I touched on the basics of muscle fiber types and motor unit activation in the last article. In this post I’ll dive further into muscle fiber types and what makes them different. As I said in the last post, we have 2 main types of muscle fibers, type I and type II.
But these fiber types can be differentiated even further.
While some researchers have identified seven different fiber subtypes based on observed myosin isoforms, for today’s purposes we will discuss three - type I, IIa, and IIb.
An important difference to understand when thinking about muscle fiber subtypes is their oxidative capacity.
Oxidative capacity refers to a muscle fiber’s ability to use oxygen (aerobic metabolism). Each of these three fiber types generally has a distinct metabolic pathway they use.
Type I have the greatest ability to use oxygen in ATP creation due to its high mitochondrial density. These are referred to as slow-oxidative fibers.
Type IIb have very little ability to use oxygen to due their lack of mitochondria and fatigue extremely fast—these are fast-glycolytic fibers.
Type IIa fall somewhere in the middle with the best of both worlds. These fibers can produce ATP through aerobic and anaerobic metabolism (with and without oxygen), making them more fatigue resistant than other fast-twitch fibers - these are referred to as fast-oxidative.
Through training, individuals can experience a transition from inefficient IIb fibers toward the more efficient IIa fibers.
Interestingly, highly sedentary individuals have a higher prevalence of type IIb fibers than their trained counterparts.
This is an extremely important point to understand.
While an individual is born with a certain fiber type composition, they will respond to the demand imposed from training.
For example, a person will move from fast twitch toward a more slow-twitch phenotype, most commonly from IIb to IIa. The opposite is also true when detraining occurs, fibers will move back toward an inefficient fast-twitch phenotype.
The dynamics of fiber type differentiation are more involved than what’s explained here, but this a good place to start in understanding how our muscles respond to training demands.