Knowing the physiology of muscle tissue should aid you in understanding how muscles can and will change with exercise training. The following presentation provides insight into the physiology of muscle tissue as well as the various types of muscle fibers that you can have in your body.
Click Here for information on Muscle Fibers
If you would like a further explanation that provides more details on what muscle fibers look like as well as the structure of the fiber itself, feel free to watch this video (I think it is entertaining... but you might think differently :)
You can stop watching at the Sliding Filament theory (around the 8th minute) if you wish... but I still think its interesting... but I digress...
Click Here for muscle fiber video
In the presentation above, it mentions slow and fast twitch muscle fibers. For a different look at these two groups of muscle tissue, watch this short video:
Click Here for muscle fiber type video
Notice in this last video that the author described the fast twitch fiber as being able to "bulk up" or get bigger. This process of a muscle being able to increase in fiber size is called Hypertrophy ("Glossary"). This is in opposition to a muscle going through Atrophy ("Glossary") because of disuse (think couch or mouse potato) or disease.