Plantaris: The plantaris is one of the exterior muscles of the superficial posterior compartment of the leg. It is formed of a thin muscle belly and a long thin tendon. The plantaris tendon is the longest tendon in the human body.
The plantaris mainly acts with the gastrocnemius.
Origin: Originates from the lateral supracondylar ridge of the femur above the lateral head of the gastrocnemius.
Insertion: Middle 1/3 of the posterior calcaneal surface, just medial to Achilles tendon
Function: Plantar flexor of the ankle joint; also flexes the knee joint.
Nerve Supply: Tibial nerve (L5, S1, S2).