Posted: March 25, 2005

Science of Sport: Muscle Fibre Typing

By Owen Anderson, Ph. D. (copyright © 2003-2005)

Way back in the dark days of exercise physiology, muscle cells were divided into three types: Type 1 ("slow twitch"), Type 2a ("fast twitch"), and Type 2b ("even faster twitch"). It was commonly believed that successful distance runners' muscles were biased toward Type-1 muscle cells, while highly competitive sprinters featured a disproportionate number of Type-2b fibers.

Indeed, research revealed that about 75% of the muscle cells in the thighs of good-quality long-distance runners were Type 1, while 25% were Type 2a; poor-old Type 2b was seldom seen, perhaps blasted out of the sinews by high-volume training (1). In contrast, good middle-distance runners were found to have a bit less Type-1 and a few more Type-2a cells, with some 2bs thrown in for good measure. A very typical middle-distance make-up was 64% Type 1, 32% Type 2a, and 4% Type 2b, for example (2). Meanwhile, successful sprinters tended to check in with approximately 30% Type-1, 50% Type-2a, and 20% Type-2-b fibers (3).

With this sort of scientific data available, some coaches, sports authorities, and journalists became interested in the idea that one could cull the best future distance runners and sprinters from a group of young athletes (or even from a crowd waiting for a bus) by taking biopsy samples from their leg muscles. Find a fellow with 95% Type-1 fibers, for example, and you had the makings of an Olympic marathoner. Retrieve a young woman with 20% or more Type-2b cells in her leg muscles, and you had the makings of a great sprinter, at least in theory. In addition, many believed that runners who had trouble deciding on their "best" distance for racing could check into a clinic or university lab, have some muscle fibers extracted, and almost instantly find out if they were cut from 800-meter or 5-K cloth.

While this simplistic thinking was attractive to many individuals (that's one of the problems with simplistic thinking), a few stains appeared in this fiber-composition paradigm. For one thing, very successful runners often did not fit the mold. The winner of a major marathon for example, was found to possess just 21% Type-1 muscle cells - and a grand total of 79% 2a and 2b! It would have been unfortunate if this excellent distance runner had been shunted toward the sprints because of his muscle-fiber make-up.

Scientists with some experience in the real world also noted that most of the muscle-fiber-composition research had utilized biopsies of the vastus lateralis muscles of the runners involved in the studies. Although the vastus lateralis is an important member of the "quad" group in the thigh, it would be hard to argue that it is the preeminent muscle for either distance, middle-distance, or sprint running; that honor might be bestowed on a calf muscle, the gastrocnemius, which is rarely biopsied. The key point is that no one really knew whether the gastrocnemius (or any other leg muscle) would actually have the same fiber composition as the vastus lateralis in a particular athlete. Complainers contended that taking biopsies of vastus-lateralis tissue in order to understand an athlete's muscle-fiber composition might be like trying to sample the bottom-feeding fish in a lake by skimming a net along the lake's surface.

Perhaps worse yet, the assigning of muscle cells into the 1, 2a, or 2b categories depended totally on something called ATPase histochemistry. Combining the techniques of biochemistry and histology, histochemistry is a science which studies the chemical constituents of living cells and tissues. ATPase, also known as adenosine triphosphatase, is an enzyme which catalyzes the breakdown of ATP (the "energy currency" for human cells), a process which releases the energy required for muscle contraction. Without ATPase, our muscles would be as lively as granite pillars.

As it turns out, there are three closely related but distinct forms ("isoforms") of ATPase found in human skeletal muscles. These isoforms react with the chemical stain applied to biopsied muscle tissue in different ways, so that cells with the slow, low-activity form of ATPase tend to become very dark, cells with the medium-activity isoform remain light, and fibers with the high-activity, fast form are in-between (usually a shade of brown). The very dark cells are then labeled Type 1, the light cells are classified as Type 2a, and the in-betweeners are called 2bs. A count of the three types is made, and the muscle-fiber "composition" for the athlete who is being biopsied is thus "determined."

One potential problem, however, is that what is actually being measured in this case is muscle-fiber percentage, not the actual contribution to the function of the overall muscle made by the differing fiber types. As you are probably aware, the diameters of the three fiber types can vary fairly dramatically, with 2-a fibers tending to be the biggest, 2bs being intermediate, and 1s being the slimmest. As a result, ATPase histochemistry might reveal that a leg muscle of a runner contained 43% Type-1 fibers and 57% Type-2 cells (including both 2as and 2bs). If the actual cross-sectional areas of the fibers were taken into account, however, the same muscle would be 35% Type 1 and 65% Type 2 (because of the larger diameters of the Type-2 fibers). Taking into account cross-sectional areas would give a truer picture of the muscle's functional characteristics than the calculation of percentages.

There is also evidence that categorization of muscle fiber types by histochemical analysis can be misleading in another way (4). What is probably far more important in determining whether a muscle cell contracts slowly, at medium speed, or with high velocity is the type of myosin found within the muscle cell; investigations indicate that myosin is the key determinant in actually regulating the contractile properties of muscle fibers (5). The specific form of myosin found in muscle cells is also a good indicator of the cells' overall metabolic characteristics and resistance to fatigue (6). One isoform of myosin, for example, tends to occur in cells with a high degree of fatigue-resistance and a proclivity for aerobic metabolism (what we would call a Type-1 cell), while another form is found predominantly in cells which fatigue in moments and have little oxidative capacity (what we might call a 2-b cell); yet a third type is located in "intermediate" cells, in terms of aerobic-anaerobic metabolism and fatigability (we might term these cells 2as).

Myosin itself is a protein composed of two high-molecular-weight, heavy-chain (MHC) components which comprise a head and tail region of the molecule; these MCH components can form "cross-bridges" inside muscle cells which interdigitate with another type of protein called actin. When energy is made available, myosin's cross-bridges actually pull actin filaments toward the center of a muscle cell, producing a contraction and the generation of force. The speed and force of this pulling depend on the type of MHC involved.

As it turns out, there are at least 10 different forms of myosin in human muscles, including a special type which is found only in our muscles of mastication and another which is specific to the muscles controlling our eyeballs. However, athletic performance hinges primarily on just three myosin types - MHC 1, MHC 2A, and MHC 2X. MHC 1 tends to take a long time to produce peak muscular force and is associated with a low velocity of shortening, but it is relatively impervious to fatigue - it can keep on pulling actin for long periods of time; it is quite similar to the MHC beta which is the primary myosin heavy chain found in the human heart. MHC 2A has a shorter time to peak tension, a higher shortening velocity, and a higher rate of force development (compared to MHC 1), but it fatigues more quickly. Finally, MHC 2X has the shortest time to reach peak tension and the highest shortening velocity and rate of force development, but it is very susceptible to fatigue, petering out in a matter of seconds when it contracts at high rates. Muscle cells which contain MHC 2X tend to be very powerful; in general, they can contract three to four times more quickly than fibers which have MHC 1.

Knowledge of the three MHC isoforms helps us understand why different muscles in our bodies behave in such diverse ways. For example, consider the soleus, vastus-lateralis, and triceps-brachii muscles in your own body. The soleus is in the calf area, lying just beneath the gastrocnemius; it is a plantar flexor which is often considered a "postural" muscle, because it works isometrically to help maintain the stability of the ankle when you are in a standing position. In contrast, the vastus lateralis muscle is a knee extensor which also has a postural function (maintaining a straight leg when you are in a standing position), and in addition it is highly involved in movement. The triceps brachii is an elbow extensor which has nothing whatsoever to do with posture and which may be used in rather rapid movements - such as throwing a baseball or controlling an explosive contraction by its antagonist - the biceps brachii.

When these three muscles are compared, the soleus has the slowest rate of force development and the greatest resistance to fatigue, while the triceps brachii has the highest rate of force production and the least resistance to fatigue (the vastus lateralis was in between). When the MHC composition of these three muscles is contrasted, the soleus is riddled with MHC 1, the vastus has an almost-even distribution of MHC 1 and MHC 2A, and the triceps brachii is dominated by MHC 2X. Interestingly enough, an MHC 1 cell from the soleus behaves in exactly the same fashion as an MHC 1 fiber found in the triceps brachii (as do MHC-2 cells from the two different muscles), which means that a muscle's overall behavior is a function of the distribution of different myosin isoforms within it - not to intrinsic differences in the properties of fiber types found in different muscles.

Although coaches and runners frequently believe that individuals are genetically endowed with a certain muscle-fiber (MHC-isomer) composition which can not be changed very much by training (witness the widespread beliefs that you can't make a distance runner out of someone with a predominance of Type-2 fibers - or a sprinter from an individual with mostly Type-1 cells), the truth is that the MHC composition of muscle cells can be quite labile. This was first noticed over 40 years ago in studies carried out with cats in which scientists re-innervated fast (Type-2) muscles with nerves which were normally attached to slow (Type-1) muscles and found that the re-innervated fast muscles very quickly became slow sinews (7). In those days, MHC compositions were not measured, but it is very likely that the muscle fibers in the fast muscles dropped their production of MHC 2A and MHC 2X and began producing MHC 1, transforming the muscles. Researchers subsequently discovered that muscle cells could easily be transformed from fast to slow by stimulating the nerve controlling the muscles with a frequency pattern which would normally be delivered to a slow muscle (8). On the other side of the coin, muscles exposed to intermittent, very high-frequency stimulation tend to reduce their expression of MHC 1 and increase their production of MHC 2A and MHC 2X (9). This is one of the reasons why plyometric training can produce substantial gains in an athlete's speed of movement.

It would make things easy if we could say, "OK - Type-1 cells must always have just MHC 1, Type-2a fibers must possess MHC 2A, and Type-2b cells must be blessed only with MHC 2X, but it just doesn't work that way. What is really fascinating is that a muscle cell extracted from the leg of a runner will often contain two of the isoforms of myosin, rather than just one (10). For example, cells which are labeled 2b by the ATPase technique often contain a mixture of MHC 2A and MHC 2X. What is even more intriguing is that the co-expression of two forms of myosin within muscle cells seems to be strongly influenced by the type, intensity and volume of training which is being carried out (11). In other words, muscle cells with just one type of MHC may blossom into fibers with two MHC forms, depending on the training which is being performed; similarly, muscle fibers which already have two isoforms of MHC may change their relative amounts of each isoform in response to a particular style of training - or even drop down to just one MHC. In addition, there seems to be considerable variation between athletes; some athletes may possess a lot of co-expression of the isoforms in their muscles and may shift MHC composition readily, while others may have muscles which are more resistant to change.

The potential presence of two different isoforms within individual muscle cells illustrates one of the problems associated with ATPase typing. For example, a group of elite sprinters was recently found to have 15% Type-2b fibers, based on ATPase histochemistry. However, when single muscle fibers from these athletes were examined, only one in 150 cells was found to contain solely MHC 2X; the other 149 had widely varying mixtures of MHC 2X and MHC 2A. In other words, the ATPase staining technique provided a misleading picture of the true nature of the muscle cells.

As mentioned, the MHC content of muscle cells can be quite responsive to training. The evidence suggests that regular, fairly heavy training seems to reduce the expression of MHC 2X, the most-powerful form of myosin (12). This appears to be true even when a training protocol consists of very short (three-second), very high-speed running intervals (13). In contrast, reductions in the amount of training performed seem to increase MHC 2X content (14). Sprinters seem to sense this, which is one reason why distance runners often complain about the light training loads enjoyed by sprint athletes. At any rate, this kind of transformation might account for some of the increases in footspeed enjoyed by runners after the completion of a tapering period. What is particularly exciting is that there is evidence that when runners drive down MHC 2X by undergoing a period of heavy resistance training, combined with their regular run training, and then embark on a period of lighter overall training, MHC 2X levels do more than return to normal during the lighter phase - they may rise to concentrations which are greater than those observed before the heavy-training period (15).

Just to make things even more intriguing, there is now evidence that athletes whose muscles contain primarily MHC 1 may train in ways which boost the muscles' contents of MHC 2A, no doubt coinciding with gains in speed. From the other end of the spectrum, it is clear that athletes blessed with MHC 2X can work in a manner which increases the contribution of MHC 2A. This convergence raises some interesting questions. What would happen, for example, if a distance-runner's muscles began to co-express MHC 2A, while decreasing production of MHC 1? Would the advance in speed of muscle contraction offset the potential for an increase in muscle fatigability? What is the optimal blend of MHC 1, MHC 2A, and MHC 2X for the middle- and long-distance runner? And - how can the sprinter engaged in serious training avoid producing too much MHC 2A, at the expense of MHC 2X? Does a normal tapering period provide adequate time for those darned MHC 2Xs to reappear?

We don't know the answers to those questions, and - as we have already mentioned - a complicating factor is that some individuals respond to various forms of training by changing the isoform composition of their muscles quite readily, while other athletes have more stable isoform levels. In a very recent study, six young male athletes underwent a three-month period of heavy resistance training involving small numbers of reps completed with near-maximal muscle contractions against a heavy external load (16). As the study began, the athletes were very similar in terms of training, activity levels, and maximal aerobic capacity (VO2max), and they also performed equally well during various tests of muscular strength. However, at the beginning of the investigation three of the subjects featured very low levels of MHC 2X in their thigh muscles, while the other three possessed high concentrations of MHC 2X.

As it turned out, the three months of heavy-resistance training (with low reps, great resistance, and near-maximal muscular forces) nearly obliterated all traces of MHC 2X in all of the athletes' muscles. When the heavy-load strength work was abandoned, however, and the training shifted to two months of very fast, short-interval running, the MHC 2X magically reappeared in the leg muscles of the runners who had originally had decent levels of the stuff. In contrast, the three runners with initially low MHC 2X concentrations were unable to stoke their muscles with this fast isoform during the intense period of interval training. This is preliminary evidence, but it would appear that athletes who are MHC-2X "responders" (like the subjects who first lost MHC 2X and then got it back again) and who depend primarily on maximal speed to achieve their top performances would want to cycle their training in a way which led to the greatest-possible production of MHC 2X shortly before the major competition of the year. It would also appear that high-load strength training is a MHC-2X suppressor, but this does not mean that high-intensity resistance training is a bad thing. For one thing, remember that MHC 2X can vault up to greater-than-usual levels after it has been suppressed for awhile. Remember, too, that high-load strength training can be carried out early in a season, before the major competitive period, and that no one completely understands how the combination of high-level strength training and fast interval training influences the MHC-2X make-up of muscles.

When training changes in a significant way, how long does it take muscle fibers to begin transforming themselves (i. e., to begin expressing different amounts of MHC 1A, MHC 2A, and/or MHC 2X) in response to the new training? Again, knowledge is scarce, but there is some evidence that the changeovers may occur to a significant degree in as little as 11 days (17)! This information comes from muscle-cell pull-outs from the legs of astronauts, however, not from the lower limbs of athletes involved in strenuous training. However, the 10- to 11-day changeover period is supported by research carried out with rabbits and rats, in which significant changes in myosin isoforms occurred during such brief periods of time (18 & 19). Much more research needs to be carried out in this area with sprinters, middle-distance, and long-distance runners.

Here are the bottom-line points about muscle composition:

(A) A "Type-1" muscle cell may actually contain a mix of MHC 1 and MHC 2A isomers, and the relative frequency of each form of myosin has an effect on performance and depends on the volume and intensity of training carried out. Similarly, "Type-2a" fibers may express different frequencies of MHC 1 and MHC 2A - or of MHC 2A and MHC 2X, depending on the amount and intensity of training. Finally, "Type-2b" cells may possess varying quantities of MHC 2A and MHC 2X; increases in MHC 2X may make a runner faster, while advances in MHC 2A may make a runner more fatigue-proof. These changes are a function of the training which is actually undertaken.

(B) A comprehension of the myosin compositions of the world's-best runners at various distances might help us understand which frequencies of MHC 1, MHC 2A, and MHC 2X are optimal, but we presently have no knowledge of the fiber-type compositions of world-class runners when they are in peak condition. True, some biopsies have been taken from world-standard harriers (18 & 19), but these have been obtained in the off-season or during periods when the runners were not in top form. Since myosin-isoform concentrations are a function of training (at least for many runners), the fiber-type compositions of these world-class athletes were probably different when they were in peak shape, compared to the off- or early-season, when biopsies were taken. In short, we have no idea what isoform compositions are associated with the highest-possible performances. Hopefully, this deficiency will be at least partially corrected in the future.

(C) It's true that there is a correlation between the percentage of Type-2b muscle fibers in an athlete's leg muscles and sprinting success, at least when sprinting success is based on 100-meter performance time (20). However, for sprinters with similar frequencies of 2-b cells, success might very well depend on the mix of MHC 2A and MHC 2X in their fibers, and at this point no one knows exactly how to optimize concentrations of MHC 2X.

(D) It is not absolutely certain what would happen if a 10-K or marathon runner suddenly began expressing a lot more MHC 2A - or even MHC 2X - in his/her muscle cells. Such a runner might PR in blazing fashion - or fall flat on his/her face (figuratively, we mean). At any rate, understanding how such transformations could be made (and the impact they might have on performance) is an exciting area for future research.

(E) As you can see, coaches and runners are - from the perspective of muscle-fiber composition - "working in the dark" as they plan their training schedules. Knowledge of optimal myosin-isoform composition and of how different training schemes influence myosin expression in muscles will help athletes and their mentors create training programs which have a more positive impact on performance. Coaches of the future, if they really want to understand how their runners are responding to their training, will have to monitor the MHC contents of their muscles. In addition, future coaches who care about performance will have to target unique MHC compositions as specific goals of training. ©

References

(1) "Torque-Velocity Relationship and Muscle Fiber Composition in Elite Female Athletes," Journal of Applied Physiology, Vol. 47, pp. 388-392, 1979

(2) "Morphology, Enzyme Activities, and Buffer Capacity in Leg Muscles of Kenyan and Scandinavian Runners," Scandinavian Journal of Medicine and Science in Sports, Vol. 5, pp. 222-230, 1995
(3) "Sprinters and Runners. Does Isokinetic Knee-Extensor Performance Reflect Muscle Size and Structure?" Acta Physiologica Scandinavica, Vol. 130, pp. 663-669, 1987
(4) "The Muscle Contractile System and Its Adaptation to Training." In Human Muscular Function during Dynamic Exercise, P. Marconnet et al, Eds., Medicine and Sports Science Basel, Vol. 41, pp. 82-94, 1996
(5) "Molecular Diversity of Myofibrillar Proteins: Gene Regulation and Functional Significance," Physiological Review, Vol. 76, pp. 371-423, 1996
(6) "Training Effects on the Contractile Apparatus," Acta Physiologica Scandinavica, Vol. 162, pp. 367-376, 1998
(7) "Interactions between Motoneurones and Muscles in Respect of the Characteristic Speeds of Their Responses," Journal of Physiology (London), Vol. 150, pp. 417-439, 1960
(8) "The Influence of Activity on Some Contractile Characteristics of Mammalian Fast and Slow Muscles," Journal of Physiology (London), Vol. 210, pp. 535-549, 1969
(9) "Slow-to-Fast Transformation of Denervated Soleus Muscles by Chronic High-Frequency Stimulation in the Rat," Journal of Physiology (London), Vol. 402, pp. 627-649, 1988
(10) "Co-Existence of Myosin Heavy Chain I and IIa Isoforms in Human Skeletal Muscle Fibers with Endurance Training," Pflugers Arch., Vol. 416, pp. 470-472, 1990
(11) "Myosin Heavy-Chain Isoforms in Single Fibres from M. Vastus Lateralis of Sprinters: Influence of Training," Acta Physiologica Scandinavica, Vol. 151, pp. 135-142, 1994
(12) "Skeletal Muscle Myosin Heavy Chain Composition and Resistance Training," Journal of Applied Physiology, Vol. 74, pp. 911-915, 1993
(13) "Myosin Heavy Chain Composition of Single Fibres from M. Biceps Brachii of Male Body Builders," Acta Physiologica Scandinavica, Vol. 140, pp. 175-180, 1990
(14) "Strength and Skeletal Muscle Adaptations in Heavy-Resistance-Trained Women after Detraining and Retraining," Journal of Applied Physiology, Vol. 70, pp. 631-640, 1991
(15) "Muscle Fibre Type Characteristics of the Runner." In Running and Science in an Interdisciplinary Perspective, Jens Bangsbo and Henrik B. Larsen, Eds., pp. 49-65, Munksgaard, 2000
(16) "Plasticity of Myosin Heavy Chain Isoforms in Human Skeletal Muscle: Effects of Training and Detraining," Ph. D. Thesis (J. L. Andersen), Department of Human Physiology, August Krogh Institute, Copenhagen Muscle Research Center, University of Copenhagen, Denmark, 1997
(17) "Myosin Heavy Chain Isoforms of Human Muscle after Short-Term Spaceflight," Journal of Applied Physiology, Vol. 78, pp. 1740-1744, 1995
(18) "Enzyme Activity and Fiber Composition in Skeletal Muscle of Untrained and Trained Men," Journal of Applied Physiology, Vol. 33, pp. 312-319, 1972
(19) "Morphology, Enzyme Activities, and Buffer Capacity in Leg Muscles of Kenyan and Scandinavian Runners," Scandinavian Journal of Medicine and Science in Sports, Vol. 5, pp. 222-230, 1995
(20) "Relationships between the Maximal Running Velocity, Muscle Fiber Characteristics, Force Production, and Force Relaxation of Sprinters," Scandinavian Journal of Sports Science, Vol. 3, pp. 16-22, 1981

To learn about Owen-Anderson's running camps in California, please send a note to Owen at owen@rrnews.com.

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