Muscle Fiber Types Explained - Type I, Type IIa, Type IIx

Aug 05, 2022

Edited By: Danielle Abel, MSN, CSCS

Muscle Fiber Types Explained

Muscles themselves are compromised of many smaller units. It's kind of like those little stackable dolls that fit inside one another; muscles are structurally very similar. 

Muscular Anatomy

Muscles are compromised of multiple smaller structural subunits layer inside of one another. 

So it looks something like this:

  • Muscle - for example the bicep
    • Muscle fascicles - made up of muscle fibers organized into groups 
      • Muscle fibers - made up of myofibrils  
        • Myofibrils - made up of myosin, actin, & mitochondria 

Each fascicle is innervated by 1 nerve. This subset is classified as a motor unit. A motor unit is essentially a group of muscles controlled by 1 nerve that is activated as a group. 

The Type I, Type IIa, & Type IIx are all different types of fibers. They are distributed in groups to make up a full muscle belly. The unique combination of muscle fiber types within each different muscle is referred to as mosaic distribution.

There are many Type I fibers, many Type IIa fibers, & many Type IIx fibers. Each set of myofibrils is controlled together, but sequentially, as one group. So a few Type I fibers are activated, then a few more Type I fibers, etc. Once all the Type I fibers are activated, the Type IIa myofibrils start to become activated.  

Activation Sequence 

Type I myofibrils are always activated & recruited first (also referred to as Henneman's Size Principle). Then Type IIa is activated, then lastly, Type IIx myofibrils are activated. 

Type I fibers are oxidative and fatigue resistant, so it is efficient for the body to use these first. 

*Note: There is a principle called selective recruitment. There is debate around this topic, but well trained lifters MAY be able to preferentially recruit their type 2 fibers first. In most cases, however Henneman's Size Principle holds true. 

Slow Twitch 

Type I myofibrils are referred to as slow twitch because the nerve conduction velocity is slow. Additionally, the time that it takes to cause a muscle twitch (the muscle to move) is slow. 

Fiber properties

  • Small fibers
  • Fewer total fibers
  • Red fibers (high capillary density)
  • High mitochondrial density (oxygen delivery)
  • Recruited first
  • Low force activity
  • Low power
  • Low oxidative capacity
  • High aerobic enzyme content
    • More enzymes present for aerobic glycolysis 
  • High in myosin ATPase (amount of contractile force & the rate of force production)
  • High resistance to fatigue

Athlete Types

  • Endurance athletes
  • Cross country
  • Half marathon, marathon
  • Cycling
  • Distance swimming

Fast Twitch

Type IIa have mixed properties, meaning they have oxidative and glycolytic properties. The fatigue resistance and force capacity is mixed. So they are less fatigue resistance compared to Type I fibers but more fatigue resistance than Type IIx fibers. Their capacity to produce force is also mixed where they can produce more force than Type I fibers but less force than Type IIx fibers. 

There are also more Type IIa myofibrils compared to Type I within a fascicle. 

  • Ex: 100 Type I myofibrils
  • Ex: 200 Type IIa myofibrils 

Fiber properties

  • Larger fiber type
  • More total fibers
  • Low myoglobin (takes oxygen from the blood and delivers it to the muscle)  
  • Faster contraction
  • Faster relaxation
  • Higher nerve conduction velocity
  • Less fatigue resistance
  • High anaerobic enzyme content
    • More enzymes present for anaerobic glycolysis 

Athlete Types

  • Basketball
  • Baseball/Softball
  • Soccer
  • Short Distance Swimming

Type IIx is the biggest fiber type with the most amount of power and force production

Fiber properties

  • Largest fiber type
  • Largest motor units
  • Large fiber diameters
  • White fibers
  • Low blood supply (limited capillaries)
  • Low mitochondrial density (oxygen delivery)
  • Highest force production
  • Highest power output
  • Very low fatigue resistance
  • Requires longer rest periods
  • Highest recruitment threshold (recruited last)

Athlete Types

  • Sprinters
  • Olympic Lifters
  • Powerlifters
  • Long Jumpers

Well-trained lifters tend to show a very small percentage of type 2x fibers after years of training and the following adaptations have occurred.

Muscular Adaptations to Training

The most common muscle fiber type transition occurs from Type IIx to Type IIa fibers. Both aerobic and resistance training produce more Type IIa fibers. Type IIa fibers are more of an athletic fiber overall, meaning they can produce high force production yet are also fatigue resistant. They are a great intermediate fiber type. 

If an athlete is training a great deal of oxidative, endurance style training, they will also have Type I muscle fiber hypertrophy along with increased oxidative enzymatic activity as well. 


Detraining occurs when the nerves that innervate the muscles are not activated as frequently or forcefully.  Meaning the connection from the brain to the nerves, to the muscles atrophies. Detraining can occur if an athlete stops training or lack of movement occurs due to the aging process. The process of denervation occurs in which the connection is reduced or lost, specifically high threshold Type IIx and Type IIa fibers. 

The muscle fibers that are no longer being recruited lose their innervation.

Innervated fibers can 

  • Atrophy
  • Pick up innervation from Type I fibers 

If more and more Type I nerves begin to pick up more and more Type IIa and Type IIx fibers, a lack of dexterity can occur because motor control overall is reduced, and our ability to move well is affected. So the thought here is that being able to continue to train and recruit Type IIa and Type IIx fibers can help reduce the aging process and maintain movement into older age.  

Muscle Fiber Transitions

The type of myosin heads that are present on a fiber referred to as myosin isoforms determine the fiber type. As resistance training occurs, myosin heads are damaged. When the myosin heads are repaired, the head can be repaired as a myosin heavy chain or a myosin light chain myosin. The myosin heads have the capability to change the fiber type and the associated properties over time. It's not clear from the research to what extent this occurs. 

So as training occurs and muscle breaks down, muscle protein synthesis occurs to repair the damage. The reparative process stimulates an increase in myosin, and how the myosin head is repaired determines the fiber type transitions. These processes are referred to as an increase in myofibrillar volume, aka muscle hypertrophy. 

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