There are two types of muscle fibers, fast twitch (FT) and slow twitch (ST). Fast twitch fibers are used for explosive type movements and are easily fatigued. Slow twitch muscle fibers contain more mitochondria than Fast twitch. Mitochondria are cell structures that contain specific enzymes, which are required by the cell in order to use oxygen for energy production.
Fast twitch muscles fibers have less mitochondria and therefore less capacity for oxygen utilization in the production of energy within the muscle. This makes them better suited to anaerobic activities such as weight training, sprinting, jumping and other explosive type activities. FT fibers create energy anaerobically, that is, without oxygen. This system uses glucose as a prime energy source. The byproduct of this anaerobic energy production is heat and lactic acid. Lactic acid accumulation in the muscle causes fatigue and soreness. The anaerobic energy system is a limited system for energy production.
Slow twitch fibers are used for endurance type activities and are particularly suited to aerobic type activities. These type fibers contain an increased number of mitochondria and therefore are capable of utilizing oxygen for the production of energy within the muscle. This system uses glucose or fat in combination with oxygen to produce energy. The by-product of this system is carbon dioxide, water and heat.
Each person has a specific ratio of FT to ST fibers. A person with a high ratio of FT fibers may find it easier to train for specific activities that involve explosive movements. Conversely, a person with a higher ratio of ST fibers might find it easier to train and excel at endurance type activities.
There is a third type of muscle fiber that exists only in humans. It is considered a FT fiber of type IIA. These fibers are less powerful than the type IIAB discussed above. What makes these type IIA FT fibers unique is that they can adapt somewhat to aerobic activities. These fibers provide the capability to alter our original genetic FT/ST ratio.