Plantaris is long, thin muscle extending behind the knee and into the sural region (calf) of the posterior leg. It forms, together with gastrocnemius and soleus, the superficial group of the posterior compartment of the leg.
The muscle belly is variable both in thickness and length. Sometimes the muscle has two bellies separated by a tendon, and in around 10% of the population, plantaris is absent altogether. There is also an ongoing debate about its function. Plantaris gets its name because in many mammals it inserts into the plantar aponeurosis. However, in humans, the plantaris comes nowhere near it. As the muscle crosses both the knee and ankle joints, it weakly assists with knee and plantar flexion.
This article will cover the anatomy and functions of the plantaris muscle.
|Origin||Lateral supracondylar line of femur, oblique popliteal ligament of knee|
|Insertion||Posterior surface of calcaneus (via calcaneal tendon)|
Talocrural joint: foot plantar flexion
Knee joint: knee flexion
|Innervation||Tibial nerve (S1, S2)|
Superficially: lateral sural and popliteal arteries
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.
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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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.
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.