of the hip, supplied by the femoral nerve.
- Medial compartment: adductors of the hip, supplied by the obturator nerve.
- Posterior compartment: flexors of the knee/extensors of the hip (hamstrings), supplied by the sciatic nerve (tibial division, mostly).
Iliopsoas (Iliacus + Psoas Major)
| Muscle | Origin | Insertion | Innervation | Action |
|---|---|---|---|---|
| Psoas major | Transverse processes and bodies of T12–L5 vertebrae | Lesser trochanter of femur (joins iliacus) | Ventral rami of L1–L3 (direct branches, not via a named peripheral nerve) | Hip flexion; trunk flexion when the limb is fixed |
| Iliacus | Iliac fossa | Lesser trochanter (with psoas major tendon) | Femoral nerve (L2, L3) | Hip flexion |
- Together form the primary hip flexors; psoas major also acts on the lumbar spine.
- Relations: pass beneath the inguinal ligament into the thigh; psoas major is retroperitoneal, closely related to the lumbar plexus (which forms within its substance) and the ureter.
Sartorius
- Origin: ASIS.
- Insertion: Pes anserinus, proximal medial tibia (with gracilis and semitendinosus).
- Innervation: Femoral nerve (L2, L3).
- Action: Flexes, abducts, and laterally rotates the hip; flexes the knee, the classic "sit cross-legged" or tailor's position movement (longest muscle in the body).
- Relations: Forms the lateral border of the femoral triangle; roofs the adductor canal.
Quadriceps Femoris
| Muscle | Origin | Insertion | Innervation | Action |
|---|---|---|---|---|
| Rectus femoris | Straight head: AIIS; reflected head, superior acetabular rim | Common quadriceps tendon → patella → patellar ligament → tibial tuberosity | Femoral nerve (L2–L4) | Knee extension; hip flexion (only quadriceps component crossing the hip) |
| Vastus lateralis | Greater trochanter, linea aspera (lateral lip) | Common quadriceps tendon | Femoral nerve (L2–L4) | Knee extension |
| Vastus medialis | Intertrochanteric line, linea aspera (medial lip) | Common quadriceps tendon (distal fibres: VMO, stabilise the patella medially) | Femoral nerve (L2–L4) | Knee extension; medial patellar stabilisation |
| Vastus intermedius | Anterior/lateral shaft of femur | Common quadriceps tendon | Femoral nerve (L2–L4) | Knee extension |
- Collectively the primary knee extensor group, converging on the patella via the quadriceps tendon, continuing as the patellar ligament to the tibial tuberosity.
- Articularis genus: small slips of vastus intermedius, insert on the suprapatellar bursa/synovial capsule, retract the capsule during extension to prevent pinching.
Pectineus
- Origin: Pectineal line of pubis.
- Insertion: Pectineal line of femur (between lesser trochanter and linea aspera).
- Innervation: Femoral nerve (L2, L3), dual/variable innervation, occasionally also receives a branch from the obturator nerve.
- Action: Hip flexion and adduction.
- Relations: Forms part of the floor of the femoral triangle; transitional muscle between anterior and medial compartments both anatomically and functionally.
Femoral Nerve
- Origin: Posterior divisions of the lumbar plexus, L2–L4.
- Course: Emerges from the lateral border of psoas major, descends in the groove between psoas major and iliacus, passes deep to the inguinal ligament (lateral to the femoral sheath, NOT within it), divides into anterior and posterior divisions within the femoral triangle.
- Motor supply: Anterior thigh muscles, iliacus, pectineus (variable), sartorius, quadriceps femoris.
- Sensory supply: Anterior/medial thigh (via anterior cutaneous branches) and the medial leg/foot (via its terminal branch, the saphenous nerve).
- Full detail (branches, lesion patterns) covered in the Peripheral Nerves section.
Femoral Artery
- Origin: Continuation of the external iliac artery at the inguinal ligament (midinguinal point).
- Course: Descends through the femoral triangle, then the adductor canal, exiting via the adductor hiatus to become the popliteal artery.
- Key branch in the thigh: Profunda femoris (deep femoral) artery: largest branch, arises just distal to the inguinal ligament, gives medial and lateral circumflex femoral arteries and perforating branches supplying the thigh musculature and contributing to the trochanteric/cruciate anastomoses.
- Full detail covered in the Blood Supply section.
| Muscle | Origin | Insertion | Innervation | Action |
|---|---|---|---|---|
| Adductor longus | Body of pubis (anterior) | Middle third of linea aspera | Obturator nerve (L2–L4) | Hip adduction; assists flexion |
| Adductor brevis | Body and inferior ramus of pubis | Upper linea aspera (proximal to adductor longus) | Obturator nerve (L2–L4) | Hip adduction; assists flexion |
| Adductor magnus | Adductor part: ischiopubic ramus; Hamstring part, ischial tuberosity | Adductor part: linea aspera, medial supracondylar line; Hamstring part, adductor tubercle | Adductor part: obturator nerve (L2–L4); Hamstring part, tibial division of sciatic nerve (L4) | Powerful hip adduction (largest adductor); hamstring part assists hip extension |
| Gracilis | Body and inferior ramus of pubis | Pes anserinus, proximal medial tibia | Obturator nerve (L2, L3) | Hip adduction; knee flexion (weak); medial rotation of flexed knee |
| Obturator externus | External surface of obturator membrane and surrounding bone | Trochanteric fossa of femur | Obturator nerve (L3, L4) | Lateral rotation of the hip; minor adduction |
- Adductor magnus is functionally and developmentally dual, its adductor part is innervated like the rest of the adductor group (obturator nerve), while its hamstring part shares tibial nerve innervation with the true hamstrings, reflecting its intermediate position between the medial and posterior compartments.
- Adductor hiatus: gap in the distal adductor magnus insertion, transmitting the femoral vessels from the adductor canal into the popliteal fossa.
Obturator Nerve
- Origin: Anterior divisions of the lumbar plexus, L2–L4.
- Course: Descends within psoas major, exits the pelvis via the obturator canal (superior part of the obturator foramen), divides into anterior and posterior branches within the thigh (separated by adductor brevis).
- Motor supply: Adductor longus, adductor brevis, adductor part of adductor magnus, gracilis, obturator externus.
- Sensory supply: Small area of skin over the middle medial thigh (variable, often minimal clinically).
- Full detail covered in the Peripheral Nerves section.
| Muscle | Origin | Insertion | Innervation | Action |
|---|---|---|---|---|
| Biceps femoris (long head) | Ischial tuberosity | Head of fibula | Tibial division of sciatic nerve (L5, S1, S2) | Knee flexion, hip extension; lateral rotation of flexed knee |
| Biceps femoris (short head) | Linea aspera, lateral supracondylar line | Head of fibula (joins long head tendon) | Common fibular division of sciatic nerve (L5, S1, S2) | Knee flexion only (does not cross the hip) |
| Semitendinosus | Ischial tuberosity | Pes anserinus, proximal medial tibia | Tibial division of sciatic nerve (L5, S1, S2) | Knee flexion, hip extension; medial rotation of flexed knee |
| Semimembranosus | Ischial tuberosity | Posteromedial tibial condyle (with expansions forming the oblique popliteal ligament) | Tibial division of sciatic nerve (L5, S1, S2) | Knee flexion, hip extension; medial rotation of flexed knee |
- Collectively the hamstrings, all originating from the ischial tuberosity (except biceps femoris short head).
- Biceps femoris short head is the exception to the "all hamstrings are tibial-innervated" rule: it is supplied by the common fibular division, a clinically and embryologically important distinction (its actions and development pattern with the posterior compartment despite different innervation).
- Hamstring part of adductor magnus: see Medial Compartment; functions with the hamstrings in hip extension despite obturator nerve innervation of its main (adductor) part.
Sciatic Nerve (in the Thigh)
- Descends through the posterior thigh, deep to the long head of biceps femoris, typically splitting into tibial and common fibular nerves at or above the apex of the popliteal fossa (variable level; occasionally splits within the pelvis).
- Supplies all hamstring muscles (and the hamstring part of adductor magnus) in the thigh, plus (via its terminal branches) the entire leg and foot below the knee.
- Full detail covered in the Peripheral Nerves section.
- Medial intermuscular septum: separates anterior and medial compartments, gives partial origin to vastus medialis.
- Lateral intermuscular septum: separates anterior and posterior compartments, gives origin to vastus lateralis and short head of biceps femoris; the site where the profunda femoris perforating branches pierce to reach the posterior compartment.
- Posterior septum (less well-defined), separates medial and posterior compartments.
- Key relation: The femoral vessels travel through the anterior compartment (via the femoral triangle and adductor canal) but reach the posterior knee region (popliteal fossa) by passing through the adductor hiatus in adductor magnus, a functional "bridge" between compartments.
- Sciatic nerve lies posterior throughout the thigh, deep to the hamstrings, making it relatively protected from anterior thigh trauma but vulnerable to posterior approaches (surgical, injection-related) and to posterior hip dislocation.
