Anatomy of the Lower Extremity: Compartments, Muscles, Functions, Innervation, and Blood Supply
Overview of Compartmental Organization
The lower extremity is organized into distinct fascial compartments that contain specific muscle groups, each with dedicated neurovascular supply. Understanding this anatomy is critical for recognizing compartment syndrome, which requires immediate fasciotomy to prevent tissue necrosis and amputation 1.
THIGH COMPARTMENTS
Anterior (Extensor) Compartment of the Thigh
Muscles:
- Quadriceps femoris group (four muscles):
- Rectus femoris (crosses both hip and knee)
- Vastus lateralis
- Vastus medialis
- Vastus intermedius
- Sartorius (longest muscle in the body)
- Iliopsoas (iliacus + psoas major)
Primary Functions:
- Hip flexion (rectus femoris, iliopsoas, sartorius) 2
- Knee extension (all quadriceps components) 2
- The quadriceps provides work avoidance during weight-bearing through isometric contraction, promoting horizontal translation of the center of mass 2
Innervation:
- Femoral nerve (L2-L4) supplies all anterior thigh muscles 3
Arterial Supply:
- Femoral artery and its branches (profunda femoris, lateral and medial circumflex femoral arteries) 4
- The superficial femoral artery continues as the popliteal artery through the adductor canal 4
Medial (Adductor) Compartment of the Thigh
Muscles:
- Adductor longus
- Adductor brevis
- Adductor magnus (has dual innervation)
- Gracilis
- Obturator externus
Primary Functions:
- Hip adduction (all muscles)
- Hip flexion assistance (adductor longus and brevis)
- Hip extension (posterior portion of adductor magnus)
Innervation:
- Obturator nerve (L2-L4) supplies most medial compartment muscles 3
- Tibial portion of sciatic nerve supplies the posterior (hamstring) portion of adductor magnus
Arterial Supply:
- Profunda femoris artery (deep femoral artery) and its perforating branches 4
- Obturator artery (minor contribution)
Posterior (Flexor) Compartment of the Thigh
Muscles (Hamstrings):
- Biceps femoris (long and short heads)
- Semitendinosus
- Semimembranosus
Primary Functions:
- Hip extension (all hamstrings except short head of biceps femoris) 2
- Knee flexion (all hamstrings) 2
- Critical dual function: The hamstrings provide economical work supply through eccentric and concentric contractions throughout stance phase while simultaneously contributing to work avoidance through antagonistic coactivation with quadriceps—this resolves Lombard's paradox 2
Innervation:
- Tibial division of sciatic nerve (L5-S2) supplies semitendinosus, semimembranosus, and long head of biceps femoris
- Common fibular (peroneal) division of sciatic nerve supplies short head of biceps femoris
Arterial Supply:
- Profunda femoris artery via perforating branches 4
- The semimembranosus pump pushes blood from deep femoral veins into the common femoral vein 5
LEG (BELOW KNEE) COMPARTMENTS
Anterior Compartment of the Leg
Muscles:
- Tibialis anterior (primary dorsiflexor)
- Extensor hallucis longus
- Extensor digitorum longus
- Fibularis (peroneus) tertius
Primary Functions:
- Ankle dorsiflexion (tibialis anterior is dominant) 6
- Toe extension (extensor hallucis longus and extensor digitorum longus)
- Ankle inversion (tibialis anterior)
- The tibialis anterior has a physiological cross-sectional area of 19 cm² with maximum anatomical cross-sectional area of 9 cm² 6
Innervation:
- Deep fibular (peroneal) nerve (L4-S1) supplies all anterior compartment muscles 3
Arterial Supply:
- Anterior tibial artery (continuation of popliteal artery) 3
- Continues as dorsalis pedis artery in the foot 3
Clinical Pitfall:
- Absence or asymmetry of dorsalis pedis pulse should raise suspicion for peripheral arterial disease, though palpable pulses do not exclude significant ischemia due to collateral circulation 3
Lateral Compartment of the Leg
Muscles:
- Fibularis (peroneus) longus
- Fibularis (peroneus) brevis
Primary Functions:
- Ankle eversion (primary evertors)
- Weak plantarflexion assistance
- Lateral ankle stabilization
Innervation:
- Superficial fibular (peroneal) nerve (L5-S1) supplies both lateral compartment muscles
Arterial Supply:
- Fibular (peroneal) artery (branch of posterior tibial artery)
- The lateral veins of the soleus drain vertically into the fibular veins 5
Superficial Posterior Compartment of the Leg
Muscles:
- Gastrocnemius (medial and lateral heads)
- Soleus
- Plantaris (small, often absent)
Primary Functions:
- Powerful ankle plantarflexion (all muscles) 5
- Knee flexion (gastrocnemius only, as it crosses the knee joint)
- The calf pump mechanism: The medial gastrocnemius veins form a large collector that ends uniquely above the knee joint in the popliteal vein, creating a powerful jet effect during muscular systole that generates a Venturi aspiration effect on deep veins below 5
Muscle Architecture:
- Soleus volume: 489 cm³ with physiological cross-sectional area of 230 cm² (largest plantarflexor) 6
- Medial gastrocnemius: 245 cm³ volume, 68 cm² PCSA 6
- Lateral gastrocnemius: 140 cm³ volume, 28 cm² PCSA 6
- The soleus has unusually short fibers relative to muscle volume, potentiating its force generation capacity 6
Innervation:
- Tibial nerve (S1-S2) supplies all superficial posterior compartment muscles 7
- The soleus receives dual innervation: ramus anterior supplies the portio anterior (bipenniform part unique to humans) and portions of portio posterior through anastomoses with ramus posterior 7
Arterial Supply:
- Posterior tibial artery (larger terminal branch of popliteal artery) 3
- Fibular (peroneal) artery contribution
Deep Posterior Compartment of the Leg
Muscles:
- Tibialis posterior
- Flexor digitorum longus
- Flexor hallucis longus
- Popliteus
Primary Functions:
- Ankle plantarflexion (tibialis posterior is primary)
- Ankle inversion (tibialis posterior)
- Toe flexion (flexor digitorum longus and flexor hallucis longus)
- Knee flexion and unlocking (popliteus)
Innervation:
- Tibial nerve (L4-S1) supplies all deep posterior compartment muscles 8
Arterial Supply:
- Posterior tibial artery (main supply) 3
- Fibular (peroneal) artery
- The medial veins of the soleus join the posterior tibial veins horizontally 5
Anatomical Variations:
- Variant fascial attachments of flexor digitorum longus can contribute to chronic exertional compartment syndrome and tarsal tunnel syndrome 8
- Supernumerary muscles, thickened fascial bands, or variant nerve and vessel courses may manifest as acute or chronic conditions 8
CRITICAL CLINICAL CONSIDERATIONS
Compartment Syndrome Recognition
Monitoring is essential after revascularization in acute limb ischemia, as reperfusion injury causes cellular edema within fascial compartments, elevating compartment pressures and leading to tissue necrosis 1. The lower leg is by far the most common site, though thigh compartment syndrome can occur 1.
Key diagnostic features:
- History of prolonged ischemia duration 1
- Elevated serum creatine kinase 1
- Elevated compartment pressures on measurement 1
- Physical examination findings consistent with compartment syndrome 1
Immediate intervention:
- Prophylactic fasciotomy is reasonable in patients with acute limb ischemia category IIa or IIb (threatened but salvageable limb) based on clinical findings 1
- Prompt fasciotomy of all involved compartments prevents devastating complications including tissue necrosis, infection, limb amputation, and systemic metabolic toxicity 1
Vascular Assessment Pitfalls
Do not rely exclusively on palpable pedal pulses to rule out peripheral arterial disease; significant ischemia can exist despite palpable pulses due to collateral circulation 3. An ankle-brachial index (ABI) <0.9 confirms PAD, while ABI >1.3 suggests arterial calcification and mandates toe-brachial index measurement 3. A toe-brachial index ≥0.75 effectively excludes clinically significant PAD 3.
Venous Pump Function
The veno-muscular pumps create a chain of events during walking that function as a peripheral heart 5. The foot pump initiates venous return, the leg pump (soleus veins) and popliteal pump (gastrocnemius veins) provide powerful propulsion, and the thigh pump (semimembranosus) completes the sequence 5. Ankle stiffness or dispersion of gastrocnemius collectors can impair this pump mechanism, worsening venous return and causing severe chronic venous insufficiency 5.