Lower Limb Muscles

Lower Limb Muscles

Fascial Compartments

The lower limb muscles are organised into fascial compartments. The thigh has three compartments: anterior (quadriceps + sartorius; femoral nerve), medial (adductors + gracilis; obturator nerve), and posterior (hamstrings; sciatic nerve). The leg has four compartments: anterior (dorsiflexors; deep fibular nerve), lateral (evertors; superficial fibular nerve), superficial posterior (plantarflexors; tibial nerve), and deep posterior (long toe flexors + tibialis posterior; tibial nerve). The fascia lata envelops the thigh; the iliotibial tract (IT band) is its thickened lateral portion. Crural fascia envelops the leg.

Hip Muscles

Iliopsoas (iliacus + psoas major) is the primary hip flexor; it originates from the iliac fossa and lumbar vertebrae and inserts onto the lesser trochanter. Gluteus maximus (inferior gluteal nerve, L5–S2) is the strongest hip extensor; it inserts into the gluteal tuberosity and iliotibial tract and is active during stair climbing and running. Gluteus medius and minimus (superior gluteal nerve, L4–S1) abduct the hip and medially rotate it; they are critical for stabilising the pelvis during single-leg stance. When these muscles are weak or paralysed, the Trendelenburg sign is positive: the pelvis drops to the contralateral side during single-leg stance on the affected side.

Thigh Muscles

Quadriceps femoris (rectus femoris, vastus lateralis, vastus medialis, vastus intermedius) extends the knee; the common tendon inserts onto the tibial tuberosity via the patella and patellar ligament. The femoral nerve supplies all four. Hamstrings (biceps femoris, semitendinosus, semimembranosus) flex the knee and extend the hip; they originate from the ischial tuberosity. Biceps femoris also laterally rotates the leg; semimembranosus and semitendinosus medially rotate it. All are supplied by the sciatic nerve.

Leg Muscles

Gastrocnemius (two heads from femoral condyles) and soleus (from fibula and tibia) together form the triceps surae and plantarflex the ankle via the calcaneal (Achilles) tendon. Gastrocnemius also flexes the knee. Tibialis anterior (anterior compartment) dorsiflexes and inverts the foot. Tibialis posterior (deep posterior) inverts and plantarflexes; important for maintaining the medial arch. Fibularis (peroneus) longus and brevis (lateral compartment) evert the foot and weakly plantarflex; fibularis longus also supports the transverse arch. Compartment syndrome occurs when pressure within a fascial compartment exceeds perfusion pressure; the 5Ps (pain, pallor, paraesthesia, paralysis, pulselessness) signal impending ischaemia — fasciotomy is the emergency treatment.