Triceps Surae
The triceps surae is the three-part calf muscle group composed of the two heads of gastrocnemius and the soleus, which converge into the Achilles (calcaneal) tendon to plantarflex the ankle (point the foot downward) and provide the primary propulsive force in walking and jumping. (The small plantaris is an accessory muscle in the same compartment, and an accessory soleus is a rare variant.)
Components
Muscle Type
“Triceps surae” refers to the three-headed calf muscle: the gastrocnemius muscle has two heads (medial and lateral) and the soleus muscle forms the third head. Gastrocnemius is a large, superficial fusiform muscle visible as the calf’s contour, while soleus is a broad, flat muscle deep to gastrocnemius. Together, they compose the superficial posterior compartment of the leg. The long calcaneal (Achilles) tendon is the common tendon of insertion for these muscles onto the heel. Plantaris, a small fusiform muscle with a long tendon, is also in the superficial compartment – it is vestigial in humans and often considered functionally insignificant. An accessory soleus muscle may be present in a small percentage of individuals as an anatomical variant.
Origin
Gastrocnemius originates by two heads from the distal femur. The lateral head arises from the lateral condyle of the femur, and the medial head from the medial condyle (posterior surface above the condyle). Soleus originates from the proximal tibia and fibula: the soleal line on the posterior tibia, the upper posterior fibula (head and proximal shaft), and a tendinous arch between them. Plantaris (when present) arises from the inferior lateral supracondylar line of the femur (just above the lateral head of gastrocnemius). An accessory soleus, if present, usually originates from the anterior aspect of the soleus muscle or from the tibia and fibula near the soleus origin.
Insertion
Gastrocnemius and soleus fuse into the Achilles tendon (tendo calcaneus), which inserts onto the posterior surface of the calcaneus (heel bone). The Achilles tendon is the thickest and strongest tendon in the body, attaching to the calcaneal tuberosity. Plantaris has a very long, thin tendon that typically runs medial to the Achilles and inserts on the calcaneus as well (sometimes merging with the calcaneal tendon). An accessory soleus, if present, may insert via a fleshy or tendinous attachment to the calcaneus on its medial side or into the Achilles tendon from its deep surface.
Action
The triceps surae is the primary plantarflexor of the ankle. When gastrocnemius and soleus contract, they elevate the heel (plantarflex the foot) as in standing on tiptoe or pushing off during gait. Gastrocnemius, crossing the knee, also flexes the leg at the knee joint. Soleus does not act on the knee (it originates below the knee) and is a pure ankle plantarflexor, active especially in postural maintenance and endurance activities. Together, these muscles provide propulsive force for walking, running, and jumping. The triceps surae also helps control dorsiflexion of the ankle (eccentrically) during the stance phase of gait. Plantaris assists marginally in plantarflexion of the ankle and knee flexion, but its contribution is minimal due to its small size (it is often deemed vestigial). An accessory soleus, when present, would also contribute to plantarflexion.
Synergists
The deep posterior compartment muscles assist in ankle plantarflexion: tibialis posterior, flexor hallucis longus (FHL), and flexor digitorum longus (FDL) are synergists to triceps surae for plantarflexion. In foot inversion, tibialis posterior works with triceps surae to supinate the foot during toe-off. The fibularis (peroneus) longus and brevis in the lateral compartment also assist in plantarflexion of the ankle (while primarily everting the foot). At the knee, the hamstrings (particularly the short head of biceps femoris for lateral rotation and the other hamstrings for flexion) can assist gastrocnemius in knee flexion. Plantaris (if present) acts as a very weak synergist to gastrocnemius.
Antagonists
The main antagonists of the triceps surae at the ankle are the dorsiflexors in the anterior compartment of the leg: tibialis anterior, extensor digitorum longus, and extensor hallucis longus, which lift the foot upward. At the knee, gastrocnemius’ flexion is opposed by the quadriceps femoris, which extend the knee. Additionally, during the gait cycle, the tibialis anterior and other dorsiflexors counterbalance the plantarflexion by triceps surae to allow controlled foot placement.
Spinal Innervation
Triceps surae receives innervation from S1 and S2 spinal nerve roots (with some contribution from L5) via the tibial component of the sciatic nerve. These sacral segments are tested clinically via the ankle jerk (Achilles) reflex, which primarily checks the S1 nerve root.
Peripheral Innervation
The tibial nerve (branch of the sciatic nerve) innervates gastrocnemius, soleus, and plantaris. Specifically, branches of the tibial nerve (originating from the popliteal fossa region) supply each head of gastrocnemius and soleus. An accessory soleus, if present, is also innervated by the tibial nerve. The tibial nerve fibers to these muscles stem mostly from the S1–S2 segments.
Vasculature
The blood supply to the calf muscles is via the popliteal artery and its branches. The gastrocnemius is supplied by the sural arteries (branches of the popliteal artery) and muscular branches of the posterior tibial artery. The posterior tibial artery and peroneal (fibular) artery supply the soleus muscle extensively. The popliteal artery’s muscular branches and the first few perforating branches of the deep femoral artery also contribute to the proximal blood supply of gastrocnemius and plantaris. Venous drainage is through the posterior tibial veins and small saphenous vein (for the gastrocnemius via sural veins), ultimately into the popliteal vein.
Clinical Application
The Achilles tendon is prone to injury – Achilles tendinitis and Achilles tendon ruptures commonly occur in activities with abrupt plantarflexion (jumping, sprinting). A complete rupture leads to inability to plantarflex the foot against resistance (positive Thompson test). Gastrocnemius strain (a “tennis leg”) often involves a tear at the musculotendinous junction of the medial head of gastrocnemius, causing acute calf pain. Chronic triceps surae tightness can contribute to plantar fasciitis or calf cramps. The Achilles reflex (tapping the tendon) tests S1–S2 nerve integrity: absence of this ankle jerk reflex can indicate S1 nerve root damage. An accessory soleus may present as a soft-tissue mass in the medial ankle or cause pain by compressing the tarsal tunnel; it can be confirmed by imaging and is usually managed conservatively unless symptomatic. In cerebral palsy or other spastic conditions, triceps surae spasticity leads to equinus deformity (toe-walking). Strength training of triceps surae (e.g. calf raises) is crucial for explosive movements, and flexibility exercises (calf stretching) help prevent Achilles injuries. Surgical lengthening of the gastrocnemius-soleus (Achilles tendon) may be done to correct contractures. Plantaris, though often vestigial, can be harvested for tendon grafts if needed.
