What genetic factors limit muscle growth?
There are four main genetically determined factors that limit muscle growth:
- the number of fibers in your muscles
- the ratio of Type I to Type II fibers in your muscles
- your hormonal balance, especially your testosterone level
- your body type.
The number of fibers in your muscles
You were born with a specific number of muscle fibers per motor unit, and, unfortunately, there is nothing you can do to increase the number. If you have an above-average number, you will have the potential to gain more strength and size than someone with an average number of muscle fibers. Conversely, if you have a below-average number, you will lack the potential to gain as much strength and size as someone with an average number of muscle fibers.
The ratio of Type I to Type II fibers in your muscles
As explained in the lesson Muscle structure, there are three types of muscle fiber: Type I, Type IIa, and Type IIb. The muscle fiber types possess different characteristics and properties. For example, Type I fibers have the most endurance, and Type IIb fibers can generate the most tension.
Each skeletal muscle in your body has a mixture of fiber types. The ratio of fiber types in each muscle, which is largely genetically determined, dictates your muscles’, and therefore your, abilities. For example, if you have a high ratio of Type II to Type I fibers, you will be able to gain muscle mass relatively fast and have the potential to do well in sports that require a lot of strength, speed, and power. On the other hand, if you have a high ratio of Type I to Type II fibers, you will make slower gains in muscle size, and have the potential to do well at sports that require endurance.
While it is not possible to change your ratio of fiber types, it is possible to change the function of some Type II fibers and make them more like Type I fibers. With regular aerobic training, Type IIa and Type IIb fibers can become more aerobic, develop greater endurance, and be more like Type I fibers. However, it is not possible for Type I fibers to assume the characteristics of Type II fibers.
Level of testosterone
Testosterone and growth hormone (GH) play an important role in the growth of muscle (hypertrophy). If you have high levels of testosterone and GH, you will respond to training more readily, and make greater gains in muscle and strength. Since men possess ten times more testosterone than women do, hypertrophy is much easier for men. The only way that women can achieve muscle growth near the same level as men is by taking additional testosterone in the form of anabolic steroids, which is not recommended.
There are a number of ways in which men can increase their testosterone level without taking anabolic steroids, the most effective ways of which will be explained in the following lesson.
Generally speaking, there are three main body types (aka somatotypes): ectomorph, endomorph, and mesomorph (Figure 1). These somatotypes apply to women, too. Each somatotype has a different potential to add muscle and fat.
Ectomorphs are lean and thin with little bulk and low levels of body fat. They have long limbs, narrow shoulders and hips, and a fast metabolism that makes it difficult to gain muscle or fat. The majority of professional basketball players are ectomorphs.
Endomorphs have a naturally stocky and rounded body with wide shoulders and wide hips. They have an even distribution of fat, and gain muscle and fat easily. Many football players and wrestlers are endomorphs, as are many strongmen.
Mesomorphs have a naturally athletic build with wide shoulders and narrow hips. They gain muscle readily and find it easy to lose fat. Mesomorphs therefore make the best bodybuilders.
Most people are a mixture of these body types, but tend to lean towards one more strongly than the others. Whichever one you are, don’t worry; you can still achieve great results. Just follow the nutrition guidelines and an effective training program.