Post by John A. Casler on Jul 4, 2007 18:22:15 GMT -8
home.hia.no/~stephens/fibtype2.htm
Below is from Stephen Seilers site listed above
Like most things, there is the simple story, and the real story. Physiological investigations in the late 60s and early 70s have done a great deal to shape our knowledge of skeletal muscle function and fiber type. The biopsy technique, enzyme histochemistry, and physiological studies all advanced this issue. From this work, we now know the fiber types differ: 1) in contractile speed, 2) in myosin ATPase enzyme characteristics, and 3) in metabolic enzyme profile. From these three differences, three different fiber type classification schemes have emerged.
Dr. Phil Gollnick and colleagues studied differences in contractile speed in different muscles. They found that the fiber types were distinguishable based on the time it took them to reach peak tension when stimulated. That difference is graphically demonstrated here.. They proposed the distinction slow, and fast. This turned out to be an oversimplification.
Meanwhile, even before this study, Brooke and Engle distinguished the fiber types based on differences in Myosin ATPase enzyme activity. They arbitrarily divided the muscles into two groups and called them Type I and Type II.
Around the same time Gollnick and colleagues were classifying muscles based on contractile speed, Dr. J.B. Peter and his group investigated the properties of the two categories of fibers established by Brooke and Engle. They proposed another set of terminology created by combining tension generating and metabolic properties. Type I cells were termed Slow Oxidative. That was simple. The slow fibers had a lot of mitochondria (containing oxidative enzymes) and capillaries. However the Type II or Fast fibers had to be further divided into two sub-categories. Type II cells were either Fast Glycolytic (FG) or Fast Oxidative Glycolytic (FOG). The FG fibers stored lots of glycogen and had high levels of enzymes necessary for producing energy without oxygen, but contained few mitochondria. The FOG fibers had the best of both worlds, high speed and glycolytic capacity, plus high levels of oxidative enzymes. These INTERMEDIATE fibers were termed type IIA fibers by a fourth research group (Brooke & Kaiser, 1970). The pure fast fibers (FG) were termed Type IIb. This last lingo system seems to have stuck within the physiological research community.
For the athletic community, the important information is this. It does appear that pure fast (Type IIb) fibers can transition to "hybrid" (Type IIa) fibers with chronic endurance training. Biopsies of elite endurance athletes reveal that after years of training, they have almost no IIb fibers, but often have a significant percentage of the intermediate, IIa fibers. BUT, the majority of the available research suggests that Type IIa fibers do not transition to Type I. This is the more accurate way of saying what I said at the end of Part I of the Fiber type discussion.
Below is a table depicting the characteristics of the three fiber subtypes described above in comparitive fashion.
Below is from Stephen Seilers site listed above
Like most things, there is the simple story, and the real story. Physiological investigations in the late 60s and early 70s have done a great deal to shape our knowledge of skeletal muscle function and fiber type. The biopsy technique, enzyme histochemistry, and physiological studies all advanced this issue. From this work, we now know the fiber types differ: 1) in contractile speed, 2) in myosin ATPase enzyme characteristics, and 3) in metabolic enzyme profile. From these three differences, three different fiber type classification schemes have emerged.
Dr. Phil Gollnick and colleagues studied differences in contractile speed in different muscles. They found that the fiber types were distinguishable based on the time it took them to reach peak tension when stimulated. That difference is graphically demonstrated here.. They proposed the distinction slow, and fast. This turned out to be an oversimplification.
Meanwhile, even before this study, Brooke and Engle distinguished the fiber types based on differences in Myosin ATPase enzyme activity. They arbitrarily divided the muscles into two groups and called them Type I and Type II.
Around the same time Gollnick and colleagues were classifying muscles based on contractile speed, Dr. J.B. Peter and his group investigated the properties of the two categories of fibers established by Brooke and Engle. They proposed another set of terminology created by combining tension generating and metabolic properties. Type I cells were termed Slow Oxidative. That was simple. The slow fibers had a lot of mitochondria (containing oxidative enzymes) and capillaries. However the Type II or Fast fibers had to be further divided into two sub-categories. Type II cells were either Fast Glycolytic (FG) or Fast Oxidative Glycolytic (FOG). The FG fibers stored lots of glycogen and had high levels of enzymes necessary for producing energy without oxygen, but contained few mitochondria. The FOG fibers had the best of both worlds, high speed and glycolytic capacity, plus high levels of oxidative enzymes. These INTERMEDIATE fibers were termed type IIA fibers by a fourth research group (Brooke & Kaiser, 1970). The pure fast fibers (FG) were termed Type IIb. This last lingo system seems to have stuck within the physiological research community.
For the athletic community, the important information is this. It does appear that pure fast (Type IIb) fibers can transition to "hybrid" (Type IIa) fibers with chronic endurance training. Biopsies of elite endurance athletes reveal that after years of training, they have almost no IIb fibers, but often have a significant percentage of the intermediate, IIa fibers. BUT, the majority of the available research suggests that Type IIa fibers do not transition to Type I. This is the more accurate way of saying what I said at the end of Part I of the Fiber type discussion.
Below is a table depicting the characteristics of the three fiber subtypes described above in comparitive fashion.