Muscle Fiber Types

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Skeletal Muscle Fiber Type

Have you ever sat down for Thanksgiving dinner and found yourself wondering why turkeys have some dark meat and some white meat? Well, you were not the first. A scientist named Ranvier reported differences in muscle color within and among animal species back in 1873. The explanation for the color differences is pretty simple and has a basis in physiology. The dark meat of the turkey, or chicken, is “red” or slow-twitch muscle. The white meat is “white” or fast-twitch muscle. Most animals have some combination of these two fiber types, though the destinctions may be less obvious. Why are they differently colored? The slow muscles have more mitochondria (full of red pigmented cyctochrome complexes), and more myoglobin packed within the muscle cells. This gives them a darker, reddish color. Humans also have dark and white meat. Some of our muscles, like the soleus in the lower leg are almost all slow twitch fibers. Others such as those controlling eye movements are made up of only fast twitch fibers. Function dictates form in these highly specialized muscles. The majority of human muscles contain a mixture of both slow and fast fiber types. From an evolutionary standpoint this makes sense. Our not so very distant ancestors’ daily survival sometimes dictated a long walk or jog in search of food. Other times, a fast sprint or jump may have kept one out of harm’s way. The exact composition of each muscle is genetically determined. On average, we have about 50% slow and 50% fast fibers in most locomotory muscles, with substantial intra-individual (and muscle to muscle) variations. This variation helps make sports interesting!

Olympic Champions are Oddballs

If you want to win an Olympic medal in the 100 meter dash, you had better be born with about 80% fast twitch fibers! Want to win the Olympic marathon? Put in an order for 80% slow twitch fibers in your quads. The fast twitch fibers benefit the absolute sprinter because they reach peak tension much faster than their slow twitch counterparts. Gram for gram, the two types are not different in the amount of force they produce, only their rate of force production. So, having a lot of fast twitch fibers only makes a positive difference when the time available for force production is very limited (milliseconds), like the 100ms or so the foot is in contact with the ground during a sprint or long jump. It makes no difference to the powerlifter who may use 3-4 seconds to execute a slow, smooth lift.. In cycling, the only event that they are decidedly advantageous for is the match sprint, analogous to the track 100 meter dash, but with more anticipatory tactics and theatrics.

For the pure endurance athlete, more slow twitch fibers that are advantageous. These fibers give up lightning contraction and relaxation velocity for fatigue resistance. Lots of mitochondria and more capillaries surrounding each fiber make them more adept at using oxygen to generate ATP without lactate accumulation and fuel repeated contractions, like the 240 or so in a 2000 meter rowing race, or the 15,000 plus in a marathon.

Does Fiber Type Change with Training?

This has been one of the 10,000 dollar questions in exercise physiology. It has been documented that elite endurance athletes possess a higher percentage of slow twitch fibers in the muscles they use in their sport, compared to untrained individuals. Is this due to genetic endowment or years of rigorous training? The answer is difficult to get at directly because we don’t have comparative muscle biopsies of great athletes before and after they started training and excelling in their sport. However, good basic investigation using experimental models has helped generate some answers. The critical knowledge to remember is that fiber type is controlled by the motor nerve that innervates a fiber. Unless you change the nerve, you won’t change fiber types from fast to slow or vice versa. Just this type of experiment has been performed in animals (generally rats). So, remember, there is no compelling evidence to show that human skeletal muscle switches fiber types from “fast” to “slow” due to training..

Then Why Am I Training So Hard?

Two reasons; first, skeletal muscles respond to chronic overload (training), by trying to minimize the cellular disturbance caused by the training. With intense endurance training, fast fiber types can develop more mitochondria and surrounding capillaries. So can the slow fibers. So training improves your existing fiber distribution’s ability to cope with the exercise stress you create for it.

Second, even among a group of elite endurance athletes, fiber type alone is a poor predictor of performance. This is especially true in the intermediate duration events. There are many other factors that go in to determining success! In fact, there is also evidence to suggest that a mixed fiber composition is ideal for success in an event like the mile run, or if good performances are to be possible in a range of events.

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.  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 sub-types described above in comparative fashion.

TYPE of FIBER
CharacteristicSlow Oxidative (I)Fast Oxidative ( IIa)Fast Glycolytic (IIb)
Myosin ATPase activityLOWHIGHHIGH
Speed of ContractionSLOWFASTFAST
Fatigue ResistanceHIGHIntermediateLOW
Oxidative CapacityHIGHHIGHLOW
Anaerobic Enzyme ContentLOWIntermediateHIGH
MitochondriaMANYMANYFEW
CapillariesMANYMANYFEW
Myoglobin ContentHIGHHIGHLOW
Color of FiberREDREDWHITE
Glycogen ContentLOWIntermediateHIGH
Myoglobin ContentHIGHHIGHLOW
Fiber DiameterSMALLIntermediateLARGE
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