Cycling NZ #8- Will I ever be a sprinter?
I can probably count on one hand the number of sprints I have won in my life. It is a rare enough occurrence that when I do win, others are sure to hear about it. For instance, recently I was rubbing it in to my slayed competitor how pathetic it was that they lost to me, a guy with 3 fast twitch fibers in his whole body. The 6-year old victim replied, "it is especially bad since 2 of them are in your jaw." The little girl had a point. The truth is that I have never been accused of being a "sprinter". Although technique and skill has something to do with that, the vast majority of the blame goes on my parents. Remember, in cycling never blame yourself; first blame equipment, then blame your parents, then blame karma. In reality though, genetics plays a large role in our success at different disciplines, and one of the key genetic determinants is muscle fiber type. In this column I will discuss a little bit of the physiology behind muscle fiber types, address issues of trainability, and then blame my parents.
Muscle fibre type staining by Dr. Heather Smith University of Auckland.
As science has become more sophisticated, so have the methods to determine muscle fiber type. The end-result is that we now have many different ways to classify the same thing. Some will refer to slow twitch, fast twitch oxidative, and fast twitch glycolytic, or Type I, IIa, and IIb, or sometimes IIx or IIab and so on. For this column I prefer to keep it simple and talk about I (slow twitch), IIa (fast twitch oxidative), and IIb (fast twitch glycolytic). Type I muscle fibres are the fibres we use for most endurance activities. These fibres are designed to be used over long periods of time and rely on oxidative metabolism - oxidative metabolism meaning they use mitochondria to produce energy from fat and carbohydrate. These are the first fibres recruited for any task and in some places, like postural muscles of your back, are almost always active. For the purpose of discussion, this is the thoroughbred horse fibre type. Type IIb are the most explosive of the fibre types. These fibres have very few mitochondria so can only use phosphocreatine (yes, the stuff you supplement with) or glucose for energy. Since they do not produce energy aerobically, muscle contraction can only work for a short period of time. These are the last muscle fibres to be recruited and are only recruited at the highest force generation. For the purpose of discussion, this is the lizard fibre type. Finally, type IIa muscle fibres lie somewhere between types I and IIb. These fibres are aerobic, but they can generate a high amount of force. I distinguished between the thoroughbred horse and the lizard fibre types because it gives you an idea what each is suited for. A horse can sustain high workloads by aerobic energy production for a prolonged period of time. In fact the VO2max of a thoroughbred racehorse is around 150 ml/kg/min, or nearly double Lance Armstrong. Conversely, a lizard can perform short bursts of energy, or sprints, but then has to rest and recover to be able to sprint again. Humans are in general a mix of all the fibre types – both throughout the body and within a specific muscle. What then distinguishes an ultra-endurance cyclist from the sprinter is the proportion of each of those fibres they have.
So, the next question is then, if I have a lot of type I fibres can I train to add type II fibres to become a better sprinter, or vice versa? This is actually a tricky question and to discuss it two important terms need to be introduced; genotype and phenotype. Genotype refers to our specific genetic code that determines what is built in the body. Phenotype is the actual observable outcome of that coding after it interacts with the environment. An example will help clarify what this means in plain English. Your genetics can tell your body to make a protein that makes the skin darker in the sun (genotype), but whether you are pale or darker is determined by whether you are in the sun or not (phenotype). Similarly, the genes in your body will tell you what kind of fibre type you have, but exercise training will determine how those fibre types function. So what this really means is that your genotype is going to determine your limits and training is going to determine the characteristics (phenotype) of those fibres.
It is important to expand on the training-induced changes a little more. When you were born, your parents dealt you a hand to work with. They dealt it to you in the form of genes. Those genes then determined what muscle fibre type you have and that cannot change. But, your parents did not dictate whether you exercise train or not, or what kind of exercise training you do. So, perhaps you were dealt a hand where you have a lot of type IIa muscle fibres. If you sat around and did nothing, they pretty much just get smaller because they do not have a lot to do (phenotype). But, lets say you get into cycling and put in long hours on the bike. Those same IIa fibres will start to increase mitochondria and the enzymes necessary to produce work aerobically. In other words, they start to resemble type I fibres (phenotype) even though they are IIa fibres. Conversely, you might start training as a track cyclist for sprints. Those same IIa fibres, instead of adding mitochondria, will add the enzymes for anaerobic energy pathways so that they start to resemble IIb fibres (phenotype) even though they are still IIa fibres. Does this mean that you can train your fibre types to do anything? Sadly the answer is no. If you are dealt a hand that is 95% Type I fibres, your ability to become a sprinter is very limited. The reason is that even though you can start to have some changes that resemble type IIb fibres, the overall ability to change is limited. They will always be Type I fibres and although their phenotype can change a little, it is ultimately limited because of the genotype.
A respected scientist published a strange paper in a scientific journal last year. It was strange because the number of subjects in it was one. In reality then, it was a case study, which is not the norm in physiological studies. The data was presented from 5 tests over a 7-year period. The tests included standard athlete physiological testing. The interesting thing about the testing was that over this period the athlete did not increase VO2max, but did increase how much power he could generate at a given VO2. The amount of power generated at a given VO2 is what is known as efficiency. Since it has been commonly observed that cyclists do NOT increase efficiency over time, it was speculated that the athlete's fibre type must have changed to cause that change in efficiency. If so, it would be the first documented case of a fibre type change. Sadly, muscle biopsies were not taken so other factors besides fibre type changes could not be ruled out. It is interesting though that this athlete went through some other significant events during this period of time…including chemotherapy. So, of course it was a thinly disguised report on Lance Armstrong, which you can read for yourself if you have access (Coyle 2005, see below).
Therefore, to answer the original question on will I ever be a sprinter – sadly no and I will just have to pick my 8-10 year old group wisely to challenge at crosswalks. But, I can take solace in the fact that my parents are to blame for that. Further, it does not preclude me from being a better sprinter than I am now. I can train those fibres so that they are better at sprinting, just not as good as I would like them to be. So, the take home message is that you have a predisposition to certain skills (genotype), but how you develop them (phenotype) is up to you.
Coyle EF. Improved muscular efficiency displayed as Tour de France champion matures. J Appl Physiol. 2005 Jun;98(6):2191-6.
Ben Miller is Senior Lecturer in Exercise Physiology. Ben did a PhD at the University of California – Berkeley and a Post-Doc at the Institute for Sports Medicine, Copenhagen before arriving in New Zealand. As a departure from his life in a closed scientific box safe from the realities of the world, he is a cyclist regularly taking his life in his own hands on the streets of Auckland and in the local club racing and criteriums. Ben's wife is much more successful at cycling having competed full-time in Europe and the US for the last 4 years.