The body does not do anything "just because". There is a principle called SAID, specific adaption to imposed demands. Bottom line is, you train a specific way, your body adapts a specific way. Short duration, high intensity exercise results in increased fast-glycolytic muscle fiber size and strength, long duration, low intensity training results in a decrease in the above fibers and an increase in fast-oxidative fibers. For example look at the physique of an olympic sprinter and that of a marathon runner, big contrast. If you try to train like both ways then you send mixed signals, which give you mixed results. The signals responsible for all these muscular changes with different types of activity is not known.
Hypertrophy is defined as the increase in muscle fiber size as well as the capacity for ATP production. (adenosine triphosphate--cells use ATP to transfer energy) Muscles don't grow because you are "ripping or tearing them" it is due to the increased synthesis of actin and myosin filaments, which form additional myofibrils. The number of muscle fibers is essentially constant throughout an adults life, so the change in size does not necessarily occur due to an increase of fibers, but in the metabolic capacity and size of existing fibers.
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