FIBER TYPE COMPOSITION OF THE RAT ILIOPSOAS MUSCLE WITH REGARD TO THE LEVEL OF ITS ORIGIN

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Background. The iliopsoas of the rat is composed of the psoas major muscle and the iliacus muscle and is generally considered to be a fast muscle. However, previous studies of the fiber type composition of the muscle showed different results. Aim. Therefore, the aim of our study was to examine the fiber type composition of the rat iliopsoas muscle with regard to its segmental origin. Material and Methods. At four different levels of its origin, using monoclonal antibodies for myosin heavy chain identification, the percentage and muscle fiber cross sectional areas (CSA) of Type I, IIA, IIB and IIX muscle fibers were identified. Results. The rat iliopsoas muscle had a predominance of fast muscle fiber types, with the highest percentage and the largest CSA of the fastest IIB muscle fibers. Moreover, our results showed changes in percentages, as well as the CSA-s of muscle fiber types in cranio-caudal direction in the psoas major muscle. There was a decrease of percentages and the CSA-s of type IIB muscle fibers from the cranial towards the caudal part. The results were published in the Journal of Anatomy (J. Anat. 2017. Vol. 230, № 4. P. 542-548.). Conclusions. The rat iliopsoas muscle is composed of all four muscle fiber types. It is a fast muscle with a predominance of fast type IIB muscle fibers with the largest CSA-s. The composition of the rat psoas major muscles changes in a craniocaudal direction thus pointing to the more postural role of the caudal part of the muscle.