Palmaris longus
Also called: Missing forearm tendon · Palmaris longus agenesis
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.
How common
Do you have this?
How it works
The palmaris longus is a long thin muscle in the forearm. It starts at the inside of the elbow and ends in a narrow tendon that runs down the middle of the wrist into the palm1. In humans it is mostly a passenger. When it is missing, grip strength and pinch strength are the same as when it is present4.
How often it is missing changes a lot with ancestry. A 2013 review of 26 studies found absence in about 26% of people of European descent3. The rate was 4.5% in East Asians and 42% in Arab Middle Eastern people3.
The muscle is called vestigial because it has lost most of its original function2. Some other primates, such as the orangutan, still use it actively1. Chimpanzees and gorillas do not1. Hand surgeons like it for the opposite reason. Because the body does not need it, they take the tendon out and use it to repair other tendons in the hand and wrist1.
How common it is
The global figure and the ancestry breakdown come from many separate anatomical surveys, some using physical exams and others cadaver dissection, going back over a hundred years135. Because studies sample different populations and use different tests for absence, figures for the same ancestry group can vary between reports. The muscle is also often missing on one arm only1.
Try it
- Rest your forearm on a table with the palm facing up.
- Touch the tip of your thumb to the tip of your little finger.
- Bend your wrist toward you a little while keeping the two fingertips pressed together.
- Look at the middle of your wrist. A tight, raised cord under the skin is the palmaris longus tendon.
- Repeat on the other arm. You may find it on one side only.
If you see two cords, the tendon is the one nearer the centre. If you see none, you are in the group without it. Nothing is wrong.
Is it a problem?
No. Missing the palmaris longus changes nothing about grip or pinch strength, since the muscle does little functional work in humans14. Having it is not better or worse than lacking it. Surgeons even remove the tendon on purpose in people who have it, to repair other tendons in the hand and wrist, without any loss of hand function1. There is nothing here to mention to a doctor, since either way is a normal, unremarkable variation.
History and culture
The absence of this muscle is a frequently studied variation in human anatomy3. Population surveys have been published for more than a century5.
Sources and further material
- Wikipedia Palmaris longus muscle
- Wikipedia Human vestigiality
- doi.org Clinical prevalence of palmaris longus agenesis: a systematic review and meta-analysis Clinical Anatomy 26(6):709-718. Meta-analysis of 26 studies. Overall pooled absence rate 20.25%.
- doi.org Does the absence of the palmaris longus affect grip and pinch strength? Journal of Hand Surgery (British & European Volume) 30(4):406-408. 418 subjects. No difference in grip or pinch strength.
- doi.org Hereditary and racial variation in the musculus palmaris longus American Journal of Physical Anthropology 4(2):205-218. An early survey of absence by ancestry.
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Related oddities
Plantaris muscle
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.
Linburg–Comstock variation
An extra tendon link in the forearm that ties the thumb to the index finger. When the thumb bends into the palm, the index fingertip bends with it. Roughly 1 in 5 people have it, but counts vary widely.
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.
Hitchhiker's thumb
A thumb whose top joint bends back past 50 degrees, once taught as a simple recessive gene. Thumb angle is really a smooth range, and the genetics are not that simple.