Muscle Fiber Type

Moving away from my tendency to speak about the biomechanics of weightlifting and such, I will this time talk about some exercise physiology that I believe you may find interesting.  I’ll say it now, I’m a bodybuilder by day and a nerd by night.  I truly enjoy the science behind the sport.  It intrigues me to see how intricately the body works and how I can change my body by diet and exercise.  Anyways, for this blog I want to talk about the two types of muscle fibers, slow twitch (Type I) fibers and fast twitch (Type II) fibers. 

Why the heck should you care about muscle fiber types right?  Well before I spoon feed you some more information about why the two different types are so great, lets first take a look at some of the basic physiology of the two types of fibers.  Type I are oxidative fibers and exhibit a large capacity for aerobic metabolism.  Aerobic metabolism is most predominant in these types I fibers due to the fact that they have lots of capillaries surrounding them, high mitochondrial enzymes and high myoglobin concentration.  Because these type one fibers are most active during and responsible for aerobic activities we see these fibers in higher percentages in distance runners as opposed to sprinters.  Why?  Because sprinters engage in explosive exercise, this explosive exercise is anaerobic.  Weightlifting is also an example of anaerobic exercise.   Type II fibers are utilized and most active during anaerobic exercise.  These fibers are fast-glycolytic fibers.  Type two fibers are categorized into type IIa and type IIx fibers.  Type IIa are sort of like an intermediate fiber or combination between type IIx and type I fibers and share characteristics with both.  Type IIx fibers, however, differ greatly from type I fibers.  Type IIx fibers have a lower number of mitochondria resulting in them being more anaerobic in powerful as opposed to type I fibers which have a high endurance to stimulus but aren’t as powerful.  The powerful component of both fiber types is result of the Vmax which is the speed of shortening of the muscle fiber itself. 

Now then the question remains, can you convert these fiber type from type I to type II or vice verse to tend to your fitness needs?  We do know that exercise induced changes occur in skeletal muscle and are specific to the type of training that is performed.  The most common example is the muscular adaptation to strength training, which is increase size and force production of the muscle.  This increase in muscular size is most likely due to hypertrophy, which I discussed in a previous blog.  Endurance exercise such as running, does not promote increased muscular size but does increase the oxidative capacity of the muscles used.  So can these two types of training induce the actual conversion of one fiber type to the other?  Recent studies have actually proven that exercise induced conversion of muscle fiber types do happen! Good news for those of us who train. However, most of the studies were conducted concerning endurance exercise and high intensity training (not in terms of resistance training).  However, this is promising research considering that older studies showed no conversion at all!