Factors affecting muscle growth: what genetics decides

Muscle building takes one man three months and another three years — and both follow the same program. The reason isn’t always a mistake in training or nutrition. Six biological factors determine your muscles’ development potential, and you can’t change them much yourself.

Why does muscle mass growth differ from person to person?

In the gym, one question comes up again and again: why is my training partner, who does the same exercises, clearly progressing faster? Many people start looking for the flaw in their program or nutrition, but the real cause often lies deeper — in genetics. The development of strength and muscle mass is shaped by six factors that we can’t control ourselves, or can control only to a very limited extent.

That doesn’t mean training is pointless. It means that to set realistic goals, it helps to understand what your body carries within it.

Muscle fibers — the foundation of muscle mass development

Our muscles contain two main types of fibers: slow-twitch and fast-twitch. Slow-twitch fibers support prolonged aerobic effort — they are fatigue-resistant, but develop little strength. Fast-twitch fibers come into play in anaerobic activities: sprinting, weightlifting, bodybuilding, throwing events. They can generate a great deal of strength, but they tire quickly.

Both fiber types respond positively to strength training, but fast-twitch fibers respond clearly better. This means that a person who is genetically predisposed to a higher proportion of fast-twitch fibers will get faster and better results from strength training. A simple truth: you don’t choose your fibers, but you can choose the training that suits them best.

In world-class marathon runners, the proportion of slow-twitch fibers is typically about 75% or more. Elite sprinters are the opposite — their proportion of fast-twitch glycolytic fibers also reaches about 75% and higher. If your slow-twitch fibers sit at around 68%, you won’t make it into the world’s sprint elite — but you can dominate endurance sports. In an ideal world, we would know this before choosing a sport.

Most people are genetically average: the ratio of fibers is roughly equal. That isn’t a bad thing. It simply means the best training direction becomes clear through time and experience, not through a lab analysis.

Age, sex and body build

Age has a noticeable effect on muscle mass development. Studies show that a well-designed training program builds muscle at any age, but the fastest development happens at ages 10–20. Once you reach physical maturity, progress no longer comes as easily — that’s normal, not a mistake.

Sex does not affect muscle quality or composition — men’s and women’s muscle fibers are biologically equivalent. However, thanks to testosterone and a larger skeletal frame, men have considerably more muscle tissue. That is why men are, on average, stronger. It isn’t a question of fairness or advantage, but of biology.

Limb length affects biomechanics. A person with shorter arms can generally lift more in biceps curls — that’s elementary physics. The same applies to the length of the muscle itself: long muscles offer better potential for growing muscle mass and strength. This too is genetically determined.

Where the tendon attaches to the bone is also decisive. Suppose Rein and Jaan have the same arm length and the same muscle length, but Rein’s biceps tendon attaches farther from the elbow. This gives Rein a biomechanical advantage — he lifts clearly more in the same exercise. Not because he tries harder, but because his lever works better.

What can you do yourself to build muscle?

Genetics sets the ceiling, but you decide how close to it you get. Here are a few practical truths that apply to every body.

The most effective technique is a slow, controlled movement — a repetition and a set that take everything the muscle has to give. Intensity matters more than volume. Half the effort gets half the result.

Rest is part of training, not an afterthought. Overtraining is a common mistake — too many sets, too little recovery, too many exercises at once. Muscles grow during recovery, not during the workout.

If your progress has stalled, take a hard look at your program. Repeating the same routine endlessly leads to a plateau. Change the exercises, the training load, the volume. The body adapts quickly and constantly needs a new stimulus.

So what actually determines whether you get in shape? Honestly — your willpower and attitude. Even a person with a predisposition to being overweight, or with genetically lower muscle mass potential, can achieve remarkable results through consistent, deliberate training.

Strength training affects the body as a whole: besides muscles, your bones, tendons and ligaments get stronger, and your overall tone improves. And — perhaps surprisingly — regular strength training also boosts confidence and self-esteem. Knowing your body is half the battle.

FAQ

Does genetics determine how much muscle mass I will ever gain?
Genetics sets the upper limit of your potential, but most people never get anywhere near it. Training, nutrition and consistency are the factors that actually decide how close to that limit you get.

Can muscle fiber type be changed through training?
A complete change isn’t possible. Long-term training can influence fiber properties and the intermediate fiber type, but the basic distribution stays genetically fixed.

At what age is muscle building most effective?
The fastest development happens at ages 10–20, but with regular training and proper nutrition, you can build muscle mass at any age.

Author: Janar Rückenberg

Source: WHO – physical activity.

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