The palmaris longus is a slender muscle that appears as a visible tendon in the wrist for many people. It is one of the more variable muscles in the human body, as a significant portion of the population lacks one or both entirely. This variability does not impact daily function or strength, making it a point of anatomical curiosity.
Anatomy of the Palmaris Longus
The palmaris longus muscle originates from the medial epicondyle of the humerus, the bony prominence on the inside of the elbow. It shares this starting point with several other muscles responsible for flexing the forearm. The muscle belly extends down the forearm, transitioning into a long, slender tendon that travels just under the skin.
This superficial placement makes the tendon visible when the wrist is flexed. The tendon passes over the flexor retinaculum at the wrist and inserts into the palmar aponeurosis, a dense tissue in the palm. When the muscle contracts, it tenses this tissue and contributes to weakly flexing the hand at the wrist.
How to Find Your Palmaris Longus
You can perform a simple test to see if you have a palmaris longus tendon. Rest your forearm on a flat surface, such as a table, with your palm facing up. Touch the pad of your thumb to the pad of your pinky or fourth finger. This action engages the muscles within the hand and wrist.
While keeping your thumb and pinky touching, gently flex your wrist upward, as if you are trying to bring your hand toward your forearm. If the tendon is present, you will see it pop up in the midline of your wrist. It will appear as a distinct, rope-like structure just beneath the skin. If you do not see this tendon emerge, you likely do not have a palmaris longus.
An Evolutionary Remnant
The palmaris longus is considered a vestigial structure, meaning it has lost its original ancestral function over the course of evolution. In many mammals, particularly primates that engage in climbing, this muscle plays a more substantial role in grip strength. As humans evolved to walk upright, the need for this specific muscle diminished.
Its non-essential nature is demonstrated by its absence in a portion of the human population. Globally, the rate of absence can range from as low as 1.5% to over 60% depending on the population studied. Studies have shown wide variation among different ethnic groups; for example, absence is reportedly lower in populations of African and Asian descent compared to those of Caucasian descent.
Surgical Use as a Tendon Graft
Because the palmaris longus is expendable, it has become a valuable resource in reconstructive surgery. Surgeons can harvest this tendon to use as a graft for repairing or reconstructing other damaged tendons or ligaments. The tendon’s length, strength, and accessibility make it a suitable choice for these procedures, and it is often considered a “spare part.”
One of the most well-known applications is in ulnar collateral ligament (UCL) reconstruction, a procedure commonly known as Tommy John surgery. This surgery is frequent among throwing athletes, like baseball pitchers, who tear the ligament on the inner side of the elbow. The harvested palmaris longus tendon is used to create a new ligament, restoring stability to the elbow joint. It is also used in various hand surgeries to repair flexor tendons or in reconstructive wrist procedures.