Plantaris muscle

At a glance

English

Plantaris muscle

Latin (TA2)

Musculus plantaris

System

Muscle

Region

Lower limb

Side

Paired, left and right

About the plantaris muscle

Plantaris is a small muscle in the superficial layer of the back of the leg. It has a short spindle-shaped belly, only a few centimetres long, and an exceptionally long, slender tendon that reaches the heel. The tendon is often mistaken for a nerve during dissection, which earned it the student nickname of the freshman's nerve. The muscle is missing in a minority of people, on one or both sides.

The belly arises from the lower end of the lateral supracondylar line of the femur, just above and medial to the lateral head of gastrocnemius, and from the oblique popliteal ligament at the back of the knee. It slants downwards and inwards across the popliteal fossa, above the popliteus. In the upper calf it gives way to its tendon, which runs between gastrocnemius in front of it and soleus behind it, along the medial side of the calcaneal tendon. It joins that tendon or attaches separately to the back of the calcaneus on its medial side.

Plantaris acts with gastrocnemius, giving weak plantarflexion of the ankle and a slight bending effect on the knee, and its contribution to force is small. Its belly holds a dense population of stretch receptors, so it may serve more as a sensor of position than as a mover. Because losing it has little effect, its tendon is a favoured graft for repairing tendons and ligaments elsewhere. Its nerve is a branch of the tibial nerve in the popliteal fossa, and its blood comes from the sural branches of the popliteal artery.

Sudden calf pain during a lunge or jump, called tennis leg, was long blamed on a tear of plantaris. Ultrasound studies show that in most cases the medial head of gastrocnemius has torn instead, and isolated plantaris injuries are uncommon. When the tendon does rupture, there may be a pop, pain and swelling in the back of the calf. A thickened plantaris tendon is also sometimes involved in pain at the inner side of the Achilles tendon.

Attachments, action and supply

Origin

Lower lateral supracondylar line of the femur and the oblique popliteal ligament.

Insertion

Medial side of the calcaneal tendon or the posterior surface of the calcaneus.

Action

Weakly plantarflexes the ankle and flexes the knee; may provide position sense for the leg.

Nerve supply

Tibial nerve (S1, S2).

Blood supply

Sural branches of the popliteal artery.