We'll tackle hypertrophy today.
Hypertrophy, simply put, is an increase the the physical size of a muscle. The volume of the muscle increases.
Muscle length or shape is genetically determined, since it depends on the length of your muscles and where they insert (attached) to your bones. But muscle size can generally be increased. Everyone has an upper limit, but while you can't shape a muscle you can generally get it to be larger.
There are two types of hypertrophy.
Sarcoplasmic Hypertrophy is an increase in the fluid volume of the muscle. Inside your muscles are both fluids and solid components. Sarcoplasmic hypertrophy is the result of your muscles increasing that fluid supply, thus swelling the size of the muscles.
Myofibrillar Hypertrophy is an increase the size of the actual muscle fibers, by increasing the size and number of myofibrils (the component parts of the fibers) in the muscle tissue. They don't increase in number, at least not in humans, but they do increase in size and therefore can contract more strongly.
Even simplified as above, the differences between sarcoplasmic and myofibrillar hypertrophy can be a little hard to grasp. Here is an even simpler definition: sarcoplasmic hypertrophy is your muscle packing in more fluids, while myofibrillar hypertrophy is your muscle increasing the size of the contractile tissue. Given a choice, you want a mix of both but an emphasis on the second.
As you can see, all size is not created equal. If your muscle size is mostly due to sarcoplasmic hypertrophy, you will be less strong than if it was mostly due to myofibrillar hypertrophy. This is why you'll hear "bodybuilders aren't as strong as they look" because their methods are generally aimed at more size of any kind, so it's largely sarcoplasmic.
Again, this is just a very basic discussion of the subject. If you want to learn more:
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