Skeletal muscle fibers are not all the same.

Traditionally, they were categorised depending on their varying color.

Red Fibers:

Those containing high levels of myoglobin and oxygen storing proteins had a red appearance. Red muscle fibers tend to have more mitochondria and blood vessels than the white ones.

White Fibers:

Those with a low content had a white appearance. 

To further confuse the issue skeletal muscle fibers are also classified, depending on their twitch capabilities, into fast and slow twitch.

Fast Twitch:

Some authors define a fast twitch fiber as one in which the myosin can split ATP very quickly.

However, fast twitch fibers also demonstrate a higher capability for electrochemical transmission of action potentials and a rapid level of calcium release and uptake by the sarcoplasmic reticulum. The fast twitch fibers rely on a well developed, short term, glycolytic system for energy transfer and can contract and develop tension at 2-3 times the rate of slow twitch fibers.

Slow Twitch:

The slow twitch fibers generate energy for ATP re-synthesis by means of a long term system of aerobic energy transfer. They tend to have a low activity level of ATPase, a slower speed of contraction with a less well developed glycolytic capacity. They contain large and numerous mitochondria and with the high levels of myoglobin that gives them a red pigmentation they have been demonstrated to have high concentration of mitochondrial enzymes, thus they are fatigue resistant.

The 2 main categories of muscle fibers become 3 when we split the white muscle fibers into 2 sections.

So we expand further:

Type I

Red fibers.

Slow oxidative (also called slow twitch or fatigue resistant fibers).

Contain:

Large amounts of myoglobin.

Many mitochondria.

Many blood capillaries.

Generate ATP by the aerobic system, hence the term oxidative fibers.

Split ATP at a slow rate.

Slow contraction velocity.

Resistant to fatigue.

Found in large numbers in postural muscles.

Needed for aerobic activities like long distance running.

Type IIa

Red fibers.

Fast oxidative (also called fast twitch A or fatigue resistant fibers).

Contain:

Large amounts of myoglobin.

Many mitochondria.

Many blood capillaries.

High capacity for generating ATP by oxidation. Split ATP at a very rapid rate and, hence, high contraction velocity.

Resistant to fatigue but not as much as slow oxidative fibers.

Needed for sports such as middle distance running and swimming.

Type IIb

White.

Fast glycolytic (also called fast twitch B or fatigable fibers).

Contain:

Low myoglobin content.

Few mitochondria.

Few blood capillaries.

Large amount of glycogen.

Split ATP very quickly.

Fatigue easily.

Needed for sports like sprinting.

Type III or IIc

White.

Fast gylcolitic

Contain:

Virtually no myoglobin content.

No mitochondria.

Few blood capillaries.

Large amount of glycogen.

Split ATP very quickly.

Fatigue easily.

Found in specialist structures like the eye.

Individual muscles are a mixture of 3 types of muscle fibers (type 1 and type 2a and b), but their proportions vary depending on the action of that muscle.Type It must be remembered that skeletal muscles, although a mixture, can only have one type of muscle fiber within a motor unit. This is demonstrated if we look at contractions. E.g. If a weak contraction is needed only the type 1 motor units will be activated. These fibers are used mainly for endurance activities. If a stronger contraction is required the type 2a fibers will be activated or used to assist the type 1 fibers. Maximal contractions facilitate the use of type 2b fibers which are always activated last. These fibers are used during ballistic activities but tire easily. With advanced EMG techniques it is possible to look at which muscle fibers are recruited when performing an exercise/test. 

The total number of skeletal muscle fibers has traditionally been thought not to change. It is believed there are no sex or age differences in fiber distribution, however, relative fiber types vary considerably from muscle to muscle and person to person. Sedentary men and women (as well as young children) have 45% type 2 and 55% type 1 fibers. People at the higher end of any sport tend to demonstrate patterns of fiber distribution e.g. endurance athletes show a higher level of type 1 fibers. Sprint athletes, on the other hand, require large numbers of type 2 b fibers. Middle distance event athletes show approximately equal distribution of the 2 types. This is also often the case for power athletes such as throwers and jumpers.

It has been suggested that various types of exercise can induce changes in the fibers of a skeletal muscle. It is thought that if you perform endurance type events for a sustained period of time, some of the type 2b fibers transform into type 2a fibers. However, this is argued about.It may well be that the type 2b fibers show enhancements of the oxidative capacity after high intensity endurance training which brings them to a level at which they are able to perform oxidative metabolism as effectively as slow twitch fibers of untrained subjects. This would be brought about by an increase in mitochondrial size and number and the associated related changes not a change in fiber type.