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With all other training factors being equal, there will always be those lucky individuals who seem to have a natural advantage over their peers when it comes to hoisting massive weights, building freaky dense muscle, or excelling in competition. Despite diligently and intelligently planning your training, nutrition, and supplementation, and doing everything you can to grow– natural or otherwise – there is little you can do to surpass these lucky few. While we can become extremely good at what we do, the hard truth is that we will only ever excel to the extent to which our genetics will allow.  Indeed, of all the variables that may be blamed for holding us back in our chosen sport, muscle fiber composition is the more culpable.

Fast Twitch : Slow Twitch

Each of us is born with a unique distribution of muscle fibers which, while perfectly suitable for day to day living, can have differential effects in our athletic endeavors.[1][6] There is a reason why some people are better suited to becoming world champion weightlifters while others are instead more adept at running ultra long distances with apparent ease.

All skeletal muscle is comprised of individual bundles of fibers called myocytes, each of which contains many myofibrils (strands of specific myosin and actin proteins which interact with each other to shorten or contract a muscle). This skeletal muscle is then made up of two main muscle fiber types: slow twitch (type I), and fast twitch (type II), the latter of which is divided into types IIa and type IIb.[6]

All human muscle contains a genetically predetermined distribution of both fast and slow twitch fibers. Our unique distribution may be a blessing or a curse depending on the ways we want our body to perform. For example, because slow twitch muscles are more efficient at using oxygen to produce the ATP (a.k.a. energy) needed to fuel muscular movement and because oxygen is more readily available as a fuel source, those with a greater number of slow twitch fibers are better able to work for longer periods without experiencing muscle fatigue — but since slow twitch fibers contract at a slower rate compared to their fast twitch counterparts, they don’t lend themselves to explosive movements.[8]  On the other hand, fast twitch fibers, which rely on anaerobic (a.k.a. without oxygen) metabolism to create ATP, move 2-3 times faster than slow twitch fibers but tire much more quickly.[5] To take this a bit further, muscle fibers capable of contracting at the highest possible rate, but which fatigue fastest, are the fast twitch IIb variety, while those which use both anaerobic and aerobic metabolism equally to produce energy are the fast twitch IIa kind – also referred to as intermediate fibers.[1][4]

Any athlete that is required to generate tremendous force will recruit fast twitch fibers, but while IIb fibers are ideal for explosive movements, such as a one rep max, or a single Olympic lift, type IIa are called upon whenever we complete a typical high intensity bodybuilding-style workout.

Slow twitch fibers do not have the same growth potential as do fast twitch fibers – thus, building massive muscles is easier for athletes with an abundance of fast twitch fibers. Marathon runners are the classic example of athletes who are slow twitch dominant, while sprinters are an example of those who are fast twitch dominant.[8] In fact, scientific analysis of muscle fiber composition has shown that Olympic sprinters possess, on average, 80 percent fast twitch fibers while marathon runners, 80 percent slow twitch.[6] The physical differences between both types of athlete clearly shows the effects that muscle fiber composition can have on a physique.

That being said, muscle fiber composition throughout the body is not uniform. Certain areas contain more of one fiber type. For example, the calves, notoriously difficult to develop for many, are comprised primarily of slow twitch fibers.[3] Which makes sense considering how they are constantly used to hold us up or help us move long distances. On the other hand, the hamstrings are more fast twitch, hence why so many people can lift some pretty big weights on deadlifts.

Red, White, and… HUGE

Ever wonder why the breast meat of chickens is lighter than the dark meat of their legs? Well, like us, a chicken’s muscles are made up of both fast and slow twitch fibers, and each plays a distinct role in a chicken’s daily activities. While a chicken’s legs use comparatively little energy (with all that idle standing around they do), their breasts are instrumental in producing brief bursts of futile flightless flapping. It may come as no surprise to learn that those high powered breasts are packed with fast twitch fibers while the less meaty legs are predominantly slow twitch.[2]

So why such a difference in color between fast and slow twitch fibers? Muscles which contain many slow twitch fibers require a constant supply oxygenated blood, and it is the presence of capillaries and the oxygen carrying molecule, myoglobin, that gives red muscle fiber its distinct color.[2][6] Because fast twitch fibers do not need oxygen to produce energy, they do not contain as many capillaries or myoglobin molecules, and as a result, are lighter in color.

Can We Change Our Muscle Fiber Composition?

For those born without the blessings of a favorable muscle fiber composition, there is still hope. It is thought that by training for strength and growth over endurance, we may develop and become more efficient at using all of our muscle fibers for a specific purpose.[4][6] One way to do this is to train with low to moderate reps (6-8), working explosively on the concentric part of each rep, and resting for 1-2 minutes between sets.[7] Despite this, we are not likely to completely change muscle fibers from one type to another, therefore, we must take advantage of our strengths and pay mind to our weaknesses.

For example, two MMA fighters may train the same way using similar training protocols, but one will stand out as being more muscular and more explosive, while the other will be more of a grinder with a greater capacity to wear their opponent down over a full three rounds. While the former may go for the fast knockout and possess more power and speed, the latter will likely have the edge the longer the fight progresses. Here we have two elite athletes training to full capacity using similar training methods with two distinctly different outcomes, both of which can end in victory. Likewise, two aspiring bodybuilders may train using the exact same methods but one may have the edge in explosiveness and muscular development due to a greater distribution of fast twitch fibers, therefore, the less blessed bodybuilder will have to work that much harder to make up the difference.

More Than Your Genes

That our fast twitch/slow twitch muscle fiber ratio determines the degree to which we will ultimately succeed, does not mean we should give up on reaching our full potential. Rather, we must work harder than ever to overcome any deficits, to correct any weaknesses, and earn out victories.

References

1. Andersen, J. L., Schjerling, P., & Saltin, B. Muscle, Genes and Athletic Performance. Scientific American. 9/2000 – page 49
2. BBC: Science – Human Body and Mind. Muscles: Fast and Slow Twitch. [Online] http://www.bbc.co.uk/science/humanbody/body/factfiles/fastandslowtwitch/soleus.shtml retrieved on 1.3.15
3. Contreras, B. cure for Cowardly Calves. T Nation.  [Online] https://www.t-nation.com/training/cure-for-cowardly-calves retrieved on 28.2.15
4. Ingalls, G. Nature vs. nurture: can exercise really alter fiber type composition in human skeletal muscle? Journal of Applied Physiology Published 1 November 2004 Vol. 97 no. 5, 1591-1592
5. Ingjer, F. Effects of endurance training on muscle fibre ATP-ase activity, capillary supply and mitochondrial content in man. J Physiol. 1979 Sep; 294: 419–432.
6. NSW HSC Online. Effect on Fast/Slow Twitch Muscle Fibers. [Online] http://www.hsc.csu.edu.au/pdhpe/core2/focus2/focus1/4007/2-1-4/fac2_1_4_6.htm retrieved on 1.3.15
7. Nilsson, N. Training to Maximize your Muscle Fiber Types. [Online] http://www.bodybuilding.com/fun/betteru25.htm retrieved on 28.2.15
8. Thayer, R., Collins, J., Noble, E. G., Taylor, A. W., A Decade of Aerobic Endurance Training: Histological Evidence for Fibre Type Transformation. Journal of Sports Medicine & Phys Fitness. 2000 Dec; 40(4).