The palmaris longus is a small, superficial muscle located in the forearm. It is notable for being variably present among individuals, as it is not always found in every person.
Anatomical Profile
The palmaris longus originates from the medial epicondyle of the humerus, a bony prominence on the inside of the elbow. It extends down the forearm, positioned superficially between the flexor carpi radialis and flexor carpi ulnaris muscles. Its long, slender tendon passes over the flexor retinaculum, a fibrous band at the wrist, inserting into the central palmar aponeurosis, a thick sheet of fascia in the palm.
Due to its superficial placement, the muscle can often be observed with a specific hand gesture. To check for its presence, touch the thumb pad to the little finger pad and flex the wrist. If present, a visible tendon will protrude in the midline of the anterior wrist. The palmaris longus is typically a long, thin muscle with a long tendon, though its morphology can vary, sometimes appearing as two muscle bundles or entirely as a tendinous band.
Functional Role
The palmaris longus primarily weakly flexes the wrist. It also helps tense the palmar aponeurosis, assisting in slightly cupping the palm. However, its contribution to overall wrist movement is minor compared to other, more powerful forearm muscles with similar actions.
This muscle is classified as vestigial, meaning it has largely lost its original function through evolution. Its absence typically has no significant impact on hand or wrist strength or function. While some studies suggest a minimal decrease in pinch strength in the fourth and fifth fingers when absent, overall grip strength remains unaffected. This functional redundancy underscores its vestigial nature in modern humans.
Clinical Relevance and Absence
The palmaris longus holds practical importance in modern medicine, particularly as a donor tendon for reconstructive surgeries. Its expendable nature, easy accessibility, and lack of significant functional loss upon removal make it a preferred choice. It is frequently used in ligament repairs in the hand, wrist, or elbow, and for tendon transfers and pulley reconstructions.
The prevalence of its absence varies widely by ethnicity, though a commonly cited range suggests it is absent in approximately 10-20% of individuals. This absence is almost always asymptomatic, meaning individuals do not experience discomfort or functional limitations. A simple clinical test, Schaeffer’s test, which involves opposing the thumb and little finger while flexing the wrist, is commonly used to determine its presence.