Are You Fast or Slow?

Fast Twitch, Slow Twitch, Red, White....What does it all mean?

Skeletal muscle is made up of different fiber types and each fiber type functions differently.  I'm sure a lot of you have heard the term fast or slow twitch muscle fibers which are sometimes referred to as red or white fiber. Basically, there are three main types: 1) Type I (aka slow twitch, red fiber type), Type IIa (aka fast twitch-a, hybrid) and TypeIIx (aka fast-twitch-x, white fiber type).  The top picture below shows what a stain of muscle tissue looks like.  The red areas represent Type I and Type IIa fibers, while the white areas represent Type IIx fibers.  As science progresses and the techniques used to measure fiber types become more sensitive, more and more isoforms (hybrids or types in-between the big 3) of these fiber types are being discovered.  The bottom picture portrays the different isoforms and how they are classified.

Each name implies a slightly different meaning and is basically just a term to characterize the different fiber types.  Fast twitch is a term given to muscle fibers that can contract very quickly (the name says it all) compared to slow twitch fibers that contract in a much slower fashion.  Because fast-twitch fibers contract so quickly and powerfully, they also fatigue easily.  Therefore they are best-suited for fast, explosive movements.  On the other hand, slow-twitch fibers are more fatigue resistant; meaning they can continue to work for extended periods of time.  That explains how muscle fibers are characterized by contraction speed but why the red/white names?  Slow twitch fibers are typically red because they have a rich blood supply.  They are built for slow contractions performed over an extended period of time, so they derive most of their energy from aerobic metabolism (oxygen consuming).  On the other hand, type II fibers are often white because they have a poor blood supply and derive the majority of their energy from anaerobic metabolism (without oxygen).  So how does this apply to performance? 

Each person has a different percentage of these fibers distributed throughout their musculature which may help them perform better at certain types of exercises/events. Your fiber type isn't necessarily going to predict what kind or how good of an athlete you will be but it can be a predictor of your genetic potential.  For example, if the majority of your muscles are made up of fast-twitch muscles, marathon running probably isn't going to be your forte.  However, you can "manipulate" your fiber type to some extent.  Research has shown that any type of exercise will shift your type IIx fibers towards more of a typeIIa isoform, meaning it becomes more aerobic in nature and will be more fatigue resistant.  So there is hope!