I saw the discussion below regarding muscle fiber composition and thought I'd post some of Dr. Mel Siff's thoughts regarding this subject for anyone interested.

D.

Is Muscle Fiber Typing Valuable?

A great deal of literature has appeared on the relationship between types of muscle fiber and types of physical activity. Generally, it has been shown that weightlifters, track-and-field athletes and other explosive type athletes tend to have a high percentage of fast twitch muscle fibers in the propulsive muscles, based on studies on characteristic muscle groups such as vastus medialis and gastrocnemiums. Conversely, distance runners and other cardiovascular endurance athletes have been shown to have a large percentage of slow twitch fibers in the same muscle groups.

Consequently, it has been deducted that genetics determine what type of athlete one is likely to become. Those accepting that there may also be a training, as well as a genetic determinant of sporting excellence, have concluded that training must be suited precisely to reflect the dominant muscle fiber types of the individual or to attempt to cause adaptive changes in those whose muscle fiber profiles are unsuitable for a given sport.

Thus, we see that Olympic-style weight training is used to stimulate fast twitch fiber changes, if these indeed occur to a significant extent in either the actin-myosin structure or in the enzyme environment of the given muscle fibers.

These deductions and recommendations assume, of course, that histological analysis of muscle correlates accurately with functional muscle performance. In other words, if biochemical and histological tests show that the fibers have a certain color, blood supply, enzyme profile, metabolic structure and so forth, then they must be able to contract, relax or hold a contraction for a certain time and at a certain intensity.

This suggests that all FT (Fast Twitch) fibers (of a given type – FT1 or FTII fibers, etc.) display the same force-time curve and contract rapidly at the same velocity. Similarly, all ST (Slow Twitch) fibers or any other types of fiber class, for that matter, contract at a given rate.

However, scientists know that all structural and functional measurements of events generally display a characteristic bell-shaped (or, Gaussian) distribution – and muscle fiber types are no exception. In other words, some FT fibers will be contracting at a very rapid rate, while others will be contracting at a much slower rate. Similarly, some ST fibers will contract at fairly high rates, while others will be thoroughly pedestrian. This leads us to wonder if some FT fibers may be contracting as slowly as the faster contracting ST fibers. If we take into account the findings of some scientists who show that, instead of there being a sequence of a few groups of muscle fiber types, there actually is a smooth continuum of muscle fiber types, constantly in flux at any given instant.

Does this not then suggest that current attempts to classify athletes largely in terms of muscle fiber types and to design training on this basis may be seriously misleading? Are we justified in correlating biochemical/histological tests? Is it accurate to state that a chemically classified muscle sample is indeed functionally very swift, while another is positively geriatric in performance? Attempts to relate muscle fiber typing to certain types of motor ability may be misleading and inaccurate.

It also has to be pointed out that the expression of muscle speed is not simply one of the fiber composition, but also one of anthropometrics. In other words, if one genetically has “favourable” speed type leverages for the relevant joint actions, then this will make an enormous difference to performance. If one happens to have a combination of both advantages, then you are much more likely to be a great speed athlete. In addition, research has revealed that elite athletes have an even greater ability for their muscles to relax after contraction, which suggest that both contraction and relaxation characteristics also have to be examined in understanding the path to excellence in sport.

By the way, studies have shown that the finest marathon runners do not necessarily have the highest V02 max, nor do lifters or sprinters have the highest proportion of certain muscle fiber types in certain thigh muscles manage to reach elite levels.

What is interesting about this topic is that no athlete in any sport has ever had extensive muscle biopsies taken from all the major muscles involved in his/her sport to allow us to make any definitive statement about involvement of the whole body in the sport under consideration.

It has also been pointed out that analysis in terms of muscle fiber types can be somewhat imprecise, since it is preferable to refer to motor unit types comprising large groups of muscle fibers of the same type. Research in which the motor nerves from fast and slow motor units have been surgically interchanged in animal studies suggest strongly that the A-alpha motor nerve of a motor unit actually determines the fiber type of the fibers within that motor unit.

Not only do Type II motor units differ in many ways from type I motor units, but there are also large differences between the motor unit subtypes within type II. For example, in addition to the familiar metabolic differences, there are also differences in levels of supraspinal excitation required to attain the firing threshold and produce impulse propagation of the motor nerves for the different types of motor unit. Type IIa (fast-oxidative) or type IIb(fast-glycolytic) need more excitation to elicit muscle contraction than do type I (slow) motor units, which is probably the result of differences in nerve fiber diameter and the smaller number of inhibitory synaptic inputs on the type 1 units.

A large part of the difference in the twitch response may be explained in terms of these differences in excitation levels and in the conduction velocities of the motor nerves (detailed near the end of this essay). This means that neurological factors probably play a more dominant role in determining twitch speed than fiber physiology. The excitation difference also plays a major role in explaining the difference in use, as well as the fiber composition of the muscles. Because slow motor units are more easily recruited and are the most common type of fiber in most muscles, they handle most of the muscular work during our daily activities. Generally, it is only when any physical activity demands greater force, speed or power, that the different fast twitch motor units are recruited.

Though I agree with most of what has been said about the characteristics of the continuum of muscle fiber types and their plasticity in response to mechanical and other stimuli, I am still concerned about possibly unwarranted extrapolations based on a few muscle biopsies from rectus femoris and one or two other leg muscles. In weightlifting and powerlifting, the hamstrings, glutei, calf muscles and erector spinae all play vital roles in lifting from the ground. How can we infer from one or two muscle samples that the part applies to the whole?

By the way, though slow fibers and fast fibers differ regarding the speed of their twitch response, this difference is not as large as some people may believe – it is around 150 milliseconds for type IIb and around 200-230 msec for type IIa and type I, respectively. In other words, the so-called “slow” fibers (or rather endurance fibers) are hardly the geriatric little aerobes which some people think that they are – characteristically, they are only some 35% slower than their faster cousins. Thus, it is not inconceivable that a significant portion of the strength advantage offered by this difference could be offset by favourable joint leverages and different patterns of muscle involvement in a given activity.

Finally, because force production depends not only on excitation of certain motor units, but also on inhibition of the motor units, very aroused mental states or extremely intense training can reduce inhibitory processes to an extent which can produce very large differences in force, power and speed production. The well-known legend of the old grandmother lifting a wrecked car to save her trapped daughter can be explained on this basis, along with the occasional breaking of world records by large margins (e.g. the long lasting long jump record of Bob Beamon).

Maybe we also have to be more circumspect in simplistically using terms such as fast and slow fibers and motor units, and focus more on neural factors, inhibitory processes, force-generating qualities and fatigue resistance characteristics in trying to understand prowess in sport.

Dr. Mel C. Siff, Facts and Fallacies of Fitness, Denver, CO, 2000, pg. 76-77.