Since muscle cells do not divide, what mechanism is
responsible for muscle growth? (ex. - body builders)
There are different types and depending on the type of exercise that a
person predominantly does certain muscle types will become more dominant.
Fast high stress exercise like in ligting heavy weights for short durations
will stimulate the increase in the size of white-fast twitch musckle fibers.
These muscles fatigue quickly though. On the other hand, if endurance
training occurs, red slow twitch fibers which have more mitochondria and
smaller bulk will predominate. There is also an intermediate fiber. The
amazing adaptions of the body to its environment.
While you don't make MORE cells, you can make more STUFF inside the
cells, called myofibrils. So as Hans und Franz used to say you can "pump up"
your cells. Then, if you don't use your muscles they will begin to atrophy.
The cells shrink, but you don't lose them.
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Update: June 2012