Anatomical and Functional Study of the Accessory Head of the Flexor Pollicis Longus Muscle
Authors: Loescher Jeremy J. , Williams‐Hatala Erin Marie , Rabey Karyne N. 
Published: November 2023

Humans are unique among extant primates in possessing a true flexor pollicis longus (FPL) muscle thought to be a key component to the evolution of human dexterity and tool making. In most non‐human primates, it is either not present or part of the flexor digitorum profundus muscle, with a tendon attaching to the first digit. Recently, researchers have noticed further variation of this unique forearm muscle in humans, specifically the presence of an accessory muscle belly. Some have documented a prevalence of up to 50% of this accessory head (AHFPL) in cadavers, and many have debated its clinical function. The goal of this study was to further assess the prevalence of this muscle belly. If present, we determined its origin, attachment, innervation and blood supply. Finally, we measured aspects of muscle architecture and mechanics, including muscle belly, tendon, and fiber lengths, muscle mass, pennation angle, and physiological cross‐sectional area (PCSA), in hopes to shed light on the functionality of this accessory head.We collected data from 80 cadavers between 23 and 98 years of age from Midwestern University. The pool prevalence of an AHFPL was 67.5% (n = 54). Contrary to past studies where AHFPL was found more commonly in men than in women, we found that 63% of the AHFPL occurrence was found in women. We also found that 72% of the cadavers had a bilateral presence of AHFPL. When present unilaterally, the left side was 50% more prevalent than the right. AHFPL has been documented to take origin proximally and medially to the FPL muscle belly, more specifically from the medial epicondyle of the humerus. However, we found that 93% of the time AHFPL actually took origin on the common flexor tendon and only reached the medial epicondyle 3% of the time. As in previous studies, we found that AHFPL inserted more often on the tendon or the muscle belly of FPL. However the location where the accessory head inserted on FPL was extremely variable (24.26 to 233.97 mm from the origin of FPL). This greatly changed the orientation, size, and shape of AHFPL. In most cases AHFPL was innervated by the anterior interosseous nerve (74%) and supplied by the artery of the same name (65%).After analyzing the muscle architectural data, we found that men had significantly longer fiber and tendon lengths as well as greater muscle mass and PCSA than women. In general though, the take home message from the functional data was variability. For example, the AHFPL ranged in weight from 0.3 to 3.05 g and in muscle belly length from 47.03 to 154.23 mm. All muscular architectural data had a huge range of variation, which was expressed as significantly in men as in women. We believe that the different insertion locations dictated the variability of the accessory head. Further analyses are needed to be able to determine the exact functionality of AHFPL. However, the high prevalence of AHFPL should be considered as part of normal human anatomy, instead of an anatomical variant. Due to the significant variations in the size and shape of the muscle, reliable anatomical knowledge is crucial to accurately diagnose and treat potential nerve entrapment, especially the anterior interosseous nerve.Support or Funding InformationThis study was generously funded by the Kenneth A. Suarez CCOM Fellowship, Chatham University and Midwestern University.