The most variable muscle in the human body is the palmaris longus muscle [3]. People can be born without one or both, have two or three in one arm, have split tendons, various insertions, and more [6]. The palmaris longus muscle is a flexor muscle that can be found in the superficial anterior forearm in most people, however, 2.5-26% of the population lacks it [3]. To determine if you have this muscle, hold your hand flat and face up, then pinch your fingers together. If you have the palmaris longus muscle, you'll notice its tendon rising on your forearm just below the wrist. This muscle is 1 out of the 5 involved in the flexion of the wrist [4]. When you type on a keyboard, open a jar, write, grip, etc., you are using your palmaris longus muscle - if you have one.
The origin of a muscle is where it is "anchored" and stabilized, while the insertion is located at the end of the muscle and attached to the bone that moves [1]. The origin of this muscle is the medial epicondyle of the humerus bone and it typically inserts into the palmar aponeurosis and flexor retinaculum of the hand. Other insertions that have been reported are the abductor pollicis brevis, forearm fascia, metacarpophalangeal joints, tendon of flexor carpi ulnaris, hypothenar muscles/fascia, and the pisiform and scaphoid bones [3].
A study in 2017 dissected 80 randomized and isolated arms, and they identified 3 distinct groups of palmaris longus morphology: type 1, 2, and 3 [5]. Type 1 consists of a palmaris longus that is inserted into the palmar aponeurosis; this is the most common type. Type 2 is characterized by tendons split into two, or bifurcated, and type 3 includes the more rare variations, such as the palmaris longus being fused with another muscle.
Due to its length, location, and limited function, the palmaris longus tendon is often harvested by surgeons for grafting purposes [3]. Common reasons for needing a tendon graft are the repair or treatment of the flexor pollicis longus, digital flexor and extensor tendon, chronic mallet finger, carpal tunnel syndrome, and carpometacarpal joint arthroplasty. The previously mentioned 2017 study highlighted the significance of the morphology types when it comes to tendon grafting, with type 1 being described as the most suitable because of how long the tendon is compared to the muscle [5]. In contrast, there is a significant risk of nerve damage when using type 2. This is because the bifurcation found in a type 2 palmaris longus overlaps with the median nerve's path.
A widely accepted hypothesis regarding the occurrence of variations is that the palmaris longus muscle is actively undergoing non-natural selective type evolution, known as non-adaptive, stochastic evolution, and that it will eventually not be found within the African Homininae anymore [2, 6]. When discussing the reasoning behind why humans may be losing this muscle, many scientists look at its presence, and lack thereof, in other great apes. The palmaris longus is always present in our distant relatives, the strictly tree-living orangutan, but is highly variable in more terrestrial primates, such as gorillas, chimpanzees, and humans. Orangutans use a significant amount of forearm, wrist, and hand movement in their daily lives, such as climbing trees and gripping branches. This pattern suggests that the palmaris longus muscle is important for environments and behaviors that involve extensive gripping, such as living solely in trees, and this may explain why humans no longer require it. Additionally, the loss of the palmaris longus muscle in the African Homininae could be the orangutan species experiencing divergent evolution right before our eyes.
References
Betts, J. G., DeSaix, P., Johnson, J. E., Korol, O., Kruse, D. H., Poe, B., Wise, J. A., Womble, M., & Young, K. A. (2013). Anatomy and physiology (2nd ed.). OpenStax.
Capdarest-Arest, N., Gonzalez, J. P., & Türker, T. (2014). Hypotheses for ongoing evolution of muscles of the upper extremity. Medical Hypotheses, 82(4), 452-456. https://doi.org/10.1016/j.mehy.2014.01.021
Cooper, D. W., & Burns, B. (2023). Anatomy, Shoulder and Upper Limb, Hand Palmaris Tendon. StatPearls. https://pubmed.ncbi.nlm.nih.gov/30137801/
The Healthline Editorial Team. (2018, January 21). Palmaris Longus muscle function, origin & anatomy | Body maps. Healthline. https://www.healthline.com/human-body-maps/palmaris-longus-muscle#1
Olewnik, Ł., Wysiadecki, G., Polguj, M., Podgórski, M., Jezierski, H., & Topol, M. (2017). Anatomical variations of the palmaris longus muscle including its relation to the median nerve – a proposal for a new classification. BMC Musculoskeletal Disorders, 18(1). https://doi.org/10.1186/s12891-017-1901-x
Throckmorton, Z., Forth, N., & Thomas, N. (2015). Non-adaptive evolution of them. palmaris longusin the Homininae. https://doi.org/10.1101/015321 Assessed and Endorsed by the MedReport Medical Review Board
All pictures and diagrams used in this article, aside from the cover art, are produced by Myr Selvage