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 in length) is the longest tendon in the human body. Not including the tendon, the plantaris muscle is approximately 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. Structure The plantaris muscle arises from the inferior part of the lateral supracondylar ridge of the femur at a position slightly superior to the origin of the lateral head of gastrocnemius. It passes posterior to the knee joint in an inferomedial direction and becomes tendinous distally to insert into the Achilles tendon. It occasionally separately inserts into the medial side of the calcaneus. Innervation The plantaris muscle is innervated by the tibial nerve, a branch of the sciatic nerve in the sacral plexus. Signaling for contraction begins in the frontal lobe of the brain with the pre-central gyrus (primary motor cortex). Upper motor neurons are stimulated and send a signal through the internal capsule and down the corticospinal tract. Decussation of the lateral corticospinal tract occurs in the medullary pyramids, then the fibers continue down the contralateral side of the spinal cord. Upper motor neurons synapse with lower motor neurons at the anterior horn of the spinal cord in the sacral plexus (formed from the anterior rami of spinal nerves L4, L5, S1–4). The lower motor neuron fibers continue down the sciatic nerve and then diverge into the tibial and common fibular nerves. The tibial nerve runs medially at the knee joint. When the tibial nerve receives an action potential, the plantaris muscle contracts, providing weak plantar flexion of the foot and weak flexion of the knee. Variation The muscle may arise from the oblique popliteal ligament. Interdigitations with the lateral head of the gastrocnemius and a fibrous extension of the muscle to the patella are not unusual. Function The plantaris acts to weakly plantar flex the ankle