Histochemical properties and muscle fiber diameters of the rectus abdominis (RA) muscle of the female rabbit were compared with those of RA muscle of (1) pregnant females at term of pregnancy (30 days); (2) pregnant females in which the stretch on the abdominal wall natural to pregnancy was artificially prolonged with a 40-day period of intraabdominal appliance; (3) virgin females subjected to intraabdominal appliance for 30, 45, 60, and 70 days. The RA muscle of the control female rabbits is composed of 29.73% type I, 12.13% type IIA, 57.59% type IIB, and 0.54% type IIC fibers. The stimulation on RA muscle due to either normal pregnancy or implant in virgin females provoked changes in muscle fiber diameters only; with the normal pregnancy plus the subsequent period of mechanical stimulation, changes occurred not only in muscle fiber diameters, but there was also a significant increase in the percentage of type I fibers and a concomitant decrease of type IIA and IIB fibers.
The fiber-type pattern in the RA muscle of male and female rabbit also was compared. The RA muscle of the male rabbit showed more type IIA and less Type IIB fibers than found in that of the female. Type I fibers were larger in the female, but type IIA fibers were larger in the male. In addition, male rabbits also were subjected to 30, 45, 60, and 70 days of RA muscle stimulation by means of an intraabdominal appliance to ascertain if the influence of such stimulation upon the RA muscle differed between the two sexes. Some differences in the response of the male RA muscle were noted.