Palmaris Longus Tendon: Is Yours Single, Double or Absent?

There are several parts of my body that I've never thought to name. I never learned the word for the spot on my back that I can't scratch, or the skin between my toes, or the second knuckle on my ring finger. I've always assumed that scientists have a word for every millimeter on our bodies, but there are plenty of areas I've never bothered to talk, write, or even think about. Until recently, the term "palmaris longus tendon" was not part of my vocabulary.

A few days ago, I was chatting with a friend when a discussion deviated into a comparison of our wrists. Just below each of my wrists, I noted a twin pair of tendons that are easy to see and feel. My friend, who has only a little more body fat than I have, does not have a tendon there at all. No matter how much she contorted her wrist, there was no tendon there. This subtle difference in our anatomy piqued my interest.

I found out that that tendon-- like every other square-inch on the human body-- does have a very specific name: the palmaris longus. And, fascinatingly, it's not at all uncommon for a person to lack this anatomical structure entirely.

In some populations, particularly Caucasians, about 16% of people have an absent palmaris longus tendon in at least one arm. These people have the same degree of grip and arm-strength as people without palmaris longus tendons, but it is impossible to see or feel a protruding tendon beneath their wrists.

As it turns out, I'm much more of a freak than my friend with an absent palmaris longus. Although I couldn't find any exact numbers, several websites mention that it is rare for a person to have a double palmaris longus. Instead of just one tendon running down the center of my wrist, I have two on each wrist. I didn't know until recently that this was at all unusual.

Regardless, there's no evidence that an absent, single or double palmaris longus has any effect on your health. As far as modern science can tell, the palmaris longus tendon serves absolutely no practical purpose to modern humans.

So why do we have a palmaris longus tendon at all? Like many vestigial structures, this tendon served a practical purpose at some point in our evolutionary history. In members of the cat family, and a few other predators, the palmaris longus tendon is involved in retracting an animal's claws. Since we haven't had retractable claws at any time in the last several million years, the structure serves no specific purpose to us.

You can find your palmaris longus tendon by looking at each wrist with your hand tilted backward. If you have one, there will be a several-inch-long, cord-like structure that rises slightly above the level of the surrounding skin. If you have a double palmaris longus, you can see a twin pair of these tendons. If yours is absent, you won't be able to see or feel one at all. Although there's no real point in discovering this, it's-- at the very least-- an interesting bit of trivia to know about your own anatomy.

Source Used: Ulster Med J. 2001 May; 70 (1) : 22-24.