The Anatomy of Strength – The Genetic and Physiological Factors That Influence Strength

What makes a person strong? Why are some people stronger than others? What’s my ultimate strength potential?

If you’ve ever found yourself asking these types of questions, this article is for you.

While we don’t have a complete understanding of all the factors that influence human strength, we have a pretty dang good idea. And we can use the knowledge we have to come up with fairly solid answers to our questions.

Some of the most important contributors to strength include the following:

  1. Nervous system
  2. Muscle fibers
  3. Muscle attachment points
  4. Body proportions
  5. Frame size
  6. Genetics

Let’s examine each one of these factors and how they can influence your current level of strength, as well as your ultimate strength potential.

How the Nervous System Influences Strength

When you start strength training for the very first time, your muscles are likely unskilled at performing the barbell movements. But as you practice the lifts, your nervous system becomes more efficient at recruiting muscles to perform the movement.

This means that a large proportion of strength gains for new lifters is attributable solely to improved coordination. In other words, increased strength is partly a matter of increased skill.

An athlete may have a large amount of muscle mass, but if their nervous system has not become skilled at using those muscles, a more skilled lifter with less muscle mass could potentially exhibit more strength. However, as the bigger lifter becomes more and more coordinated, the smaller lifter’s skill advantage starts to diminish quickly.

As you gain more experience, the benefits of skill learning reach a point of diminishing returns. From then on, increases in strength must come primarily from other factors. For advanced lifters, the main variable will be the ability to pack on more muscle (which is discussed later on).

How Muscle Fibers Influence Strength

You may already be somewhat familiar with muscle fiber types and their functions, but for the sake of completeness, we’ll review them briefly.

Muscle fibers come in two types: Type I (slow-twitch), and Type II (fast-twitch). Type II fibers have two subtypes: Type IIa and Type IIx. Each type is optimized to perform a particular kind of physical work.

Type I Type IIa Type IIx
Twitch Speed Slow Fast Very Fast
Endurance High Medium Low

As you can see, Type I fibers are not very powerful, but they don’t fatigue easily. Type IIx fibers are quite powerful, but they fatigue quickly. And Type IIa fibers fall somewhere in the middle of the spectrum on both endurance and power—a hybrid of sorts.

The comparison above can sometimes cause confusion, because there is a difference between strength and power. Strength is the ability to produce absolute force. Power is the ability to produce force quickly. A common misconception is that Type II fibers are “stronger” than Type I fibers. However, both fiber types are capable of producing the same amount of force. The main difference is power. Type II muscles are more powerful, which means they generate more explosive movements (i.e., good for sprinting and throwing).

This means that neither fiber type is necessarily advantageous when it comes to pumping iron. A high proportion of one muscle type or the other is only beneficial for sports on the extreme ends of the power/endurance spectrum.

The body utilizes both fiber types to move the bar, and both types of muscles grow in response to training. So for pure strength athletes, muscle fiber type doesn’t play a huge role in how strong you can get. More Type II muscles will help you out in certain Olympic lifts like the clean and the snatch, but that’s about it. You can’t change your fiber type composition, nor can you train specific fiber types.

What about differences in muscle fiber strength between athletes?

In this regard, not all muscle fibers are created equal. Studies have shown that muscle specific tension (a measure of force output by muscle fibers) can vary by as much as 61% between athletes. So two athletes with the exact same muscle mass and body mechanics (discussed more below) could differ in strength because of the force output of their individual muscle fibers.

Why some people’s muscle fibers are stronger than others’ is not fully understood. It may be slightly influenced by muscle architectural factors, such as fascicle length and pennation angle. It may also be explained by variation in the amount of non-contractile tissue present in the muscles, such as intramuscular fat and connective tissue.

Whatever the reason, the muscle fibers for some athletes seem to produce more force, pound-for-pound, than for others.

How Muscle Attachment Points Influence Strength

Your musculoskeletal system provides stability and produces movement using different types of levers. These levers follow the same laws of physics that govern the use of many tools like wrenches, wheelbarrows, and shovels. We won’t get into the physics of levers, but it’s pretty easy to grasp the concept.

Consider a wrench: you pull on the handle to rotate the head, which is attached to the bolt you’re trying tighten. The longer the handle, the more torque you can produce and the easier it is to make it really tight. Just think about how much harder it would be to tighten a bolt by pulling the handle close to the head instead of pulling on it toward the end of the handle:


Your muscles work in much the same way. Muscles attach to bones (via tendons), and when muscles contract, they pull on your bones like levers to produce movement. And just like the wrench example above, the point where your muscles attach to your bones (the attachment point) makes a difference in how much torque is produced and how strong you are.

Take the biceps brachii, for example. It originates from two different points on the scapula (shoulder blade) and attaches to the lower arm via the radial tuberosity. When the bicep contracts, it brings the forearm up toward the body.


Every person’s body is different, which means there is slight variability in where our muscles attach to our bones. One person’s bicep may attach 20 millimeters from the joint, while another person’s bicep may attach 25 millimeters from the joint. This may not seem like much of a difference, but because of the nature of levers, just millimeters of variation can produce significant differences in strength.

Another factor that comes into play is the angle of the attachment point. Consider the difference between two attachment points: one where force is applied more toward the joint at a smaller angle, and one where force is applied more upward from the joint at a larger angle, as illustrated below.


When the attachment angle is smaller and the force is angled more toward the joint, it takes a greater force to move the lever than when the force is angled upward. This means that, given the same force production by a muscle, the larger attachment angle is more efficient at transferring that force in the intended direction, which equates to more strength.

The differences here are exaggerated for illustration purposes. Variation in these attachment angles from one person to the next is much more subtle, but the forces at play still have an impact on strength.

How Anthropometry (Body Proportions) Influences Strength

Conventional wisdom holds that body proportions affect your ability to excel at certain lifts. For example, athletes with short arms and large chests are believed to be better at bench press, while athletes with shorter legs relative to their height are better at squats. And athletes with longer arm lengths relative to their legs are better at deadlifts.

One study  examined the relationship of body dimensions to strength performance in powerlifters and found that body dimensions were not a particularly good predictor of strength in the various lifts. The authors found some correlation, but the relationship was relatively small.

A potential explanation for this is that bone lengths and attachment points may have a strong correlation to each other. In other words, a person may have relatively short femurs compared to their total leg length (which we would assume is better for squats), but the placement of the attachment point for the muscles likely negates much of the would-be advantage.

However, this is not to say that these advantages don’t exist. While body segment lengths and attachment points may go together on average, the existence of outliers is entirely feasible. Given a hypothetical scenario with two people whose body proportions and attachment points are exactly the same, except one has shorter femurs, the one with shorter femurs would have more strength potential for the squat.

How Frame Size Influences Strength

The most obvious anatomical factor that influences strength is the amount of skeletal muscle on the body. Muscles produce force, so the more muscle you have, the more force you can produce and the stronger you are.

As was shown above, there are a number of factors that can increase the efficiency at which force is applied by the muscles. This leads to cases where a smaller athlete with optimal lifting skill, muscle force production, attachment points, and body proportions can exhibit more strength than a much bigger athlete. However, on average, the guy with more muscle has more strength potential than the guy with less muscle.

So what determines how much muscle mass a person’s body can have? Research on this subject has revealed that frame size (or bone structure) is probably the strongest predictor of a person’s (drug-free) muscular potential.

Studies of elite athletes have found that there is a maximum muscle-to-bone ratio for humans of about 5:1 (or 4:1 for women). In other words, for every pound of bone in your frame, you can support a maximum of 5 pounds of muscle. Naturally, it follows that the bigger your frame, the more muscle you can support. This means that given two athletes who are the same height, the one with the thickest bones will be able to build the most muscle.

Others have provided great resources for estimating your maximum drug-free muscle mass and strength potential, so we won’t go into that here. But you can check out the formulas if you want to get an idea of how much muscle you could have with optimal training.

How Genetics Influence Strength

The influence of genes on strength is a complicated topic. Obviously, your genes determine all of the factors discussed above—muscle fiber type, attachment points, body proportions, frame size—but we don’t fully understand the complex interaction between genes and environmental factors that affect strength and training response.

To date, science has identified 22 genes that are believed to impact human strength. However, their direct influence, and their interaction with other genes that combine to influence strength, is still not completely understood.

The presence of one or many of these genes doesn’t necessarily predict anything, and the expression of someone’s genes can be influenced by the conditions inside their mother’s womb as well as their early childhood environment. Additionally, each of the 22 “strength genes” that we know about only contribute to a 2-3% difference in overall performance.

The point is that while we know genes definitely impact strength, we can’t use them to completely predict how strong someone will be or exactly how they will respond to training. For example, the famous “sprinter gene,” known as ACTN3, codes for the production of a particular protein. The 577RR genotype is sometimes considered a requirement to be an “elite” sprinter. However, one study found that 1 in 12 international-level sprinters had no working copies of this gene.

Given the uncertainty around genetic makeup and strength, the best we can do at this point is to use genes to estimate probabilities. In other words, the presence of any given gene may slightly increase or decrease the probability of a certain trait or ability.


There’s one logical question after examining all these factors: What is the single most important contributor to a person’s strength potential?

The answer: probably frame size.

The interaction between variables that influence strength is very complex and not fully understood. This makes it hard to pinpoint the percentage of total strength potential that comes from frame size. But once you’ve exploited all your advantages not related directly to muscle mass, your ability to put on more muscle (which is a function of frame size) is the most important factor for continuing to build strength.

So what does all of this mean? What’s the point?

If nothing else, it helps us realize that while frame size and muscle mass are definitely important factors in overall strength potential, they’re not the only factors. You don’t have to let it hold you back. You never know what kind of gains you can make with dedicated, proper training.

We can also use this information to properly evaluate differences in performance between athletes. When we compare ourselves to others and see that we’re not as strong, or not making as much progress, we’re tempted to think that we don’t work hard enough or that the other person has some secret knowledge that we lack.

In reality, people are just different. Some people have more strength potential than others, and that’s okay.

It’s fun to compete and try to become the strongest. Competition can be a great motivator and character building experience. At the end of the day, however, your most relevant competitor is yourself. You would never expect a Yorkie to somehow work hard, and through sheer grit and power of will, transform itself into a Great Dane. Biology makes that impossible (at least with our current technology). We simply want our Yorkie to be the best Yorkie it can be.

We should take a similar approach with ourselves. Don’t view some aspect of your body as a detriment that holds you back. Instead, look at the different variables and try to identify your relative strengths and weaknesses. Then use that information to exploit your strengths and shore up your weaknesses to become your strongest self.