Your muscles consist of three main types of fiber: slow, intermediate and fast twitch. These fibers are also given alpha numerical names as well and are called type 1, type 2a and type 2b, respectively. The type of muscle fibers you have is genetic and dictates the activities that you are good at. While specific types of training will result in fitness improvements, your genetic potential for physical performance is directly attributable to your muscle fiber type. Sprinters, for example, have high levels of fast twitch fibers, whereas marathon runners have slower twitch fibers.
Color
Despite the fact that all muscle is red, slow twitch muscle fibers are said to be red in color, while fast twitch fibers are described as being white. Red fibers have a higher blood supply than white. A high blood supply is closely associated with a high work capacity and means that slow twitch muscle fibers receive a good supply of oxygenated blood, which allows them to work for long periods of time before becoming fatigued. Fast twitch fibers have an inferior supply of blood and, as a result, fatigue much more quickly.
Force Generation Ability
Slow twitch fibers can generate low amounts of force for long periods of time and are best suited to low intensity but long duration activities such as running and cycling. They are small in diameter and cannot generate large amounts of force to demonstrate strength or power. Slow twitch fibers do not get bigger as a result of exercise, a process called hypertrophy, whereas fast twitch fibers have the greatest capacity for growth. With training, slow twitch fibers develop an increase in work capacity and can work longer but not much harder. This is mainly due to an increase in energy producing cell density and improved blood supply.
Preferred Fuel
Slow twitch muscle fibers are aerobic by nature and use oxygen, fats and carbohydrates to produce the energy-yielding molecule adenosine triphosphate, or ATP, for short. This occurs in cells called mitochondria. Slow twitch muscle fibers contain a lot of mitochondria, which ensures that, in combination with the high oxygen supply, they can continue working for extended periods of time. Fast twitch fibers contain far fewer mitochondria and subsequently fatigue more quickly.
Distribution
The number of fast versus slow muscle fibers you have in your body is genetic, but some muscles, due to their primary function, are more likely to contain a high percentage of slow twitch fibers. The muscles responsible for holding your head up so that you can maintain a neutral horizon are predominately slow twitch. The action of balancing your head on top of your spine does not require a large amount of strength, but the force necessary to achieve this feat must be applied for long periods of time. Other muscles that consist primarily of slow twitch muscle fibers include your intercostals and your diaphragm. These muscles are responsible for every breath you take and need to generate a low amount of force all day, every day.
References
- "Essentials of Exercise Physiology"; William D. McArdle, Frank I. Katch and Victor L. Katch; 2005
- "Anatomy and Physiology for Dummies"; Donna Rae Siegfried; 2002
- "Principles of Anatomy and Physiology"; Gerard J. Tortora and Bryan H. Derrickson; 2008
