Fibre type composition of the human psoas major muscle with regard to the level of its origin

Abstract

The aim of our study was to explore the fibre type composition of the human psoas major muscle at different levels of its origin, from the first lumbar to the fourth lumbar vertebra, and to compare the muscle fibre size and distribution of different fibre types between levels with respect to its complex postural and dynamic function. Muscle samples were collected from 15 young males (younger than 35 years). Serial transverse sections (5 microm) of the samples were cut by cryomicrotome. Type I, IIA and IIX muscle fibres were typed using myosin heavy chain identification. The serial sections were analysed using a light microscope with a magnitude of 100x. The differences between measurements were evaluated using a repeated-measures anova and Scheffé test for post-hoc analysis. Our study showed that the human psoas major muscle was composed of type I, IIA and IIX muscle fibres. It had a predominance of type IIA muscle fibres, whereas type I muscle fibres had the largest cross-sectional area. Type IIX muscle fibres were present as a far smaller percentage and had the smallest cross-sectional area. Moreover, the fibre type composition of the psoas major muscle was different between levels of its origin starting from the first lumbar to the fourth lumbar vertebra. We conclude that the fibre type composition of the psoas major muscle indicated its dynamic and postural functions, which supports the fact that it is the main flexor of the hip joint (dynamic function) and stabilizer of the lumbar spine, sacroiliac and hip joints (postural function). The cranial part of the psoas major muscle has a primarily postural role, whereas the caudal part of the muscle has a dynamic role.

Fig. 1

Identification of the muscle fibre types with monoclonal antibodies against MHC isoforms: (A) BA-F8 (MHC-I; 1 : 200) stains slow, type I muscle fibres; (B) SC-71 (MHC-IIa, 1 : 500) stains both fast fibre types, type IIA and IIX, but with different intensity; and (C) BF-35 (specific to all MHC isoforms except MHC-IIx; undiluted) stains all fibre types except IIX. Therefore, we considered unstained fibres to be type IIX. Scale bar: 50 μm.

Fig. 3

The CSA of the muscle fibre types of the human psoas major muscle with regard to body side (left and right) and the levels of origin of the muscle (L1–L4 segment of the lumbar spine). Values are mean ± 0.95 confidence interval. *Significantly different from the type IIX fibres of the same muscle at the same level of origin (P < 0.01).

Fig. 2

The percentage of the muscle fibre types of the human psoas major muscle with regard to body side (left and right) and the levels of origin of the muscle (L1–L4 segment of the lumbar spine). Values are mean ± 0.95 confidence interval. *Significantly different from the rest of the fibres of the same muscle at the same level of origin (P < 0.01). †Significantly different from the type I fibres of the same muscle at the same level of origin (P < 0.01).