The plantaris muscle is often considered the most useless muscle in the leg.
The plantaris is a small, slender muscle located in the posterior compartment of the lower leg. It runs between the gastrocnemius (calf muscle) and the soleus. Its tendon is one of the longest in the body for its size.
Why is it considered "useless"?
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Limited Function: The plantaris muscle's primary function is weak plantarflexion of the ankle (pointing the toes) and flexion of the knee. However, these actions are primarily carried out by the much larger and stronger gastrocnemius and soleus muscles.
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High Rupture Rate: The plantaris tendon is prone to rupture, often referred to as "tennis leg," because it is small and relatively weak. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5643747/
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Evolutionary Remnant: It's believed that the plantaris was more important in our primate ancestors for gripping with their feet when climbing. As humans evolved to walk upright (bipedalism), the need for this muscle diminished, leading to its reduction in size and function.
Is it Completely Useless?
While often called useless, the plantaris isn't entirely without purpose:
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Proprioception: It may play a role in proprioception, which is the body's ability to sense its position and movement in space.
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Graft Material: Due to its long, thin tendon, the plantaris tendon can be harvested and used as graft material in reconstructive surgery, such as for Achilles tendon repairs.
Prevalence
Notably, the plantaris muscle is absent in 7-10% of the population. Its absence typically causes no functional deficit.
In summary, the plantaris muscle, due to its limited function and propensity for rupture, is widely considered a vestigial or useless muscle in the human leg. Its primary value seems to be as a potential source of graft tissue rather than a significant contributor to movement.
