Musculus plantaris

The plantaris is one of the superficial muscles of the superficial posterior compartment of the leg, one of the fascial compartments of the leg. It is composed of a thin muscle belly and a long thin tendon.

While not as thick as the achilles tendon, the plantaris tendon (which tends to be between 30–45 centimetres (12–18 in) in length) is the longest tendon in the human body.

Not including the tendon, the plantaris muscle is approximately 5–10 centimetres (2.0–3.9 in) long and is absent in 8-12% of the population.

It is one of the plantar flexors in the posterior compartment of the leg, along with the gastrocnemius and soleus muscles.

The plantaris is considered to have become an unimportant muscle when human ancestors switched from climbing trees to bipedalism and in anatomically modern humans it mainly acts with the gastrocnemius.