What Genetic Factors Determine the Shape of Your Muscles and How Strong They Are?

Can you really change the shape of your muscles?

You sometimes hear people say they’re trying to change the shape of their muscles but that’s not actually possible. The shape of a muscle is determined by genetics, not by training. Yes, if you’re consistent with your training, you can increase the size of a muscle and its strength by using progressive overload. Increasing the size of a muscle makes that muscle look more prominent, assuming you have a low enough body fat content that the muscle can show.

For example, when you increase the size of your biceps, they will look more prominent, but you inherited the shape that your muscles are, and training can’t change that. So, there are some limitations to what you can change about your muscles. Yet, by simply changing the size, you can change your body composition and how muscular or soft your body looks. In addition, losing body fat makes a muscle look more prominent.

Now that we know that genetics determine a muscle’s inherent shape, what are these genetic factors that determine the shape of a muscle?

Muscles vs. Tendon Ratio

For skeletal muscles, muscles connect with tendons and tendons attach to the bone. When you move a limb, the muscle contracts and pulls on the tendon and the tendon pulls on the bone and causes it to move. Tendons, and in turn muscle, attach to a bone at two points. One is the point of origin. The other is the point of insertion. The point of origin generally doesn’t move when you contract a muscle. Movement occurs at the point of insertion. Take the biceps as an example. The biceps is a two-headed muscle, so it has two points of origin. One is the top of the scapula and the other is just above the shoulder joint. The two heads meet at the elbow and insert at the radius, a bone in the forearm. When you do a biceps curl, movement occurs at the insertion point, so your forearm moves upward toward the points of origin.

The ratio of muscle to tendon length can vary at a particular site on your body. A good example is the calf muscle. Some people have a long muscle belly in the calf and a relatively short Achilles tendon. Other people have the opposite, a short muscle belly and a long tendon. The potential for a muscle to increase in size is impacted by this ratio. For example, if you have a long muscle belly in the calf and a short Achilles, you have greater potential to increase the size of your calves relative to someone who has a short muscle belly and a long Achilles tendon. But, there are advantages to having a short muscle belly in your calves and a long Achilles tendon. You’ll have to work harder to get defined calves, but you’re probably a better jumper than someone with a short Achilles tendon!

Unfortunately, you’re stuck with where your muscles insert, and this gives you certain limitations. All you can do is hypertrophy the muscle belly between the insertions. You can’t change the points where the muscle inserts to make the muscle longer. Yet, no matter how long your muscle bellies are, you can still strengthen and enlarge the muscle within the limitations of your genetics.

How the Size of Your Frame Impacts How Muscular You Look

How strong and muscular you look is also related to the size or your frame and the ratio of shoulders to hips. If you have a large frame, you’ll naturally look stronger and may have some advantages when you play certain sports. But, people with a small frame have an advantage in certain sports too, like gymnastics and long-distance running.

The size of your torso is a factor too. In terms of getting that muscular look, people who have a short torso tend to look more muscular, while those with a longer torso will typically look leaner. Height impacts how muscular you look for a given amount of weight training too. People with long, lean limbs usually look less muscular than people with shorter limbs.

The length of your limbs can also influence how much weight you can lift. Have you ever noticed how champion powerlifters tend to be short and have short limbs? Short limbs give you a strength advantage. It takes more work to complete a repetition when your limbs are longer. For example, if you do overhead presses and you have long arms, you have to cover more distance to get to the apex of the movement. You’re working harder to do 8 reps than someone who has short arms is. Therefore, you may not be able to use as much weight or complete as many repetitions.

The same is true when you do an exercise like squats. You cover more distance when you do a squat if you have long legs. However, it’s not true for every exercise. For example, having long arms gives you an advantage when you do an exercise like deadlifts. Interestingly, people with short limbs are called dolicomorphs and those with long limbs are labeled brachiomorphs. You can toss those terms around if you want to dazzle everyone with science speak! It’s easy to see now why people who excel at certain sports have similar body compositions. Short limbs give you an advantage when you weight train but won’t help a lot if you’re a long-distance runner.

The Bottom Line

Yes, there are factors you can’t control. You’re born with certain muscle shapes based on their insertion points and you also inherit a particular body size, frame, and torso and limb length. These factors you’re born with make it harder or easier to build strength and muscle size. They also are a factor in sports performance. But, you shouldn’t let them limit you. You can still build muscle if you’re long-limbed or short-limbed and regardless of your frame, but you might have to work a little harder if your anatomy is less favorable. Who said building muscle and a better body composition is easy? But, it’s worth it.



Principles and Practices of Resistance Training. Stone, Stone, and Sands. (2007)


Related Articles By Cathe:

How Resistance Training Shapes Tissues Other Than Your Muscles

Can You Build Muscle Size Through Aerobic Exercise?

Regional Muscle Hypertrophy: What is It and Can It Impact Your Workout?


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