Effect of endurance running on cardiac and skeletal muscle in rats

We studied the effect of resistance running on left cardiac ventricle size and rectus femoris muscle fiber composition. Ten male Wistar rats were trai ned on a treadmill 6 days per week for 12 weeks. Ten rats remained sedentar y and served as controls. A higher endurance time (40%) and cardiac hypertr ophy in the trained animals were indicators of training efficiency. Morphom etric analysis of the left ventricle cross-sectional area, left ventricular wall, and left ventricular cavity were evaluated. The endurance-running gr oup demonstrated a hypertrophy of the ventricular wall (22%) and an increas e in the ventricular cavity (25%); (p<0.0001). Semi-quantitative analysis o f rectus femoris fiber-type composition and of the oxidative and glycolytic capacity was histochemically performed. Endurance running demonstrated a s ignificant (p<0.01) increase in the relative frequency of Type I (24%), Typ e IIA (8%) and Type IIX (16%) oxidative fibers, and a decrease in Type IIB (20%) glycolytic fibers. There was a hypertrophy of both oxidative and glyc olytic fiber types. The relative cross-sectional area analysis demonstrated an increase in oxidative fibers and a decrease in glycolytic fibers (p<0.0 001). Changes were especially evident for Type IIX oxidative-glycolytic fib ers. The results of this study indicate that the left ventricle adapts to e ndurance running by increasing wall thickness and enlargement of the ventri cular cavity. Skeletal muscle adapts to training by increasing oxidative fi ber Type. This increase may be related to fiber transformation from Type II B glycolytic to Type IIX oxidative fibers. These results open the possibili ty for the use of this type of exercise to prevent muscular atrophy associa ted with age or post-immobilization.