There are three types of muscles: cross-striated (or skeletal), smooth muscles and cardiac muscle. Unlike skeletal muscles, heart and smooth muscles are not controlled by the will. Skeletal muscles consist of slow and fast muscle fibers. The following article aims to explain the differences between slow and fast muscle fibers and what impact training has on muscle fibers.

Depicted muscle section has been treated with ATPase to stain type 2 muscle fibers black; Type 1 muscle fibers appear light-colored. The fibers are of approximately equal diameter in this normal muscle. The proportion of type 1 to type 2 muscle fibers is genetically determined and is different between individuals. On the right side of the picture is a biopsy specimen from the same individual after receiving corticosteroids for 3 months. The type 2 fibers stained black are on the right markedly atrophied, which is characteristic of a steroid myopathy. Patients with steroid myopathy have proximal muscle weakness but do not have elevated creatine phosphokinase.


Slow muscle fibers (often referred to as type I) are fibers that contract slowly and are very durable (have great resistance to fatigue). Muscle fibers type I’s role is maintaining the posture and being dominantly active during lower intensity exercises (e.g. jogging). Why is that? One reason is that such fibers possess more mitochondria, so they have a great oxidative ability and they release ATP slower. In addition, slow muscle fibers have lower glycogen storage capacity. Because of the mitochondria, increased aerobic production of energy and high blood flow these muscular fibers look red, and are often referred to as red muscle fibers. But the amount of force that these muscles can produce is smaller than in fast muscle fibers.


Fast muscle fibers are divided into IIA and IIX (or IIB). Although, the division of muscle fibers by some authors differs from this basic and commonly used division. Namely, Staron and Hikida (1992) divide slow muscle fibers on type I and type IC, and fast muscle fibers on types IIA, IIB, IIC, IIAB and IIAC. However, for the purposes of this article our foundation will be division of fibers on IIA and IIX. Fast muscle fibers are often called white muscle fibers.

Type IIA

Type IIA fibers are also called oxidative-glycolitic since these are muscular fibers that get energy from both oxidative (aerobic) and glycolytic (anaerobic) energy mechanisms. The fibers of IIA are activated primarily during movements and activities that are fast, repetitive and somewhat lower intensity. These fibers are activated after the type I. This type of fiber possesses a plenty of mitochondria and is rapidly recovering after activity.

Type IIX

The IIX fibers are rapidly contracting and activating during high intensity activities such as certain athletic disciplines and Olympic weightlifting. A small number of mitochondria, a lower oxidative and higher glycolytic ability are characteristics that describe the fibers IIX. It is important to understand that these fibers tire very quickly and their recovery is slightly slower, which comes mainly after physical activity. However, unlike type I, type IIX fibers have the capability of generating high force and are almost exclusively responsible for overcoming maximum loads.

When we consider the composition of certain muscles it is important to understand that they are not made of exclusively slow or fast muscle fibers, but a combination of one and the other. The exact ratio depends on number of factors, but certain muscle groups often have more of one or the other fibers. Thus, in the muscles of the leaf, i.e. m. soleus possesses more slow muscle fibers than gastrocnemius. The primary functions of m. soleus are walking and maintaining the upright position of the body, i.e. activities that may last longer but are of lower intensity. Unlike him, m. gastrocnemius is predominantly composed of fast muscle fibers, so it is most activated when running, jumping and executing other rapid movements using feet.

Characteristics TYPE I TYPE II TYPE IIX
Contraction velocity Slow Fast Very fast
Oxydative capacity High High Low
Diameter Small Moderate Big
Resistance to fatigue High Moderate Low
Low Moderate-high Very high


Muscle fibers and training

One important aspect of all the above is the question, how much are actually muscular fibers influenced by training? Namely, the already known fact is that persons engaged in endurance disciplines have a higher proportion of slow muscle fibers compared to fast fibers, and that people who are engaged in Olympic weighting have a higher number of fast muscle fibers. Costill et al. (1976) found that the ratio of slow to fast fibers in inactive population is 50:50, endurance athletes 60-70% of type I fibers and sprinters 70-80% type II fibers.

Has the genetic component led these individuals to one or another sport or has the specificity of training resulted in these differences?

The plasticity of muscle fibers reveals that there is a high probability of changing the muscle fibers from one type to another, of course, when these muscular fibers are exposed to adequate stimuli. The most common change was discovered from type IIA to type IIX and vice versa, however much less evidence exists about changes from type I to type II.

One of the variables that seems to affect the change of fiber from type I to II is the rate of contraction. Liu et al. (2003) investigated changes in muscle fibers of long-head of triceps in 24 subjects who had 3-5 months of training experience. Subjects were divided into two groups, both trained 3 times a week, 6 weeks in total. The first group has performed every training 3 RM on bench press, while the second on Mondays did the same training as the first group, on Wednesdays they did 10 concentric repetitions on the bench press with 30% 1RM, and on Fridays 10 plyometric pushups, with 5 series per exercise.

The results showed the following:

  • in the first group there is a decrease in fibers IIX, increase in IIA without change in type I.
  • in the second group there is an increase in the fibers of type IIA and reduction of type I by 50% (from 18.2 to 9.2%).

These results indicate that the high speed of contraction is one of the variables that can affect the changes between the muscle fibers. Precisely, the training that involves high-speed contractions explains why sprinters and weightlifters have more fast muscle fibers than endurance athletes.

What about transition from fast to slow fibers?

Howald et al. (1985) in 10 subjects investigated the possibility of transforming fast to slow fibers. Subjects rode cycling ergometer for 6 weeks, 5 times per week. Six weeks later an increase in the number of type I fibers by 12% was detected and a reduction in the IIX type by 24%. These data also indicate the possibilities of change from type II to type I.

Certain information also tells us that physical inactivity can cause changes among muscle fibers (Hortobagyi et al. (2000)), but this data does not seem interesting as determining exercise induced changes in muscle fibers.


Based on the current knowledge it is possible to give certain recommendations for people who want to increase the amount of fast or slow muscle fibers. Namely, persons wishing to increase the amount of slow muscle fibers should execute more low intensity long duration trainings. A larger amount of slow muscle fibers will have a great impact on performance in endurance sports. People who want to increase the amount of fast muscle fibers should focus their training on high intensity, and/or a large contraction rate, with slightly lower volume. Albeit there is evidence of the possibility of changing one type of fiber into another many questions are still unanswered.

  • Do certain muscle groups have greater changing capabilities than others?
  • Once the change comes is it permanent?
  • How capable are we for changing fiber types with training, i.e. does a person who is born with 30% of fast muscle fibers can increase that amount with training to 70-80%?

Very interesting years of research are head and we should be certainly looking forward to acquiring new data that are yet to come.