Palmaris longus

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 many mammals it helps to flex the paw. In humans it is mostly a passenger. When it is missing, grip strength and pinch strength are the same as when it is present¹.

About 15% of people worldwide do not have it¹. The figure changes a lot with ancestry. Studies report absence in 5% to 25% of people of European descent, 3% to 6% of East Asians, and up to 37% in some Middle Eastern populations¹. It is often missing on one arm and present on the other¹.

The muscle is called vestigial because it has lost most of its original function³. Other primates that walk on all fours or climb still use it. 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 and ligaments¹.

Try it

1. Rest your forearm on a table with the palm facing up.
2. Touch the tip of your thumb to the tip of your little finger.
3. Bend your wrist toward you a little while keeping the two fingertips pressed together.
4. Look at the middle of your wrist. A tight, raised cord under the skin is the palmaris longus tendon².
5. 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.

History and culture

The absence rate of this muscle is one of the most studied variations in human anatomy. Population surveys have been published for well over a century¹.