Miller JZHere’s the second installment of Dr Miller’s Q&A column, Adaptions. If you have any physiology, performance, or nutrition related questions, email Ben at


Dr. Miller,


I’ve heard there are two major types of skeletal muscles fibers, fast twitch and slow twitch, and that there are as many as 3 different sub types of fast twitch fibers.  In the context of bike racing I’ve also heard that individuals who have more slow twitch fibers are better suited for endurance events and those with more fast twitch are better suited for shorter events.  I’m not sure how accurate this is and I’d like to understand more about muscle fiber types and their impact on training and racing.  I have several questions on this topic:

What are the different skeletal muscle fiber types? 

Is it possible to determine the types of muscle fibers we have?  

Is it possible to “convert” one fiber type to another?  

Is it possible to isolate and train one type over the other?

Does a preponderance of one fiber type determine the events for which an individual is well suited?  

Does fiber type impact recovery between workout sessions?   




Jeff Homan


Thank you for the question, Jeff.  Skeletal muscle is a unique tissue and I have spent many years studying it.  Some of its interesting features are that by mass it is the largest tissue in the body and therefore dictates a lot of the Miller Colmunmetabolism of your body, it is has many nuclei per cell (not just one like most cells), and it does not replicate cells (which is why you don’t find muscle cancer) since it is stuck in a cell cycle phase called G0. Skeletal muscle can function differently in different parts of the body and that is partly dictated by what fiber types are in a particular muscle.  I will do my best to answer your questions below. 

What are the different skeletal muscle fiber types?  This question is actually probably more complicated than you think.  I will present the muscle fiber types in their most basic classification to increasing complexity.  This order is also roughly equivalent to the chronological adaptation of classifications.  First, you can classify muscle as red or white.  Most people know from a very young age that there is a difference between white meat and dark meat and that is due to different muscle fiber types.  During the 1960’s it was determined that muscle fiber types differed in their speed of contraction and fatigue resistance.  When muscle was sectioned, stained for the activity of different enzymes, and viewed under a light microscope, the differential staining corresponded differences in speed and fatigue resistance.  This “histochemical” classification lead to three fiber types that are known as Type I (slow oxidative or slow twitch), Type IIa (fast oxidative glycolytic or fast fatigue resistance), and Type IIb (fast glycolytic or fast fatigable).  The fiber typing (depending on the term you use) is based on its speed, fatigue resistance, or primary energy metabolism. With the advent of molecular biology, a new classification system emerged.  In this scheme, the type of myosin heavy chain (a protein that forms part of the little arm that reaches across, engages a stationary protein called actin, and cycles to shorten the muscle – called a molecular motor, Image) is classified by a technique that uses antibodies to identify specific proteins.  From this identification scheme, a new muscle type called Type IIx was found.  The IIx fiber type had properties that put it somewhere between IIa and IIb.  What is now accepted is that different muscle fibers are really somewhere on a continuum as I ↔ I/IIA ↔ IIA ↔ IIA/IIX ↔ IIX ↔ IIX/IIB ↔ IIB, which makes for some very confusing classification.  Those wanting to know more can read this excellent and very detailed review (Schiaffino & Reggiani, 2011).  For simplicity, it is easiest to understand the Type I, IIa, and IIb scheme; with Type I being slow to fatigue and slow to contract, Type IIa being somewhat slow to fatigue and faster, and Type IIb being very quick to fatigue but fast.  Also, it is important to know that this is also the order in which fiber types are recruited during a contraction.  You recruit slow fiber types first followed by increasingly faster more explosive ones.  This recruitment order maximizes fatigue resistance and smoothness of contraction (you don’t need to engage the very explosive muscles to lift a paper clip).  The nerve that goes to the muscle from your brain dictates the recruitment. 


Is it possible to determine the types of muscle fibers we have?  Yes, you can do this by having a muscle biopsy and having the muscle fiber typed.  The fiber typing can be done by histochemical techniques or by western blotting (both described above).  I don’t believe this is a routine diagnostic procedure as it is typically used for research purposes. 

Is it possible to “convert” one fiber type to another? As with many things in science, more sophisticated methods often indicate that what was thought to be the answer, isn’t necessarily true.  The old answer was no.  What is known now is that fiber type can change and that this is often dictated by the properties of the nerve that goes to that fiber.  A given motor nerve (the name of the nerve that innervates muscle) goes to multiple muscle fibers (Figure courtesy of Dr. Brian Tracy).  The muscle fibers it goes to are all the same type.  Therefore, the firing of a motor nerve elicits the recruitment of different fibers.  It was shown in an animal model that if you switch a motor nerve that went to slow twitch fibers to fast twitch fibers, the fast twitch fibers become more like slow twitch fibers.  This experiment was also done in the reverse direction.  This experiment showed that muscle fibers could change types.  However, it did not necessarily show that this conversion does happen regularly (only that it could theoretically happen).  What is now accepted is that certain fiber types have an adaptive range.  You can think of this as becoming more like the fiber types that are most similar.  It is unlikely that a Type IIb fiber will become a Type I fiber, but it may become more like a Type IIx fiber.  As mentioned, the nerve that goes to the muscle fiber likely controls this. 

Is it possible to isolate and train one type over the other?  Of course.  As mentioned, muscle fibers are recruited in order of small low force generating fatigue resistance, to large high force generating fatigable.  Think of your back postural muscles that are active all day (low force fatigue resistant) to those that would be required to jump up on a box (explosive but fatigable).  You only recruit what is needed to do the work at hand.  Biking a long distance easily requires Type I muscle fibers, doing an all out track sprint requires the most explosive type IIb.  To train the different fiber types, just use this logic.   Some new emerging research indicates that Type IIa are the most malleable of fiber types and are likely the most trainable.  In fact performance gains during a “taper” period, may be solely to increases in Type IIa function.

Does a preponderance of one fiber type determine the events for which an individual is well suited?  Yes to a certain extent.  You are born with a genetic predisposition toward certain fiber type characteristics.  However, as mentioned above, muscle fibers are adaptive (called plastic) to the stresses imposed on them and can take on other characteristics.  The easiest way to think of this is that genetics has dealt you a hand, but how you play that hand will determine the outcome.  You can’t have different cards than what was dealt you, but you can certainly play that hand better or worse. 

Does fiber type impact recovery between workout sessions?  This question is difficult to answer because it has many variables that need to be considered, which mostly have to do with what sort of workout is being performed (maximal contraction, submaximal endurance, weights, biking, etc), but also how long the recovery period is (seconds, minutes, or days).  In general (and this is a big generality) Type IIb fibers have a greater decrease in maximal force generation and take longer to restore their maximal force when compared to Type I. 

Schiaffino, S., & Reggiani, C. (2011). Fiber Types in Mammalian Skeletal Muscles. Physiological Reviews, 91(4), 1447–1531. doi:10.1152/physrev.00031.2010