Gastrocnemius
The gastrocnemius is the most superficial muscle of the calf, forming its characteristic bulge. It has two heads and is a powerful plantarflexor of the ankle, also assisting with knee flexion.
Origin, Insertion, Action, Innervation
OOrigin
Medial head: Posterior surface of medial femoral condyle. Lateral head: Posterior surface of lateral femoral condyle.
IInsertion
Calcaneus via the calcaneal (Achilles) tendon
AAction
- • Plantarflexion of the ankle (strongest plantarflexor)
- • Flexion of the knee (weak)
NInnervation
Tibial nerve (S1, S2)
Blood Supply
Sural arteries (branches of popliteal artery)
Clinical Relevance
Achilles tendon rupture often occurs in middle-aged athletes ('weekend warriors'). The Thompson test (squeezing the calf should plantarflex the foot) is positive when ruptured. Gastrocnemius can be a site of deep vein thrombosis (DVT). The Achilles reflex tests S1-S2.
Palpation
Easily palpated on the posterior calf. Have the patient stand on tiptoes to see the muscle bulge. The two heads can be distinguished proximally.
Study Tips
- ✓Two heads from femoral condyles = crosses knee joint
- ✓Achilles tendon = strongest tendon in body
- ✓S1, S2 - 'S1 and S2 buckle my shoe' (ankle plantarflexion reflex)
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Common questions about the gastrocnemius
The Achilles tendon (calcaneal tendon) is the strongest tendon in the human body. It is formed by the combined tendons of the gastrocnemius and soleus muscles and inserts on the calcaneus (heel bone).
The gastrocnemius originates from the femoral condyles above the knee, so it crosses both the knee and ankle joints. This allows it to weakly flex the knee in addition to plantarflexing the ankle.
The Achilles reflex (ankle jerk) is elicited by tapping the Achilles tendon, causing plantarflexion of the foot. It tests the S1 and S2 nerve roots and the tibial nerve.
The Thompson test is used to diagnose Achilles tendon rupture. With the patient prone, squeezing the calf should cause plantarflexion of the foot. Absence of this movement indicates tendon rupture.