Slow Twitch , type 1

Slow Twitch Fibers are excellent in the oxidation of fat, but they are often neglected because they are less likely to hypertrophy.

Those muscles are more efficient at using oxygen to generate more fuel (known as ATP) for continuous, extended muscle contractions over a long time. They fire more slowly than fast twitch fibers and can go for a long time before they fatigue. Therefore, slow twitch fibers are great at helping athletes run marathons and bicycle for hours.

Even if you are in one way or another stimulate those fibers, you usually do not train them to burn fat. If you care about your appearance, of course, there is a big mistake to ignore them.

Fast Twitch, type 2 fibers,

Fast twitch fibers use anaerobic metabolism to create fuel, and because of that are much better at generating short bursts of strength or speed than slow muscles.

The down side is that fatigue more quickly. Fast twitch fibers generally produce the same amount of force per contraction as slow muscles, but they get their name because they are able to fire more rapidly. Those fibers are the primary target in bodybuilding. Fast twitch fibers are good to use carbohydrates, but they are not so good in burning fat.

Type II-a muscle fibers are intermediate and their properties lay between types I and type II-b.

The following Image shows the difference between fiber types:

Image taken from: http://www.lionden.com/muscle_animations.htm

So basically when you train, think of this as a contest between muscle fibers.

Practicing only with high and aggravated by the low number of repetitions to get hypertrophied fiber type 2, makes you convert part of fiber type 1 to type 2.

As a result, the number of fiber type 1 decreases, and long muscles tend to be mainly composed of fibers type 2.

The situation is even worse for doping users. Usually doping accelerated the transformation of type 1 fibers in fiber type 2.

 When speaking on the topic of growing in sizes, the difference in the Type I and Type II fibers is that Type II-b fibers are highly gycolytic and there fore tend to grow in size more than Type I fibers.

How Does A Rep Range Affect Your Muscle Fibers & Strength Gains?

Overview	Growth In Muscle Fibers Below
Repetition Range	Type I	Type IIA	Type IIB	Strength Gains
1-2 repetitions	Very Low	Low	Low	Excellent
3-5 repetitions	Very Low	Low	Decent to Good	Excellent
6-8 repetitions	Very Low	Good	Excellent	Good
9-12 repetitions	Low	Excellent	Very Good	Good Within Rep R.
13-15 repetitions	Decent	Very Good	Decent to Good	Endurance
16-25 repetitions	Very Good	Diminishing	Low	Endurance
25-50 repetitions	Excellent	Low	Very Low	Endurance