Abstract
Myofibrous organization of the M. rectus abdominis of 9, one and a half year old rats which were subjected to running exercise on the treadmill, were examined. And the results were compared with controls and some lower extremity muscles of the same animals.
The following results were achieved :
1) In male, body weight of subjects and cross-sectional area of muscle venter were greater. In females the number of muscle fibers per sq. mm. and total number of muscle fibers were higher. On the other hand, the number of red fibers per sq. mm. was greater in males than in females.
2) Compared with the lower extremity muscles, cross-sectional area, number of muscle fibers per sq. mm. and total number of muscle fibers were always : most, gastrocnemius ; then tibialis anterior ; and rectus abdominis least. But the number ratio of red muscle fiber to white fiber of rectus abdominis and tibialis anterior were about 2 times greater than that of gastrocnemius.
3) Concernning the thickness of each muscle fibers of rectus abdominis muscle, white fiber was larger than red fiber and white fiber of male was larger than female in controls. Hypertrophy of the muscle fiber in experiments were so large in white fiber of male muscle and in red fiber of female muscle, so that the differences of muscle fiber size between male and female in experiments were excellent in white muscle fiber.
4) In control, the thickness of muscle fibers were larger in order of gastrocnemius, tibialis anterior and rectus abdominis. In experiment male animals, the white muscle fiber of tibialis anterior developed larger than that of gastrocnemius, and white fiber of rectus abdominis became equal to that of gastrocnemius.
5) It was recognized that the muscles examined showed the different reaction by running experiment, even though the muscles in the same animal.
