Plantaris muscle

Origin and insertion

Plantaris is a long, slender muscle that consists of a short, fusiform belly (7-10 cm) and a long, thin tendon extending inferiorly. It originates from the inferior end of the lateral supracondylar line of femur, just superior to the lateral head of the gastrocnemius muscle. The attachment often extends onto the oblique popliteal ligament. Its tendon then travels inferomedially along the medial border of the gastrocnemius. This tendon is so thin that it often gets confused for a nerve by less experienced students during dissections. As a result, it has acquired the nickname ‘freshman’s nerve’.

The plantaris tendon inserts onto the posterior surface of the calcaneus, medial to the calcaneal tendon (common tendon of the soleus and gastrocnemius muscles, also known as Achilles’ tendon). Sometimes, plantaris might join the calcaneal tendon, or merge with the flexor retinaculum of the ankle or leg fascia.

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Relations

Plantaris is one of the most superficial muscles within the posterior compartment of leg. The only muscle overlying it is the gastrocnemius. Together with its lateral belly, the plantaris muscle forms the inferolateral border of the popliteal fossa. The plantaris descends between the gastrocnemius and soleus, the latter being located deep to the plantaris.

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Innervation

Plantaris is innervated by the tibial nerve, which is a branch of the sciatic nerve. The tibial nerve arises from the S1 and S2 spinal nerves.

Blood supply

Plantaris has a dual blood supply. Superficially it receives blood from the lateral sural artery, a branch of the popliteal artery. Its deep surface is supplied by the superior lateral genicular artery, which also stems from the popliteal artery. The distal part of the plantaris tendon receives blood from the calcaneal branches of the posterior tibial artery.

Functions

There is a level of disagreement about the function of the plantaris muscle. Due to its attachments, it is involved in plantar flexion of the foot at the ankle joint and in knee flexion at the knee joint. In reality, however, it weakly assists the gastrocnemius and soleus muscles to perform these two movements. 

The plantaris muscle contains a large number of muscle spindles (proprioceptive receptors), so it has been theorised that it acts as a proprioceptive organ for the larger flexors of the ankle joint. However, since neither proprioceptive or flexor functions are affected when removed or absent, some studies argue that plantaris might be almost vestigial in humans.