Sprinter or Distance Runner...What Do You Relate To??

Are you a better sprinter or distance runner?  Many people believe that having more fast or slow twitch muscle fibers may determine what sports athletes excel at and how they respond to training.

Let's have a brief anatomy lesson so you understand what muscle fibers are all about.  Skeletal muscle is made up of bundles of individual muscle fibers called myocytes. Each myocyte contains many myofibrils, which are strands of proteins (actin and myosin) that can grab on to each other and pull.  This shortens the muscle and causes muscle contraction. It is generally accepted that muscle fiber types can be broken down into two main types: slow twitch (Type I) muscle fibers and fast twitch (Type II) muscle fibers.  Fast twitch fibers can be further categorized into Type IIa and Type IIb fibers. 

These distinctions seem to influence how muscles respond to training and physical activity, and each fiber type is unique in its ability to contract in a certain way.  Human muscles contain a genetically determined mixture of both slow and fast fiber types.  On average, we have about 50 percent slow twitch and 50 percent fast twitch fibers in most of the muscles used for movement. 

Let's break down each type of muscle fiber so you get a better understanding of what we are talking about.

Slow Twitch (Type I)

The slow muscles are more efficient at using oxygen to generate more fuel (known as ATP) for continuous, extended muscle contractions over a long time.  They fire more slowly than fast twitch fibers and can go for a long time before they fatigue.  Therefore, slow twitch fibers are great at helping athletes run marathons and bicycle for hours. 

Fast Twitch (Type II)

Because fast twitch fibers use anaerobic metabolism (e.g., metabolism in the absence of oxygen) to create fuel, they are much better at generating short bursts of strength or speed than slow muscles.  However, they fatigue more quickly.  Fast twitch fibers generally produce the same amount of force per contraction as slow muscles, but they get their name because they are able to fire more rapidly.  Having more fast twitch fibers can be an asset to a sprinter since she needs to quickly generate a lot of force.  

Type IIa Fibers

These fast twitch muscle fibers are also known as intermediate fast-twitch fibers. They can use both aerobic and anaerobic metabolism almost equally to create energy.  In this way, they are a combination of Type I and Type II muscle fibers.

Type IIb Fibers

These fast twitch fibers use anaerobic metabolism to create energy and are the "classic" fast twitch muscle fibers that excel at producing quick, powerful bursts of speed.  This muscle fiber has the highest rate of contraction (rapid firing) of all the muscle fiber types, but it also has a much faster rate of fatigue and can't last as long before it needs rest.

So, with that being said, are you a sprinter or a distance runner (even if in theory)?  Take a look at the different body types in various sports and you'll begin to understand the difference in muscle fibers.  And that, our friends, is your anatomy lesson for the day!!