Are Muscle Fibre Types Genetically Determined Or Determined By Training?

The type of muscle fibres an athlete possesses will prove to be a limiting, or promoting factor in the sport an athlete chooses to participate in. There are two main types of muscle fibre, slow twitch and fast twitch. Slow twitch fibres enable an athlete to perform under aerobic conditions whilst fast twitch fibres enable an athlete to perform under anaerobic conditions. Therefore the percentage an athlete possesses of each can be a predictor of how well an athlete will do in a given sport.

The percentage of each type of fibre an athlete possesses is genetically determined. That is it is inherent from parents and other ancestors. This could be shown by a muscle biopsy taken, for example (although not practical in real life), at birth and then again at a later stage in the athletes life. The percentage of muscle fibre types would still be the same. Why is this important? If we look at modern day athletes, in particular 100m sprinters, it is clear to see that the majority of athletes are coloured. Their somatotypes suit sprinting and muscle biopsies have shown that they posses a large percentage of fast twitch fibres, a higher percentage than most white athletes. This means they have a genetic advantage which immediately puts them ahead of most of the white athletes.

So if our muscle fibre types are genetically determined does this mean a person with a high percentage of slow twitch fibres cannot be a sprinter or a person with a high percentage of fast twitch cannot become a marathon runner? This can be answered by looking at the main reason for athletic prowess, training.

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It is known that slow twitch fibres (type I) cannot be changed to fast twitch (type II), however we posses two types of type II muscle fibres, type IIa and type IIb. Type IIb are the fibres associated mainly with explosive power, however type IIa are an almost half way house between type I and type IIb. They have a greater resistance to fatigue and higher aerobic capacity than type IIb. These muscle fibres can be utilised through training to give an enhanced performance. They can be changed through training.

Although this does not change the percentage of genetically determined muscle fibre types in the body, it does show that training plays a part in our muscle fibre make up, and also shows that a person with high percentage of fast twitch fibres could still compete in a marathon against athletes with higher percentages of slow twitch fibres.

Training is obviously an important factor to consider. Training causes changes to the body as a whole. For endurance training one of the main adaptations is increased VO2 max. This means more oxygen can reach the muscles and so they can work harder and longer. Endurance training also increases the number of capillaries around type I muscle fibres meaning they can get more oxygen to them. This will increase an athlete’s ability to run further for longer and so they will perform better.

Training also affects the type IIa muscle fibres. It can make them adapt to suit the training being done. If an endurance training programme is being used, the type IIa fibres will get an increased number of mitochondria and increased myoglobin content, meaning they will have a better aerobic capacity and will be able to be utilised much better in an endurance event.

Alternatively if a power training programme is used the type IIa fibres will respond differently again. They will increase in size and more contractile proteins will be present meaning they can contract quicker and faster and be utilised in a power or speed event.

Therefore an athlete with a higher percentage of slow twitch fibres could still compete against another with a higher percentage of fast twitch in a 100m race if he had trained properly to develop his existing fast twitch fibres and also trained to improve his sprinting technique, starts and finishes.

After looking at both sides of the argument it is clear to see that neither is completely right. Our fibre types are indeed determined genetically and we will be stuck with what we were born with but it is not the only factor that needs to be considered. It has been shown here that through training of either type of muscle fibre it is possible to exploit our existing fibres to a better degree and that also with motivation to do well our genetically determined fibre types do not have to be a limiting factor.

Unfortunately, so far, there has not been any 100% conclusive scientific evidence that can prove that muscle fibres do or do not change with training, as studies have not been conducted over long enough time periods. It is appropriate to say that the evidence so far shows that type I cannot be changed but type IIa can modify to being like type IIb and also more like type I fibres. Thus meaning we can always train to be what we wish to be.

Elliott Owen: N0036961

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