IntroductionThe psoas muscle group is part of the posterior abdominal wall and is comprised of long muscles – major, minor, and tertius. Out of those, only the psoas major muscle is an obligatory muscle present in all individuals. The psoas minor muscle (PMM) originates as vertical fascicles inserted in the bodies of the last thoracic and first lumbar vertebrae and inserting into the iliopectineal eminence. The muscle provide flexion of the lumbar spine in a limited fashion. The aim of the study was to establish the frequency of the muscle in the Bulgarian population.Materials and methodsThis study was carried out in the Department of Anatomy and Cell Biology, Medical University, Varna, Bulgaria, by Dr. Paraskev Stoyanov in November 2017, on a total of 10 cadavers. The length, width, and circumference of the muscles were measured. The collected data were interpreted in a descriptive manner.ResultsThe PMM was present in six out of ten cadavers (60%). Out of those six cadavers, the muscle was bilateral in three, unilateral on the left side in one, and unilateral on the right side in two cadavers. The average length of the muscle was 19.66 cm (range:14.4 cm - 21.7 cm), average width was 1.73 cm (range: 1.0 cm - 3.2 cm ) and average circumference was 3.48 cm (range: 1.7 cm - maximum 5.6 cm). The male to female ratio of cadavers with a PMM was 1:1.ConclusionThe frequency of the muscle's variations considering its presence in the Bulgarian population (60%) is higher when compared to its presence in the Indian population (36.67%), virtually identical to the Brazilian population (59%), and lower than that reported in the US (65.6%). The morphometric analyses of the different populations showed a shorter psoas minor in the Bulgarian population.

INTRODUCTION: The posterior abdominal wall is a complex region of human anatomy. It is formed by the lumbar vertebrae, pelvic girdle and the five main posterior abdominal muscles-the iliacus, psoas major, quadratus lumborum, the diaphragm, their associated fascia and the variable psoas minor and tertius muscles. The psoas muscle group is comprised of the long fusiform muscles-major, minor and tertius, with only the psoas major muscle being an obligatory muscle present in all individuals. The psoas muscle group, however, is extremely variable and the muscles have differences not only in their structure and morphology, but also in their innervation. The psoas minor is an extremely variable inconstant muscle, a prime example for a vestigial structure in the human body. CASE REPORT: During a standard posterior abdominal wall dissection of an adult female cadaver a unilateral right-sided psoas minor muscle was established, with a complete absence of the same muscle on the contralateral left side. The unilateral right-sided psoas minor muscle was located superficially and laterally to the psoas major muscle; it was fusiform in shape and its body measured 96 mm in length and 16mm in circumference. CONCLUSION: The psoas muscle group is one of the most variable muscle groups in the human body. It could be bilateral-located on both sides of the vertebral column on the anterior surface of psoas major muscle. Our case is representative of one of the not so common variations of the psoas minor muscle with muscle morphology extremely representative of this type of unilateral variation.