Composition of psoas major muscle fibers compared among humans, orangutans, and monkeys

In primate species the m. psoas major, the only muscle simultaneously controlling the spinal column and lower extremity, is expected to reflect morpho-functional adaptation to diversified locomotor behavior. By using histochemical analysis with Sudan black B staining, composition of different types of muscle fibers in the psoas major was compared between 2 Japanese macaques, 2 hamadryas baboons, 2 anubis baboons, 2 orangutans, and 17 humans. The comparison has revealed unique features of this muscle in humans: 1) Type 1 or red fibers are thicker than Type 2 or white fibers in humans but vice versa in nonhumans; 2) among the species examined the number of the muscle fibers per unit cross-sectional area is largest, implying the fibers are thinnest, in humans; 3) frequency of Type 1 fibers is highest in humans, intermediate in monkeys, and lowest in the orangutan, whereas Type 2 fibers show an inverse trend among the species. These results suggest a correspondence in primates between the composition of the psoas major muscle fibers and difference in substrates inhabited as well as in their positional behaviors.