Slow and Fast Twitch Muscle Fibers

Slow twitch muscle fibers (Type 1) and fast twitch muscle fibers (Type 2) are both skeletal muscle fibers, but they behave differently.

Type 1 are oxidative in their production of ATP, meaning Type 1 muscle cells go though cell respiration using oxygen for the citric acid cycle and oxidative phosphorylation to continue the production of ATP. To support this high concentrations of blood vessels are found in Type 1 muscle so that more oxygen can be brought to the cells. This works in combination with a slower releasing of calcium ions to create a muscle that does not fatigue quickly. Any movement (rep) done more than approximately 15 times will be using Type 1 muscle almost exclusively.

Type 2 muscle fibers are broken into two subcategories, Type 2a and Type 2b. Type 2a is mainly oxidative, much like Type 1, but is still a short term use muscle; though it fatigues less quickly than Type 2b. Type 2b is glycolytic, meaning that it does not use oxygen for the production of ATP, and relies on just glycolysis. Without oxygen Type 2b cells use lactic acid fermentation to produce ATP. Doing such provides high amounts of energy quickly, but also produces large quantities of lactate. However, in both types of fast twitch muscles higher amounts of calcium ions are released which gives them their short lived, but intense, power. There also remains a significantly smaller concentration of blood vessels in both Type 2 muscle .

A good visual comparison of these two muscles is the light and dark meat on a chicken. The dark meat has its color from the higher concentration of blood vessels, and this is the slow twitch muscles on the chicken. The white meat is fast twitch muscle, and its small amount of blood vessels gives it the white color.

Muscle fibers has also been shown to adapt to fit their use. If large amounts of endurance training, like running, is done then Type 2 muscle will slowly start to alter its properties to better suit endurance. This process is largely dictated by the frequency of the signals sent out by the motor neurons which control the release of calcium ions.

The question this leaves me with is why there is not a higher concentration of blood vessels in Type 2 muscles, which would allow greater ATP production and faster removal of lactate through the blood stream. The effect of lactate on muscle function is disputed; some think lactate inhibits muscle function, but others say it is beneficial to muscle function in Type 2 muscles. The lack of blood vessels, which cause the need for lactic acid fermentation in Type2 muscle, leads me to believe that there is a benefit to having lactate in those muscles. This effect is where the next part of my research will take me.