You were born with whatever mixture you were born with, and you cannot grow different fibers, just thicken the ones you've got. Normally, young athletes should be evaluated for various events to see exactly what mix they have. Very serious coaches will actually order a biopsie for their athlete's muscle, though unless you've got lots of backing for that Olympic gold this is overkill. In any case, one person might have e.g., a bit more fast twitch fiber in his or her quadriceps than another and therefore have an edge as a sprinter. While you cannot change the composition, you can alter some of the properties, such as making fast twitch fiber grow more mitochondria and therefore increase its endurance.

Culinary aside: Chicken thighs are ST fiber and that is darker because of the myoglobin. Chicken breast meat is white becase it is fast twitch fiber. Chickens can't fly, but when in danger can jump and get a short burst of height and speed from their wings. Ducks and other migratory foul have dark breast meat to help them fly tremendous distances.

To increase muscle size most people concentrate on increasing their cross-sectional area. FT fibers have the greatest cross-section, so it makes sense that to get bigger, you want to use FT fibers (see below on how to gain strength and size). The actual mechanism runs as follows. You must increase the amount of contractile proteins, which are actin and myosin or you must increase the number and size of the myofibrils per fiber. This is normally done by overloading the muscles which breaks down the muscles. The body rebuilds the muscles stronger (just like breaking a bone, it gets stronger at the point of damage). The chief ways to overload are by having more resistance, increasing the number of repetitions per set or increasing the number of sets themselves.

You can also get increases in muscle size from non-muscle tissue, such as vascularization which is just growing more blood vessels (such as runner's legs), increasing the amount of connective tissue or increasing stored nutrients and enzymes. This last one is why people take creatine supplements, since the muscles store this and swell nicely. This is a trick of body builders who pop these (they are one of the few supplements that are supposedly harmless and no I don't take them nor ever have) for a few days before a competition so they look really pumped.

What are the ways that one can build strength and size? Through resistance training. To do this you talk about your x-rep max written as xRM which simply means that you can lift a given load x times before being unable to continue. A 1RM is the absolute most you can lift exactly once. So the schemes are

• Strength lift 2 - 3 reps at 90% - 95% of your 1RM
• Size lift 5 - 8 reps of at 80% of your 1RM
• Both, but more size than strength lift 8 - 12 reps at 60% of your 1 RM

A good set of references are the 'Hard Gainer' books by Stuart McRobert. He's an older powerlifter and has an actual maintainable scheme for lifting long term. He is wonderfully no-nonsense. His book on form has been a big help for a lot of these exercises.

• ST generates this from any of fat, protein or carbohydrate by a process of aerobic glycolysis, although the preferred way is to use the stored form of fat, triglycerides. The main factor here is that the body can keep this up for hours as long as it can get sufficient oxygen.
  
• FTA generates this via anaerobic glycolysis. This can be sustained for up to a few minutes. The basis of this is stored glycogen.
  
• FTB The final method uses creatine phosphate and can last only a few seconds. the basis for this too is stored glycogen.

One other factor here is recovery time. While you cannot sustain many of these reactions, you can train to recover from the effort quicker, allowing you to continue. This is actually one thing trained in taiso.