Changes in rodent muscle fibre types during post‐natal growth, undernutrition and exercise.

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Abstract
Using histochemical staining methods for myosin ATPase oxidative and glycolytic enzymes, 3 major types of muscle fiber were identified in the skeletal muscle of hamsters and mice. Muscle fiber counts showed that the proportions of the different fibers were not entirely stable with age. In the hamster biceps brachii, which is predominantly composed of ATPase-high fibers, there was a decrease in the number of ATPase-low fibers. In the soleus muscle which is predominantly composed of ATPase-low fibers there was a decrease in ATPase-high fibers with age. Although there was a change in the total proportion of fiber types, there was no change in the number of fibers within the muscles with age. Some reinnervation may take place during growth and this is why the less dominant fiber type decreases. The response of the different fiber types to partial starvation was studied. The ATPase-high fibers showed the greatest decrease in size. Of these, the ATPase-high glycolytic type responded more than the ATPase-high oxidative type. The effects of the under nutrition on the different fiber types were completely reversible. Starvation did not affect the total number of fibers or the numbers of any fiber type. The response ot the different fiber types to high intensity exercise (weight lifting) was studied. This type of exercise resulted in hypertrophy of all 3 major fiber types. The extent of the response varied according to the fiber type and the exact nature of the exercise. In most cases the ATPase-high fibers underwent hypertrophy more readily than the ATPase-low fibers. Where distinction was made between the 2 types of ATPase-high fibers, the ATPase-high glycolytic hypertrophied more than the ATPase-high oxidative fibers. The effects of post-exercise recovery (return to relative inactivity) were also studied and the changes in fiber size were completely reversible.