Born With it – INFS Health Series: Muscle Fibers Part 2

Author: Utsav Agrawal, Visiting Faculty, INFS


  • It has been found that pre-birth period is more important for muscle fiber numbers than a post-birth period but postnatal nutrition is equally important in respect of growth of muscle fibers size. Malnourishment in both the period causes severe implications.
  • Genetics play a key role in muscle fiber composition, but to what extent, is still unknown.
  • Ratio of fiber types of skeletal muscles is probably not significantly altered by physical training in adults. The area of a given fiber type may however, change in response to training.


In the last article, we discussed the concept and types of muscle fiber and hypertrophy. Now let’s have a look at the effect of postnatal (after-birth) and prenatal nutrition (pre-birth) on muscle fibers, gender variability on muscle fibers and how muscle fibers composition is dependent on physical activity and Genetics.

It has been established that skeletal muscles are comprised of following muscle fiber types:

  1. Pure Types: Type I, Type IIa, Type IIx (historically known as Type IIb)
  2. Hybrid Types: Type I/IIa, Type IIa/IIx, and Type I/IIa/IIx

The total number of muscle fibers is mainly determined prenatally. In general, muscle fibers number remains almost unchanged during postnatal growth. In popular textbooks on exercise physiology, one finds two major and different points of view typically expressed: either the percentage of type I fibers in human skeletal muscle is set by the genes or fiber type distribution is partly genetically determined and thus perhaps amenable to change with exercise training. For example, the slow-twitch content of vastus lateralis(one of the muscles in quadriceps) ranges from 5-90%.

This variability, in turn, may determine individual’s potential to perform different types of resistance training. Accordingly, data show that Type I muscle fibers have high resistance to fatigue and are thus suited for low-intensity resistance or aerobic (endurance) training, IIA fibers are better suited for medium-term anaerobic(resistance) exercise and type IIX fibers are adapted for high-intensity (power and strength) exercise.


  • Difference between the sexes:

In most cases males exhibited higher muscle fiber number compared to female, it has been concluded that differences in muscle fiber number between males and females can arise by hormonal action if differences in androgen hormones (male hormones) are sufficiently high during the period of prenatal fiber formation.

Additionally, differences in fiber number have been related to different physical activity between male and female muscles. Some studies also suggested that the women have more type 1 muscle fiber than men. However, to what extent these differences are due to hormonal actions, different patterns of activity and/or other factors remains unclear.

  • Inheriting the fiber composition

It is generally believed that muscle fiber composition is dependent on genetics, especially type -1 muscle fibers. To investigate the question of the influence of genetic factors on the proportion of fibers types, a study was conducted where muscle fiber samples were obtained from 32 pairs of brothers, 26 pairs of male and female dizygotic twins(developed from different egg), and 35 pairs of male and female monozygotic twins(developed from same egg). Although brothers and dizygotic twins share about one-half of their genes, it also appears by this study that there is increased resemblance for the proportion of type 1 muscle fibers. Even the difference between type 1 fibers in monozygotic twins was not huge. These investigations clearly suggest that some individuals have a high or a low prevalence of muscle type I or type II fibers because of genetics.

At last this study concluded that approx 40 to 50% of type 1 muscle fiber in total composition is dependent on genetics. But it has to be considered that this study is done only on twins hence, it lacks enough evidence and can-not hold true in every condition.

  • Transforming the fibers with training:

Sedentary individuals are known to have almost 20-40% of hybrids muscle fiber in the total fiber composition. It has been observed that when they start training, the muscle fiber areas for all the types increases and these hybrids can transition to pure types (i.e. Type I/IIa converts to Type I or Type IIa) as well.


To understand the effect of the kind of physical activity on muscle fiber composition, a study was conducted on 74 men between the age of 17 and 58 years. These subjects were selected to represent  different age groups and states of physical fitness and to encompass men who were participating in a variety of sport activities that used different muscle groups and training programs. It concluded that the type of training also influences  the relative size and number of the two fiber types. Weightlifters had more type 2 muscle fiber in both leg and arm whereas the swimmer had more type 1 muscle fibers in their arm and leg. Also that a particular muscle group which involves actively in endurance activity has more type 1 muscle fiber.

These results can be justified if we consider that fast twitch fibers are only called upon when the resistance becomes too heavy for the slow twitch fibers. Hence, muscle fibers need to be fatigued, not just stimulated, in order to grow. Finally that fast twitch fibers need a higher intensity to grow than slow twitch fibers.

In a recent review paper, the concept was put forward that the proportion of slow twitch muscle fibers in human is resistant to change with response to endurance training, but in this study, only 10 subjects were involved, hence the results found were not convincing enough. Whereas at least 5 different studies suggest that regular exercise has an influence on the proportion of slow twitch muscle fiber in human but the changes are generally modest in magnitude.

Thus, training may contribute to shifting in the fiber types, but this shift is primarily from hybrid to pure or within the same types (Type IIa to IIb and vice-versa, depending on the nature of the physical activity). There is not enough data to conclude the possibility of conversion of type I to type II (or type II to I), although there is some evidence that this could be possible.

  • Eat to transform?

The prenatal period of muscle development is more sensitive to nutritional deficiencies in reducing muscle fiber number than a postnatal period. Fiber formation is largely completed around the time of birth in mammals. During postnatal growth, the increase in skeletal muscle mass is mainly due to an increase in muscle fiber size (hypertrophy). After birth total muscle fiber number has been reported to remain unchanged in mammals and birds. Postnatal muscle fiber hypertrophy strongly depends on the total number of muscle fibers within a muscle.

Adequate nutrition is essential for normal skeletal muscle growth. Malnutrition during postnatal  growth reduces body weight and skeletal muscle weight. Malnutrition both in quantity and in quality (protein deficient diets) has been reported to lead to decreases in muscle fiber diameters.  With regard to changes in muscle fiber number in response to postnatal dietary restriction, contradictory results have been reported. Fiber numbers were not influenced by postnatal malnutrition.

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Figure showing extensor digitorum muscle characteristics in response to the post- wearing quantitative feed restrictions.

In summary, whether postnatal malnutrition is able to induce muscle fiber loss seems to depend both on the intensity and on the time period (developmental stage and duration) of dietary restriction. Only severe restriction (starvation, 75% restriction) seems to cause fiber loss, whereas moderate undernutrition exclusively affects muscle development (hypertrophy). The prenatal period of muscle development is much more sensitive to nutritional deficiencies because this period includes muscle fiber formation.


The muscle fiber composition differs massively from person-to-person and is different for every muscle. Most data suggest that we’re born with a certain ratio of Type I to Type II. The limited evidence also indicates the gender-based difference in the muscle fiber type ratio and it’s unlikely that this ratio can change significantly through training or diet, though their relative size may change. Now the question arises why it’s important to understand these concepts and the most straightforward answer is these factors directly or indirectly affect our training/sports performance along with the results we get from training. Hence, it’s important to understand what all muscle fibers are dominant in our body and how to train them for best results.


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  5. Gollnick, P. D., Armstrong, R. B., Saltin, B., Saubert 4th, C. W., Sembrowich, W. L., & Shepherd, R. E. (1973). Effect of training on enzyme activity and fiber composition of human skeletal muscle. Journal of Applied Physiology, 34(1), 107-111.
  6. Costill, D. L., Daniels, J., Evans, W., Fink, W., Krahenbuhl, G., & Saltin, B. (1976). Skeletal muscle enzymes and fiber composition in male and female track athletes. Journal of applied physiology, 40(2), 149-154.
  7. Miller, A. E. J., MacDougall, J. D., Tarnopolsky, M. A., & Sale, D. G. (1993). Gender differences in strength and muscle fiber characteristics. European journal of applied physiology and occupational physiology, 66(3), 254-262.

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