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.[1]
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.
The plantaris muscle 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.[4]
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's nerve"), the muscle was useful to other primates for grasping with their feet.[5]
Clinical significance
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 one clinical study, 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%).[6]
Injury may occur from running, jumping, or pushing off one leg in sports such as tennis, basketball and soccer, which require quick foot movement in a certain direction. Isolated plantaris muscle strains are rare, and ruptures normally occur in conjunction with injury to other muscles in the posterior compartment of the lower leg.[7] Symptoms of a plantaris muscle rupture may include an audible popping sound in the area during physical activity, swelling, pain in the back of the lower leg, and persistent soreness. Ankle flexion may also be painful.[8]
^Saladin, Kenneth S. Anatomy and Physiology The Unity of Form and Function. 6th ed. New York: McGraw-Hill Science Engineering, 2009. Print.[page needed]
^Freeman, A. J.; Jacobson, N. A.; Fogg, Q. A. (2008). "Anatomical variations of the plantaris muscle and a potential role in patellofemoral pain syndrome". Clinical Anatomy. 21 (2): 178–81. doi:10.1002/ca.20594. PMID18266282. S2CID21873763.
^Moore, Keith L; & Dalley Arthur R (2008). Clinically Oriented Anatomy (6th ed.). Lippincott Williams and Wilkins. ISBN978-1-60547-652-0[page needed]
^Andor, W.J.M., Glaudemans, Rudi A.J.O. Dierckx, Jan L.M.A. Gielen, Johannes (Hans) Zwerver (2015). Nuclear Medicine and Radiologic Imaging in Sports Injuries. Springer. p. 762
^Delgado, Gonzalo J.; Chung, Christine B.; Lektrakul, Nitaya; Azocar, Patricio; Botte, Michael J.; Coria, Daniel; Bosch, Enrique; Resnick, Donald (2002). "Tennis Leg: Clinical US Study of 141 Patients and Anatomic Investigation of Four Cadavers with MR Imaging and US1". Radiology. 224 (1): 112–9. doi:10.1148/radiol.2241011067. PMID12091669.
