Plantaris muscle
Also called: Freshman's nerve
A small calf muscle with a very long, thin tendon that medical students mistake for a nerve. Between 7% and 20% of people do not have it.
How common
Do you have this?
How it works
The plantaris is a small muscle at the back of the knee. Its belly is only 5 to 10 centimetres long1. Its tendon is another matter. It runs down the whole calf between the two large calf muscles and joins the Achilles tendon near the heel1. It is one of the longest tendons in the body.
In other primates, the plantaris helps the foot to grip1. In humans the muscle adds very little to pushing off the ground1. It is listed as vestigial for that reason2. It may still have a role as a sensor. The muscle is rich in stretch receptors, so it may report the position of the ankle to the brain1.
Some people do not have it1. Missing it has no known effect on walking, running, or jumping. Surgeons use the tendon as spare material for repairing tendons elsewhere in the body1.
The muscle can tear during a sudden push-off in sport1. This is one rare cause of tennis leg. Most tennis leg is a tear in the gastrocnemius, the large calf muscle1. Tennis leg usually heals without surgery4.
How common it is
The 7% to 20% range comes mostly from dissection studies3. Rates differ between studies because methods differ3. A thin muscle, or one fused with a neighbouring muscle, can also be missed3.
Is it a problem?
No. Being born without a plantaris muscle has no known effect on walking, running, or jumping1. It is a normal variation, not a deficiency. The muscle can tear during a hard push-off in sport. This causes sudden calf pain and is one rare cause of tennis leg1. Tennis leg usually heals without surgery4. See a doctor for any sudden, sharp calf pain during exercise, mainly to rule out a more serious tendon injury.
History and culture
The tendon is thin, pale, and cord-like. First-year medical students often mistake it for a nerve in dissection classes, which is why it is nicknamed the freshman’s nerve1.
Sources and further material
- Wikipedia Plantaris muscle
- Wikipedia Human vestigiality
- NCBI The Plantaris Muscle Is Not Vestigial: Developmental, Comparative, and Functional Evidence for Its Sensorimotor Role Biology (Basel). Review. Gives the 7–20% absence range and the reasons why studies differ.
- NCBI The plantaris muscle: anatomy, injury, imaging, and treatment J Can Chiropr Assoc. Review. Says that conservative treatment is sufficient for tennis leg.
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Related oddities
Palmaris longus
A thin tendon that stands out in the wrist when you press your thumb and little finger together. About 1 in 7 people do not have it at all, and they lose nothing.
Darwin's tubercle
A small bump on the folded rim of the ear, which Darwin thought was the leftover tip of a pointed ancestral ear. Between 10% and 58% of people have one, depending on the population.
Accessory ossicles
Small extra bones, most often in the foot, that form when a growth centre never fuses with its main bone. They are usually harmless but can be mistaken for fractures on an X-ray.