The mucous sheaths of the tendons around the ankle. Medial aspect. (Tendon of Plantaris labeled at bottom right.)
The plantaris is visible under the gastrocnemius.
|Lateral supracondylar ridge of femur above lateral head of gastrocnemius|
|Tendo calcaneus (medial side, deep to gastrocnemius tendon)|
|Actions||Plantar flexes foot and flexes knee|
|Tibialis anterior muscle|
|Anatomical terms of muscle|
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 to 45 cm in length) is the longest tendon in the human body. Not including the tendon, the plantaris muscle is approximately 5-10 cm long and is absent in 7-10% of the human 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 an unimportant muscle and mainly acts with the gastrocnemius.
Passing inferomedially posterior to the knee joint, it becomes tendinous while passing distally to insert into the tendo calcaneus, or occasionally separately inserting into the medial side of the calcaneus.
The plantaris m. is innervated by the tibial nerve (S1,S2). Signaling for contraction begins in the frontal lobe with the pre-central gyrus (primary motor cortex of the brain). Upper motor neurons (UMNs) 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. UMNs synapse with lower motor neurons (LMNs) at the anterior horn of the spinal cord in the sacral plexus (formed from anterior rami of nerves L4, L5, S1-4). The LMN fibers continue down the sciatic nerve and then diverge into the tibial and common fibular (peroneal) nerves. The tibial nerve runs medially at the knee joint. When the tibial nerve receives an action potential, the plantaris m. contracts, providing weak plantar flexion of the foot and weak flexion of the knee. 
Also, it may arise from the oblique popliteal ligament.
Plantaris acts to weakly:
- plantarflex the ankle joint
- flex the knee joint
Plantaris may also provide proprioceptive feedback information to the central nervous system regarding the position of the foot. The unusually high density of proprioceptive receptor end organs supports this notion.
Its motor function is so minimal that its long tendon can readily be harvested for reconstruction elsewhere with little functional deficit. Often mistaken for a nerve by new medical students (and thus called the "freshman nerve"), the muscle was useful to other primates for grasping with their feet.
A common injury that is normally attributed to the plantaris muscle is a condition called "tennis leg." Although pain in the calf can be attributed to a rupture of the plantaris muscle, recent ultrasound research has shown that "tennis leg" more commonly arises from tears in the musculotendinous junction of the medial gastrocnemius. In a clinical study at the University of San Diego, California, 94 out of 141 patients (66.7%) diagnosed with tennis leg were found with a partial rupture of the gastrocnemius muscle while rupture of the plantaris tendon was only seen in 2 patients (1.4%). 
Injury may occur from running, jumping, or pushing off one leg. Sports such as tennis, basketball, soccer, etc. which require quick foot movement in a certain direction. Isolated plantaris muscle strains are rare and ruptures to this structure normally occur in concurrence with injury to other muscles in the posterior compartment of the lower leg. Symptoms of a plantaris muscle rupture may include an audible popping sound in the area during physical activity, swelling, pain in the posterior side of the lower leg, and persistent soreness. It may also be painful when trying to flex the ankle. 
- DR. K. Musculoskeletal cases website: Plantaris tendon tear[dead link]
- Saladin, Kenneth S. Anatomy and Physiology The Unity of Form and Function. 6th ed. New York: McGraw-Hill Science Engineering, 2009. Print.
- Moore, Keith L; & Dalley Arthur R (2008). Clinically Oriented Anatomy (6th ed.). Lippincott Williams and Wilkins. ISBN 978-1-60547-652-0
- Delgado, G. J., C. B. Chung, N. Lektrakul, P. Azocar, M. J. Botte, D. Coria, E. Bosch, and D. Resnick. "Tennis Leg: Clinical US Study of 141 Patients and Anatomic Investigation of Four Cadavers with MR Imaging and US." Radiology 224.1 (2002): 112-19. JSTOR. Web. 7 Dec. 2014.
- Spina, Andreo A. "The Plantaris Muscle: Anatomy, Injury, Imaging, and Treatment." The Journal of the Canadian Chiropractic Association 51.3 (2007): 158-65. NCBI. Web. 7 Dec. 2014.
|Wikimedia Commons has media related to Plantaris muscle.|
- Anatomy photo:15:st-0412 at the SUNY Downstate Medical Center
- -1274675120 at GPnotebook
- plantaris+%28muscle%29 at eMedicine Dictionary