Muscle Fiber Types

There are three major fiber types: Slow Twitch (Type I), Fast Twitch Oxidative Glycolytic (FOG) (Type IIa), and Fast Twitch Glycolytic (Type IIb) fibers. This is via staining by way of the Myosin ATPase assay. The myosin corresponding to these fiber types is actually on a continuum meaning that the Myosin ATPase assay is a way to force them into categories. More recent analyses include gel electrophoresis, and clearly there is a continuum from slow to fast rather than distinct delineations. Nonetheless, the Myosin ATPase method of classification has stood the test time. Slow twitch fibers have low contraction velocity, low force, and generally not very fatigable. FOGs have high contraction velocity, high force, and low to moderate fatigability. Fast twitch glycolytic fibers have a high contraction velocity, high force, and high fatigability. Typically, during exercise of increasing intensity, slow twitch fibers are recruited first, followed by FOG, and by fast twitch glycolytic fibers. Training can result in a fast twitch glycolytic fiber becoming an FOG fiber. Under situations of extreme endurance training an FOG fiber can become a slow twitch fiber. Based on the sum toto of all the data it does not appear that a slow twitch fiber can become an FOG fiber or a fast twitch glycolytic fiber although there have been scant reports of this occurrence. Generally speaking, endurance training will increase the number of capillaries in skeletal muscle capillary beds that are used, increase the number and function of mitochondria, and increase the amount of oxidative enzymes in skeletal muscle. These adaptations will improve arteriovenous oxygen difference in one of the components of VO₂max/peak.