Genetics Series: Muscular Size

January 4, 2017

Being in the fitness industry, I hear the same goals and aspirations all the time:


Females: "I don't want to get too bulky!"


Males: "I want bigger muscles!"


Obviously this is a generalization, because I am sure there are plenty of women whose goal is to increase the size of their muscles, and plenty of men who wish to stay lean rather than get "bulky." What this blog will hopefully inform you is that regardless of your goals, much of your size and strength is already genetically pre-determined and you won't be able to do much about changing that.

I'll explain.


I'm sure I could write many blogs on the social significance of how we are raised, about what an ideal man or woman looks like. How entertainment, the media, professional athletes, etc. all shape the way we perceive the ideal physique should look like. Truth is, many of the professionals you see - whether they are athletes, actors, or models, are either genetically prone to look the way they look, or have had some *ahem* pharmaceutical help to assist their transformation.


I know, I know, this sounds like a whiny cop-out excuse to success. Don't get me wrong, I know that hard work pays off. I advocate hard work and effort to my clients and followers every single day. That is not what I'm saying here. But just as stretching yourself every single day isn't going to make you any taller, training a certain way because someone told you to isn't necessarily going to get you the big huge muscles you think you want. That is the primary focus of this Genetics Series.


Going beyond the hormonal differences between men and women, and the difficulty for most men, let alone women, to grow any measurable amounts of muscle, there are vast differences in the determination and makeup of our muscles. Case in point:

Genetic determinism of fiber type proportion in human skeletal muscle.




Skeletal muscle fiber type distribution is quite heterogeneous, with about 25% of North American Caucasian men and women having either less than 35% or more than 65% of type I fiber in their vastus lateralis muscle. To what extent human skeletal muscle fiber type proportion is under the control of genetic factors is examined in this paper. The results summarized here suggest that about 15% of the total variance in the proportion of type I muscle fibers in human is explained by the error component related to muscle sampling and technical variance, that about 40% of the phenotype variance is influenced by environmental factors, and the remaining variance (about 45%) is associated with inherited factors. These estimates suggest that a difference of about 30% in type I fibers among individuals could be explained exclusively by differences in the local environment and level of muscular contractile activity. However, unidentified genetic factors would have to be invoked to account for the observation that the skeletal muscle of about 25% of the North American Caucasian population have either less than 35% or more than 65% of type I fibers.


This study found a 45% variance in muscle fiber distribution from genetic factors. For those of you unaware, type-I muscle fibers are considered "slow-twitch." They don't generate as much force as type-II fibers, tend to be more "aerobic" in nature, or in other words, they can contract for longer periods of time since they don't generate as much force. Elite marathon runners, for example, tend to have higher ratios of type-I to type-II muscle fibers.


On the other hand, type-II muscle fibers ("fast-twitch") generate more force, but can't contract for as long, or for as many repetitions as type-I muscle fibers. Elite strength and power athletes tend to have a greater ratio of type-II to type-I muscle fibers. Type-II muscle fibers have the greatest affinity for muscular hypertrophy or growth. Type-I fibers can enlarge, but not to the extent of type-II fibers. Strength athletes and bodybuilders have larger muscles for this very reason.


So, as the above study found, genetic variance plays a large role in the ratio of type-I to type-II muscle fibers, and can therefore determine not only the strength and performance of said muscles, but also the size of them too.

This study actually focused on creating and testing workout programs specifically designed for genotypes. What they found is exactly what we would expect - participants who are more well-suited for a specific training stimulus based on their genes saw greater results.


The takeaway? Well, not everyone is able to, or should even worry about a genetic test just to determine their optimal training program. Most of you reading this will probably already have a pretty good understanding of what works for you. Some folks are good runners. Some are not. Some lift weights with ease, while others struggle. Regardless of your current situation, however, focus on improving you and stop comparing yourself to others, regardless of how similar you think you may be to someone else.

Key points to focus on:

  • Size isn't necessarily better - bigger muscles aren't necessarily stronger, so even if you can't build big muscles doesn't mean you can't be strong, relative to your size.

  • Your body frame is a pretty good indicator of the size of your muscles and the strength you can gain. If you have big, broad shoulders, for example, you are more likely to also have large, strong muscles.

  • Despite your genetic advantages or disadvantages, hard work will always improve your current situation.



A couple key points can be summarized from this type of research. First, we know that certain individuals have a greater affinity for muscular growth due to a number of reasons, including their ratio of type-II to type-I muscle fibers, genetic potential for growth, hormonal response, etc.


Changing gears a bit, however, we can also begin to understand the differences in training stimuli and how they affect hypertrophy. Research that tries to understand optimal hypertrophy training styles are quite variable. If there was a one-size-fits-all approach to training, then research would be far more specific in its findings. For example, lower intensity, higher repetition training may be more beneficial for some individuals whereas higher intensity, lower repetition training may be more beneficial for others. What works well for you may not work well for me.


It is impossible to eliminate all variables when it comes to scientific studies, especially genetic variations. That is why whenever you hear of a health study, the results are presented as percentages or trends. Unfortunately, at times, media outlets like to sensationalize headlines. This results in many jumping to conclusions and taking something as truth, rather than understanding that results need to be generalized and applied to a broad populous.


This is applicable to fitness professionals as well. Just because an individual has large muscles or "looks the part" when it comes to training doesn't necessarily mean they are well-suited to coach another individual on how to duplicate those results. They have found an optimal training style that has helped them personally strive for their goals - be it muscular growth, however that does not mean that what worked well for them would also work well for you.


Also, be weary of coaches or fitness professionals who paint a broad stroke, offering fitness plans and/or diet plans that is applicable to the masses. Every individual is different, goals are different, health status is different, genetics are different. A well-rounded coach should be one that is educated and stays up-to-date with current trends and research to offer you the most optimal training and nutritional planning.



Tyler Robbins B.Sc., CSCS
Director of Fitness
Head of CrossFit

Share on Facebook
Share on Twitter