Why Are Marathon Runners Legs So Skinny?

It’s a common observation that most marathon runners have legs that are disproportionately thin compared to the rest of their body. This begs the question, why are marathon runners’ legs so skinny?

I’ve discovered that there are several reasons why marathon runners tend to have skinny legs. One of the main reasons is their sheer amount of running.

Marathon runners typically run anywhere from 30 to 120 miles per week, which can lead to a significant loss of body fat, including in the legs. Additionally, marathon runners tend to have a low body fat percentage, which can further contribute to the appearance of skinny legs.

Muscle Composition and Function

As a marathon runner, I’ve often been asked why my legs are so skinny.

The truth is, it’s not just about losing weight but also about the composition and function of the muscles in my legs. So let’s take a closer look at the science behind it.

Slow Twitch vs. Fast Twitch Muscle Fibers

Our muscles comprise two types of fibers: slow-twitch and fast-twitch.

Slow-twitch fibers are used for endurance activities like long-distance running, while fast-twitch fibers are used for explosive movements like sprinting.

As a marathon runner, I rely heavily on my slow-twitch fibers, which are highly efficient at using oxygen to produce energy.

These fibers also have a high fatigue resistance, allowing me to maintain a steady pace for long periods.

On the other hand, fast-twitch fibers are larger and more powerful, but they fatigue quickly. This is why sprinters, who rely on explosive power, tend to have larger, more muscular legs than marathon runners.

Role of Mitochondria in Endurance

Another factor contributing to marathon runners’ skinny legs is mitochondria’s role in endurance activities. Mitochondria are the powerhouses of our cells, responsible for producing energy in the form of ATP.

During endurance activities, such as long-distance running, our muscles require a steady supply of ATP.

This is where mitochondria come in. For example, Marathon runners have a higher density of mitochondria in their muscles, allowing them to produce more ATP and sustain their energy levels over long periods.

While this may not result in larger, more muscular legs, it makes marathon runners highly efficient at using their energy stores and maintaining a steady pace for hours.

Training and Nutrition

As a marathon runner, I can attest that our legs are skinny and incredibly strong. But why are our legs so slender? It’s a combination of factors, including our training and nutrition habits.

Impact of Long-Distance Running on Muscle Mass

Marathon runners typically train for long hours, which can lead to a decrease in muscle mass.

This is because our bodies use glycogen stores for energy when we run long distances.

If we don’t replenish those stores, our bodies will start burning fat and muscle for fuel. This is why consuming enough calories and protein is essential to maintain muscle mass.

However, it’s important to note that not all runners will experience a decrease in muscle mass.

For example, elite runners who train at a high volume and intensity are more likely to experience muscle loss. In contrast, recreational runners who train less frequently and at a lower power may not see a significant decrease in muscle mass.

Dietary Habits of Marathon Runners

Marathon runners must consume many calories to fuel their bodies for training and racing. However, consuming the right foods is essential to ensure that we get the proper nutrients and not just empty calories.

Many runners follow a carbohydrate diet, as this macronutrient is the primary fuel for endurance exercise.

However, consuming enough protein to maintain muscle mass and aid recovery is also essential. Runners should also consume healthy fats, such as those in nuts and avocados, to support overall health.

It’s also important to listen to our bodies and consume enough calories to support our training.

If we don’t consume enough calories, our bodies will start breaking down muscle for fuel, which can decrease muscle mass.

Body Type and Genetics

As a fitness coach, I have observed that marathon runners come in all shapes and sizes. However, many of them indeed tend to have skinny legs.

In this section, I will discuss the role of body type and genetics in determining leg size.

Ectomorphs and Marathon Running

Some marathon runners have a body type known as an ectomorph. Ectomorphs are naturally lean and have difficulty gaining weight, including muscle mass.

In addition, they tend to have a smaller bone structure, longer limbs, and fast metabolism.

This body type is well-suited for endurance activities like marathon running but may also contribute to skinny legs.

Another factor is the type of training that marathon runners typically do. Long-distance running burns many calories, and it can lead to a calorie deficit if runners do not consume enough food to fuel their workouts.

This deficit can result in the loss of fat and muscle mass, making the legs appear even skinnier.

Genetic Factors in Leg Size

Genetics also play a role in determining leg size. Some people are born with a predisposition to have thinner legs.

This can be due to various factors, including the distribution of muscle and fat in the body, the shape and size of the bones, and how the muscles attach to the bones.

However, it is essential to note that genetics is not the only factor determining leg size.

Environmental factors like diet, exercise, and lifestyle choices can also impact.

For example, a person genetically predisposed to have thicker legs may still end up with skinny legs if they do not exercise regularly or eat a healthy diet.

In conclusion, while body type and genetics can play a role in determining leg size, many other factors can contribute to skinny legs in marathon runners.

Therefore, it is essential to focus on overall health and fitness rather than trying to change the shape of one’s body.

Final Thoughts

After researching and analyzing why marathon runners have skinny legs, I have come to a few conclusions.

First, marathon training involves much running, which results in burning calories and fat. This, in turn, leads to a reduction in body weight, including the legs.

Second, marathon runners often follow a strict diet to maintain weight and improve performance.

This diet involves consuming fewer calories, which may contribute to skinny legs. Third, genetics may play a role in determining the body type of a marathon runner. Some individuals may be predisposed to having slender legs, regardless of their training regimen or diet.

Finally, it’s important to note that having skinny legs does not necessarily mean a person is unhealthy or weak.

Marathon runners have strong and efficient leg muscles, which allow them to run long distances without getting tired quickly.

In conclusion, the skinny legs of marathon runners result from a combination of factors, including training, diet, and genetics.

While having slender legs may not be desirable for everyone, it’s essential to appreciate the hard work and dedication that goes into marathon training and recognize these athletes’ strength and endurance.



Slightly obsessed middle aged runner.