Gastrocnemius

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Gastrocnemius
Muscle Type
Origin	Lateral head – lateral condyle of femur (posterior aspect); Medial head – medial condyle of femur (popliteal surface above medial condyle)
Insertion	Posterior calcaneus via calcaneal (Achilles) tendon
Action	Plantarflexes ankle (especially with knee extended); flexes knee
Synergists
Antagonists	Tibialis Anterior, Extensor Digitorum Longus, Quadriceps Femoris
Spinal innervation
Peripheral Innervation	Tibial nerve (S1–S2)
Vasculature	Posterior tibial artery; sural arteries

The gastrocnemius is the most superficial calf muscle, forming the bulk of the calf’s contour with its two heads (medial and lateral). It spans two joints, therefore acting to plantarflex the ankle and flex the knee. Gastrocnemius provides the propulsive force in activities like running and jumping (fast, powerful movements) and is active during the push-off phase of gait. Together with soleus, it forms the triceps surae and inserts via the Achilles tendon. Its prominence and strength make it a key muscle for locomotion and posture (preventing the body from falling forward at the ankle).

Structure

Gastrocnemius is a fusiform muscle with two heads (biarticular). It is part of the superficial posterior compartment of the leg. It has a high percentage of fast-twitch fibers for explosive power but also supports posture. The two heads give it the "diamond" shape of the calf.

Origin

• The medial head originates from the popliteal surface of the femur above the medial condyle, and the capsule of the knee joint (posterior side).
• The lateral head originates from the lateral aspect of the lateral femoral condyle (also from the lateral supracondylar line region and knee capsule).

These two heads form the lower boundaries of the popliteal fossa.

Insertion

The two heads combine into a single calcaneal (Achilles) tendon, which inserts into the posterior surface of the calcaneus The gastrocnemius tendon merges with the soleus tendon (and plantaris if present) to form the Achilles tendon. This is the thickest and strongest tendon in the body.

Action

Plantarflexion of the ankle: Gastrocnemius powerfully points the foot downward (especially when the knee is extended, because if knee is flexed, gastroc is shortened over knee and contributes slightly less). This action is critical in toe-off phase of gait, jumping, running, climbing stairs.

Knee flexion: It aids in bending the knee (though not as strong as hamstrings). When standing, if ankles are fixed, contraction can help initiate knee flexion or prevent knee hyperextension.

It also stabilizes the knee in the sagittal plane and prevents hyperextension (especially the lateral head working with ACL in preventing anterior tibial translation).

Due to crossing knee posteriorly, an extended knee allows gastroc to act efficiently on the ankle; a flexed knee slackens it, shifting load to soleus for plantarflexion.

Also, it helps raise the heel during gait (with soleus) and helps control the body’s forward momentum on the tibia (eccentric contraction in mid-stance to late stance to control dorsiflexion).

Synergists

The soleus is the primary synergist for plantarflexion; together they are the triceps surae. Soleus is more for endurance/posture, gastroc for power; but they usually act together (e.g., rising on tiptoes). Plantaris, a small muscle, also assists plantarflexion (though minimally). For knee flexion, gastroc synergizes with hamstrings (semimembranosus, semitendinosus, biceps femoris) especially in positions where hamstrings might be slack (like knee flexion from an extended hip). However, in deep knee flexion, gastroc is slack and hamstrings dominate.

• In preventing forward sway (postural control), gastroc and soleus act eccentrically opposite tibialis anterior.
• In jumping, gastroc synergizes with gluteus maximus and quadriceps to produce powerful extension of lower limb (quad extends knee, glute extends hip, gastroc plantarflexes foot).
• For knee flexion, it can help the hamstrings especially when starting knee flexion from full extension (when hamstrings are elongated at hip if hip is flexed).

Antagonists

• For plantarflexion: the tibialis anterior, extensor hallucis longus, extensor digitorum longus (dorsiflexors) are antagonists. During stance, tib ant eccentrically resists foot slap (gastroc is off then), but during swing, dorsiflexors lift foot (antagonizing gravity and passive tension from gastroc).
• For knee flexion: the quadriceps femoris muscle group (knee extensors) are antagonists. When standing, quads prevent knee from buckling (flexing) against gravity, whereas gastroc can't flex knee strongly because it's maintaining ankle position (in that case, it's synergy with quads to stabilize knee). In knee extension (like kicking a ball), quads contract while gastroc is stretched and quiet.
• It's worth noting that at the ankle, dorsiflexors also invert typically (tib ant) whereas gastroc doesn’t invert/evert, so tibialis posterior (a plantarflexor and inverter) is not an antagonist but a synergist in plantarflexion and an antagonist in inversion (not directly relevant as gastroc is neutral re: inversion).
• In knee, antagonists are quads, but if foot is fixed, gastroc and quads can co-contract to stabilize knee.

Spinal Innervation

S1–S2 via tibial nerve (branch of sciatic).

Peripheral Innervation

Tibial nerve innervates gastrocnemius (both heads). This nerve travels through the popliteal fossa, giving off sural branches, then runs between gastroc heads into the calf. Injury to tibial nerve at popliteal fossa (e.g., trauma, Baker’s cyst compression, entrapment) would paralyze gastroc and soleus => inability to plantarflex (can't stand on tiptoes, gait push-off weak) and decreased knee flexion strength. But hamstrings would still flex knee, so knee flexion not completely lost.

Vasculature: The sural arteries (branches of popliteal artery) supply each head of gastroc. Additionally, the posterior tibial artery (and peroneal artery branches) supply it from below. There is rich collateral supply. The two sural arteries are often cut in below-knee amputations or used as landmarks. Venous drainage is into popliteal vein; gastroc can develop venous clots (e.g., DVT often starts in the calf muscle veins, including gastroc veins).

Clinical Relevance

• Calf Strain ("Tennis Leg"): Gastroc, particularly the medial head, is prone to strain or tear, especially in explosive push-off or abrupt dorsiflexion (like lunging for a tennis shot – hence tennis leg). The injury often involves the musculotendinous junction of the medial head or a tear involving plantaris tendon. Presents as sudden sharp pain in calf, with difficulty plantarflexing (standing on toes) and maybe a palpable defect. Treatment: RICE, heel lifts to shorten the muscle during healing, and gradual rehab with stretching and strengthening. A complete gastroc tear may require longer immobilization or rarely surgery if very retracted (though usually managed conservatively since soleus can take over much function).
• Achilles Tendonitis/Tear: Chronic overuse of the triceps surae can cause Achilles tendinopathy. The gastrocnemius (with soleus) being tight or overused predisposes to microtears at the tendon insertion. A traumatic rupture of the Achilles often involves sudden push-off with gastroc contracting (common in middle-aged recreational athletes). The Thompson test (squeezing calf and seeing absence of foot plantarflexion) is used to diagnose Achilles rupture. Surgical tendon repair or conservative casting can be done. Since gastroc is major contributor to Achilles, a rupture functionally eliminates gastroc/soleus action (leading to inability to plantarflex strongly – can't do a single-leg toe raise).
• Spasticity & Contracture: In neurologic conditions (stroke, CP), gastroc-soleus spasticity leads to equinus foot (tiptoe walking, inability to dorsiflex foot). This can cause gait issues and muscle contracture. Treatment may involve physical therapy stretching, bracing (ankle-foot orthosis to keep foot neutral), Botox injections to gastroc, or surgical lengthening (gastroc recession or tendo-Achilles lengthening) if severe. The gastrocnemius specifically can be selectively lengthened with a Strayer procedure (gastroc recession) which cuts gastroc aponeurosis and leaves soleus intact, allowing improved dorsiflexion range while preserving some strength.
• Compartment Syndrome (Deep vs. Superficial): Gastroc is in the superficial posterior compartment. Isolated compartment syndrome here is less common than deep compartment (which holds soleus, tibialis posterior, FDL, FHL), partly because gastroc has more room to expand. But trauma can cause a superficial compartment syndrome (like contusion to calf). Signs would be tight calf, pain on passive dorsiflexion of ankle (stretching gastroc), weakness of plantarflexion. A fasciotomy of superficial compartment (medial and lateral incisions to release each head) might be needed. More commonly, deep compartment is involved (pain with toe flexion if tibial nerve or those muscles affected).
• Knee Hyperextension Gait: Some individuals with weak quadriceps lock their knee in stance by hyperextending it. The gastroc can contribute to knee hyperextension if the ankle is fixed and it contracts (since it will pull femur posteriorly relative to tibia). Usually, gastroc co-contracts with quads to stabilize knee at a slight flexion (like in stance). In genu recurvatum problems, sometimes addressing gastroc tightness (with stretches or heel wedge) can reduce hyperextension tendency by not pulling tibia and femur together from behind.
• Triceps Surae Strength: Testing gastroc/soleus strength is done by asking patient to do heel raises (calf raises). A single heel raise tests ~calf strength against body weight – inability indicates significant weakness. For subtle weakness, repeated heel raises (norm is >20 single leg raises). Electromyographically, gastroc is more active in fast or power tasks, soleus more in slow endurance or posture. So for specific rehab: e.g., after Achilles repair, both heads of gastroc should regain mass, so exercise includes both straight knee (gastroc+soleus) and bent knee (soleus focused) calf raises.
• Sural Nerve: The sural nerve runs down the calf between gastroc heads (receiving branches from tibial and common fibular nerves). It's sensory to lateral foot. Not directly tied to gastroc function, but surgeons doing gastroc release or Achilles surgery watch for sural nerve.
• Aesthetics and Walking: Gastroc is often targeted in fitness for shape of calf (e.g., calf raises). However, genetics largely define calf size. Women sometimes have gastroc tightness from prolonged high-heel use (foot in plantarflexion shortens gastroc). This can contribute to Achilles tendon contracture and discomfort when switching to flat shoes. Stretching exercises focusing on gastroc (straight knee wall stretch) and soleus (bent knee stretch) are recommended to maintain ankle dorsiflexion flexibility.