Muscle fibers can be put into one of three categories: Slow twitch or type 1, type 2a and fast twitch or type 2b. Type 2a are essentially fast-twitch fibers but their ability to adapt the traits of both type 1 and 2b fibers according to the training you perform means that they are normally classified as intermediate fibers. There are a number of chemical, functional and structural differences between slow- and fast-twitch muscle fibers.
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Slow-twitch fibers have a very good blood supply -- so much so that they are often referred to as red fibers. This plentiful supply of blood ensures that slow-twitch fibers receive a large amount of oxygen, which allows them to work for a long time before becoming fatigued. In contrast, slow-twitch fibers have a relatively poor blood supply and are subsequently referred to as being white in color. The lack of blood results in relative oxygen restriction, so fast-twitch fibers tend to fatigue much faster than the better oxygenated slow-twitch fibers.
Density of Mitochondria
Mitochondria are energy-producing cells, which create adenosine triphosphate or ATP for short. ATP is your body's essential chemical fuel and is necessary for all energetic reactions. Slow-twitch fibers have a high number of mitochondria, which provide them with an almost limitless amount of energy. Combined with their high blood supply, slow-twitch muscle fibers are ideally suited to long, endurance-type activities. On the other hand, fast-twitch fibers have considerably fewer mitochondria and, as a result, fatigue far quicker than their slow-twitch counterparts.
Diameter and Force Production
Fast-twitch muscle fibers have a larger diameter than slow-twitch fibers. They also have the greatest hypertrophy or growth potential. The larger the cross-sectional size of a muscle fiber, the greater the amount of force it will be able to produce. Fast-twitch fibers are best suited to high-intensity but short-duration activities such as lifting heavy weights or sprinting. Slow-twitch fibers are much smaller in diameter and are far less powerful. While they cannot generate large amounts of force, they can generate low amounts of force for long periods of time. They have very little potential for hypertrophy and are best suited to endurance activities such as walking and cycling long distances.
Preferred Fuel Source
Fast-twitch fibers use ATP derived from glucose for fuel. For very short, intense contractions lasting up to 10 seconds, the ATP is supplied by stores within your muscles. In activities lasting from 10 seconds up to around three minutes, ATP is produced during the incomplete breakdown of stored glucose -- a substance called glycogen. This process results in the production of the fatiguing waste product lactic acid.
In contrast, slow-twitch fibers use a mixture of glucose and fats for energy. The lower the intensity of the activity, the greater the amount of fat used. As intensity levels rise, fat metabolism decreases while carbohydrate metabolism increases. If intensity continues to rise, slow-twitch fibers are unable to continue working and fast-twitch fibers will take over.