Morphometric and Statistical Analysis of the Palmaris Longus Muscle in Human and Non-Human Primates

Abstract

The palmaris longus is considered a phylogenetic degenerate metacarpophalangeal joint flexor muscle in humans, a small vestigial forearm muscle; it is the most variable muscle in humans, showing variation in position, duplication, slips and could be reverted. It is frequently studied in papers about human anatomical variations in cadavers and in vivo, its variation has importance in medical clinic, surgery, radiological analysis, in studies about high-performance athletes, in genetics and anthropologic studies. Most studies about palmaris longus in humans are associated to frequency or case studies, but comparative anatomy in primates and comparative morphometry were not found in scientific literature. Comparative anatomy associated to morphometry of palmaris longus could explain the degeneration observed in this muscle in two of three of the great apes. Hypothetically, the comparison of the relative length of tendons and belly could indicate the pathway of the degeneration of this muscle, that is, the degeneration could be associated to increased tendon length and decreased belly from more primitive primates to those most derivate, that is, great apes to modern humans. In conclusion, in primates, the tendon of the palmaris longus increase from Lemuriformes to modern humans, that is, from arboreal to terrestrial primates and the muscle became weaker and tending to be missing.

A Statistic Method for Anatomical and Evolutionary Analysis

“…We reported SMCA analyses previously [7,[9][10][11] and the SMCA could satisfactorily incorporate many qualitative data numerically. In conclusion, the main features of SMCA are as follows:…”

“…It is desirable to quantitatively assess any kinds of data, even in gross anatomy [7,10], which is important for more precise discussions and more reliable conclusions [17]. Indeed, according to Lord Kelvin, "When you can measure what you are speaking about, and express it in numbers, you know something about it."…”

“…tant when the study includes comparative anatomy. For example, the palmaris longus could be defect in humans [8,10,12] and its prevalence is around 90% [13]; therefore, RF might be 90% among total individuals. However, in the analyses of innervation, vascularization, origin, or insertion of the palmaris longus, only 90% receives atention and the data from the remaining 10% are sometimes discarded.…”

“…The 8 specimens of the Sapajus sp, 16 forearms with a total of the 304 muscles, were analyzed [9], and the data derived from the previous literatures using chimpanzees, gorillas, baboons, and humans were compared with the muscles in the control species (Sapajus sp). For other examples of SMCA application to muscles, see AversiFerreira et al [7,[9][10][11]. In the same way, we also analyzed 4 Japanese monkeys for 4 muscles in the arm resulting in a total of the 16 structures [11], which were compared with the data of the modern humans and other primates obtained from previous studies except those for Table 2.…”

“…We previously compared SMCA with other nonparametric methods including cladistics [7,10]. In fact, the SMCA accept more variables to be analyzed for each structure than the cladistics method.…”