Muscle Fiber Types

There are two main types of muscle fibers:  slow-twitch fibers and fast-twitch fibers.  Within the fast-twitch fibers there are two sub-types:  Type IIx and Type IIa.  Below are the difference and what you  need to know about each.

Type I Slow-Twitch Fibers
   *Slow Oxidative (or aerobic)
   *Resistant to fatigue and capable of sustaining aerobic metabolism (endurance fibers)
   *Smaller than fast-twitch fibers
   *Contract more slowly than fast-twitch fibers

Type IIx Fast-Twitch Fibers
   *Limited capacity for aerobic metabolism
   *Have high number of glycolytic enzymes which provides a considerable anaerobic capacity
   *Fatigue more easily than slow-twitch fibers & cannot sustain their efforts for more than a few
     seconds (under two minutes at most)
   *Largest and fastest muscle fibers and capable of the most force

Type IIa Fast-Twitch Fibers
   *Intermediate  or fast-oxidative glycolytic fibers
   *Possess speed fatigue and force-production capabilities somewhere between slow-twitch and Type
     IIx fast-twitch fibers (up to three minutes)
   *Used for strength and power activities
   *Are highly adaptable and are capable of increasing their oxidative capacity to levels similar to
     those in slow-twitch fibers

"A muscle's fiber-type composition is typically an equal mixture of both fast- and slow-twitch fibers, though some muscle groups are known to be made up of primarily fast- or slow-twitch fibers.  The percentage of specific fiber types contained in skeletal muscle may be influenced by genetics, hormones, and the activity and exercise habits of the individual."  (ACE's Essentials of Exercise Science for Fitness Professionals)

Even though people respond to specific exercise programs based on their own individual genetic composition, studies have shown that a combination of high-intensity resistance training and short-interval speed work can cause the conversion of Type I fibers into Type IIa fibers (Anderson et al., 2000; Staron et al., 1990)  Therefore, muscle-fiber composition is only one variable that determines success in overall physical performance.