Genetics - Blame Your Ancestors 

Change your exercises? Great. Alter your training schedule? Good idea. Tighten your eating practices or switch supplement brands? Go ahead! You can even change the color of your hair or skin. But there is nothing, absolutely nothing, you can do to alter your genes. From the moment of conception your DNA blueprint has been set and cannot be changed. Within a few months after you begin training you will quickly discover that certain muscle groups respond better than others. You may even notice that one side grows slightly faster than the other. Also,your chest may grow by simply looking at a barbell, while your back has to be pounded into growth.

Many bodybuilders have to accept that they have weak bodyparts. However, acceptance doesn’t equate to giving up. It took Arnold Schwarzenegger over 10 years to bring his legs – his calves in particular – up to the superior level of his upper body. Don’t let race influence your mindset either. While African Americans are generally considered to have poor genetics when it comes to building calves, some of the greatest calves of all time belong to such African American bodybuilders as Ronnie Coleman, Chris Dickerson, Vince Taylor and Dexter Jackson.

Everyone is different genetically. Just take a glance at the lineup of the most recent Mr.Olympia. Few bodybuilders can compare their front double biceps with Ronnie Coleman. Jay Cutler has brought his back up to the point that other bodybuilders pale in comparison in back shots. Lee Priest has more muscle on his 5‘4” frame than many bodybuilders who are a foot taller. And while few would refer to Lee as symmetrical, or aesthetically pleasing, you can’t deny that he has one awesome physique. 

Factors For Muscle Growth

Conversely, Dexter Jackson has that Greek statue look.Even among these greats we have incredible diversity. Arnold Schwarzenegger’s single double biceps pose (on the cover of his book Education of a Bodybuilder) has few equals. Franco Columbu’s rear lat spread has yet to be duplicated. And when it comes to legs – Tom Platz was in a class by himself.

Genetics is the study of how traits are passed from one generation to the next. When relatives say you have your mom’s eyes, or your dad’s hair, they aren’t trying to embarrass you– they are noticing inherited physical characteristics. Plenty of physical and personality traits become more and more visible as we age – some good, some bad. For example, cancer can“run” in some families. It doesn’t mean that you’ll definitely get cancer, but you will have a higher risk of developing it than someone whose family has no history of cancer.

Genetic Marker - Bone Size

Generally speaking, there is a direct relationship between skeleton size and muscle mass. A small-boned individual will have far more difficulty developing the same degree of muscle size as a larger-boned individual. A flip through a recent copy of MuscleMag International will confirm this. There may be a 20- or 30-pound difference in weight between two bodybuilders of the same height. Numerous bodybuilding authorities have suggested that one of the best predictors of eventual arm size is wrist circumference. 

Generally speaking, you can expect to build your upper arms to about 10 inches larger than your wrist size. Keep in mind that this is adult wrist size, not teenage wrist size. If you’re in your early to mid teens you still have many years of growth ahead of you, so don’t let a seven-inch wrist depress you! By the time you’re in your early 20s, it may be nine or ten inches in size. And even if it does max out at seven or eight inches, you could still develop large upper arms. 

A classic example of this is the great French bodybuilder and Pumping Iron star, Serge Nubret, who built 20+ inch arms with wrists measuring just seven inches. I should add that there is an advantage to having small bones and joints. Nothing looks as impressive as large, full muscles separated by small joints. Extremely large-boned bodybuilders may have more muscle mass, but they often appear blocky and chunky. Small boned individuals tend to exhibit far better proportions and symmetry. Superstar Flex Wheeler built what many considered the greatest physique of all time and yet had a medium-sized bone structure at best.

Number of Muscle Cells

As you are probably aware, muscles are made up of smaller sub units called muscle cells (more commonly called muscle fibers). It was once believed that individuals were born with a fixed number of muscle cells and that weight training simply enlarged them. Therefore, a person with 100 million cells in a given muscle would potentially be able to build more size than someone with 50 million cells. We all know guys who walk into a gym with muscular arms although they have never lifted a barbell in their lives. 

Such lucky individuals probably have an overabundance of muscle fibers in their arms – yet another blessing of good genetics! These are the guys who could one day grace the pages of MuscleMag International – assuming they train hard and eat plenty of clean food.But as with wrist size, don’t let this genetic variable curtail your hopes and dreams. Since the first edition of this book was written over 10 years ago, research has determined that weight training may be able to split muscle fibers, which can then enlarge.

Source:
Robert Kennedy: Encyclopedia of Bodybuilding, The Complete A-Z Book On Muscle Building. 2008
Nick Evans: Bodybuilding Anatomy. 2012
Arnold Schwarzenegger: The New Encyclopedia of Modern Bodybuilding, 2013