Muscle of the Day: Palmaris Longus
The palmaris longus muscle is a unique and unusual muscle due to the fact that it is absent in 10-15% of the population. It originates on the medial epicondyle of the humerus and inserts into the anterior aspect of the flexor retinaculum (the fibrous sheath that forms the top of the carpal tunnel) and into the palmar aponeurosis. It functions as a superficial flexor of the hand at the wrist and tightens the palmar aponeurosis.
To check and see if you have a palmaris longus muscle, follow these steps to palpate its tendon.
Touch your pinky fingertip to your thumb tip
Flex or bend your wrist with the palm side facing up
The tendon of Palmaris longus will appear as a raised ridge running along the middle of the anterior aspect of your wrist.
Don’t be alarmed if you do not see this tendon pop up and discover that you do not have a palmaris longus muscle. Absence of the palmaris longus does not affect grip strength at all.
There are a few theories that try to explain the absence of this muscle. Some suggest that the palmaris longus muscle was inherited through common descent of a primate ancestor, and at some stage the thumb was developed along with the thenar muscle group, which led the palmaris longus becoming vestigial. Close primate relatives (such as the chimpanzee and gorilla) also do not actively employ the muscle, and also demonstrate the same variability as in humans.
There are so many fascinating things about the human body and the absence of this small muscle in our forearms is one of them. So next time you are at a party, you can gather everyone and do a quick anatomy study, and try to see if anyone is in the 10-15% of the population without a palmaris longus.
Written by Daniel Le, DPT