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 By: David Robson
Athletic success depends to a degree on fast-twitch/slow-twitch muscle fibre composition. Whether it be sprinting 100 meters in 9 seconds, running a marathon, or throwing a well timed punch in boxing to knock out ones opponent, we depend upon a certain percentage of each fibre type.
Depending on the type of sport, and its concomitant duration, intensity and skill level, we will use a combination of both fast-twitch and slow-twitch fibres. However, one of these fibre types will usually predominate. High intensity, anaerobic, short-burst activity, engages the fast-twitch fibres, which are lighter in coloring than slow-twitch fibres due to the low levels of myoglobin (the iron-containing protein found in muscle cells that stores oxygen for use in cell respiration) and mitochondria they possess. Slow-twitch fibres, on the other hand, possess, not surprisingly, more mitochondria and myoglobin, and are red in coloring as a result. These fibres help to sustain one over a long activity period: a marathon or triathlon event for example. The average person has about 50% slow and 50% fast fibres in most locomotory muscles. Those who dominate the athletic world, however, are blessed with muscle fibre composition variation compatible with their chosen sport. You can almost guarantee that newly crowned 100-meter Olympic sprint champion Justin Gatlin has around 80% fast twitch fibres in his quadriceps muscles.
Essentially We Have Two Fibre Types:
The fast-twitch fibres, however, were re-classified due to observed differences in their properties. Fast twitch fibres now have two sub-categories: Fast-Glycolytic (FG) or Fast-Oxidative-Glycolytic (FOG), or 2b and 2a fibres respectively. FG fibres differ from FOG fibres in their pure fast-twitch properties. They have few mitochondria and high levels of stored glycogen and the enzymes necessary for producing energy without oxygen. The FOG fibres have the stored glycogen and enzymatic properties of FG fibres, in addition to high levels of oxidative enzymes which assist aerobic metabolism. They have the best of both worlds. Fast-twitch fibres exert their effects due to, as their name might suggest, peak-tension facilitation. A sprinter, for example, will reach phenomenal speeds due to the force production this peak-tension allows. It is worth noting here that fast-twitch fibre force production is no greater than slow-twitch fibre force production. The difference lies in the rate to which these fibres produce force. Fast-twitch fibres allow one to generate a relatively high amount of force in a short period. The take-off motion in a sprint and long jump (when the foot is in contact with the ground for a millisecond) are perfect examples of this. Over a longer distance (whether it be cycling, running or rowing), fast-twitch fibres are useful primarily for their ability to activate during the sprint phase. Slow-twitch fibres, on the other hand, come into play during the pure endurance aspect of these events. Fatigue resistance, at the expense of fast rate force production, is the hallmark of these fibres. A 2000-meter rowing race, or 26-mile marathon are instances where the slow-twitch fibres predominate.
Chickens use their legs (red meat) for walking and standing for large periods of time, while their wings (white meat) are used for brief bursts of activity. This perfectly illustrates the compelling differences between fast and slow-twitch fibres. The process of contraction can also provide us with an example of fast-twitch/slow twitch fibre engagement. Within a motor unit (a group of muscle fibres and the single motor nerve that activates the fibres) there can only be one type of muscle fibre. Therefore, when a group of muscles contract there is generally an activation of either slow or fast twitch fibres. If the contraction is weak, the slow-twitch (type 1) fibres will take control. Conversely, when the contraction is maximal, the fast twitch (type 2b) fibres will predominate. If the contraction is only moderately stronger than our weak contraction, the fast twitch type 2a fibres will activate.
Given that the exact composition of ones muscles is genetically determined, there is probably not a lot one can do to change the extent to which fibre types influence their training, during the early stages of their athletic career. However, over time, purely fast twitch (Type 2b; FG) fibres can, it appears, transition to fast twitch with slow twitch properties (2a; FOG). This is great news for endurance athletes. However, it will take years of chronic endurance training to achieve this. Indeed, biopsies of elite endurance athletes reveal that after years of training, they have almost no 2b fibres, but often have a significant percentage 2a fibres. It is further shown that type 2a fibres will not transition to type 1. This means that one who has fibres with a combination of fast twitch/slow twitch properties will be unable to change these to purely slow twitch. So, it seems that by training intensely over long distances one can develop the mitochondria and surrounding capillaries of the muscle. More mitochondria and increased capillary density is the muscles attempt to minimize the cellular disturbance caused by the intensity of activity. This leads to an increased ability of the fibre to cope with successive training sessions. However, in saying all of this, here are many more variables that dictate athletic success. Indeed, it has been said that fibre type composition, in the scheme of things, is a poor predictor of athletic performance. Hydration, mental state, nutrition and rest remain profoundly important indicators of success in the athletic sphere. Below is a table depicting the characteristics of the three different sub-types of muscle fibres.
As shown, athletic success may depend largely on ones muscle fibre composition. With the greatest sprinters requiring a large number of fast-twitch fibres and marathoners needing a predominance of slow-twitch fibres, it is clear that athletes are blessed with the genetic goods to dominate in their respective sports. However, although fibre types are probably very important, diet, training type, mental attitude and rest are ultimately deciding factors. References
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To simplify things somewhat, a nice analogy can be used to explain the various actions of these two fibre types. Chickens, like us, have both fast and slow-twitch fibres: their darker (red) meat is composed of slow-twitch fibres, and the white meat of fast-twitch fibres, in line with that of their human counterparts.
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