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Why Do People Who Participate in Marathons Tend to Have Smaller Muscles?

Why do people who participate in marathons tend to have smaller muscles?

So, you want to run a marathon. 

First off, yikes! That’s 26.2188 miles. I’m already exhausted just typing that number. 

But second off, congratulations! Committing to run a marathon is no small task. 

Maybe you have a few friends who invited you along, or maybe this is just part of your personal journey. Either way, it’s a huge moment when you decide to take on the mammoth challenge of running a marathon! 

But you have a few reservations: 

  • It seems like a huge commitment! 

  • It’s such a long time to run. 

  • It’s so much practice time. 

  • And also…will it make me skinnier

Whether getting skinnier is a positive or negative concept for you personally, we can all just ask the elephantine question in the room: Why do people who participate in marathons tend to have smaller muscles? Or in other words: Why are runners so skinny? 

We’ve all thought it, right? 

And we’ve all watched the Olympics. Their marathon runners are super fit, and there’s literally not an ounce of fat on their bodies! 

Or maybe you’ve been to a marathon or a cross-country run and noticed the typical slim physique of just about everybody there. 

There’s no way around it: marathon runners are skinny, and before you embark on your personal marathon journey, it’s worth knowing the facts about what will happen to your body as you train. 

6 Reasons Why People Who Participate in Marathons Tend to Have Smaller Muscles

Why are runners so skinny?

1. ​​Longer, More Frequent Training Sessions

That’s a Long Time to Run!

If you’re considering running a marathon, you probably already know this, but training for a marathon is a huge time commitment! Most training plans last 12-20 weeks, so holy cow– we’re talking over 3-4 months or even longer! 

Not only that, but most marathon runners train frequently during the week: 3-5 days per week–running a total of 25 miles per week via those trainings. [1] 

Before you drop to the floor and freak out about those numbers, I want to remind you that you can do this! In fact, 11 million people throughout the world are estimated to have run a marathon in 2022. [2] Crazy, right? But encouraging. If they can do it, you can too! 

So now that you’re back off the floor: let’s talk about how this frequency and level of intensity impacts your muscle mass and fat levels. 


Why Are Runners So Skinny: The Nerdy Science Stuff 

Ever get stuck on the side of the road because you let your gas tank run empty? Yeah. I’m sheepishly raising my hand, too. We’ve all been there. 

Now, imagine if there was a reserve tank to use! Amazing, right? You’d no longer be stuck on the side of the highway. 

Only thing is, there’s one caveat: your car gets smaller and smaller the longer you use this reserve gas tank. 

This is what it’s like to be a marathon runner. 

Running for such extended periods at a steady pace tends to develop leaner muscle mass rather than bulky, hypertrophied muscles. This happens because, during the run, glycogen is consumed by the body as fuel. 

During a longer run, inevitably the glycogen is gonna run out. Once this happens, the body turns to fat as its fuel rather than the glycogen. This process helps maintain a slimmer form, depleting the body of any excess fat stores. [3]

The muscles themselves also get impacted through this process. With limited amounts of glycogen actually converting the fat, the muscles have less of a chance to bulk. Instead, the runner just gains lean muscle mass. 

2. Slow-Twitch vs. Fast-Twitch Muscle Fibers 

You heard me right. Those are some weird names for muscle fibers, but trust me: they’re correct. In fact, they have a lot to do with why people who participate in marathons tend to have smaller muscles. 

Slow-Twitch Muscle Fibers

  • These guys are used during long-duration, low-intensity workouts–such as marathons! 
  • Since these muscle fibers are basically designed for aerobic exercise, you’ll be using a ton of these during your marathon, and if you regularly train this way, your legs will consist of more slow-twitch fibers than fast-twitch. 
  • As you might have guessed, these fibers are associated with leaner, less bulky muscles! 

Fast-Twitch Muscle Fibers

  • Your fast-twitch muscle fibers are used when you perform a more explosive movement in a shorter burst, such as sprinting or lifting. 
  • Distance runners typically don’t have a ton of these fibers; however, if you are lifting weights with your legs, such as performing leg presses, you’ll end up with more of these. 
  • These fast-twitch fibers do end up leading to a bulkier muscle mass–hence why people who participate in marathons seem to lack the bulk. 

3. Energy Efficiency 

Why do people who participate in marathons tend to have smaller muscles?

Energy efficiency plays a huge part in why people who participate in marathons tend to have smaller muscles. Think of it this way: When you begin running for such long periods of time, your body has two choices: 

  • Use its excess stores of energy to gain muscle mass. 

  • Use its excess stores of energy to build efficient muscles built around endurance rather than mass. 

  • Of course, our bodies aren’t overthinking this one, and saying, “Hmmm, I’ll really need to look into this one.” They already know exactly what to do. 

    At the core of it all: it’s survival, and it's pretty amazing if you think about it. Our bodies are designed in a way to conserve energy and adapt to our environments. And that includes adapting to marathons! 

    4. Oxygen Consumption

    Breathing is kind of important. Especially on a 26-something-mile run. And you better believe it has a ton to do with how your body develops around its marathon-ish environment. 

    Now that you’re a pro at twitch muscle fibers, you’ll get this: slow-twitch muscle fibers are leaner and require less oxygen than a larger, bulkier muscle. This makes it more efficient for a runner to have smaller, leaner muscles–creating efficient oxygen flow that is sustainable for the whole run. 

    If you had all larger muscles requiring more oxygen, you would feel fatigue earlier during your marathon run. 

    5. Nutrition

    Caloric Deficit

    On average, people burn nearly 100 calories per mile on a run. [4] For a pro distance runner, this means they’re burning so many calories on days when they practice, that it becomes very difficult to eat enough to match the amount of calories burned. 

    It’s no crazy math, but this puts marathon runners in a caloric deficit, meaning they’re losing more calories than they’re gaining–a huge contribution to why people who participate in marathons tend to have smaller muscles. 

    Healthy Eating

    Add a typical runner’s diet to this, and you’re asking for a slim build with smaller muscles rather than a bulkier weight-lifting build. 

    Part of training for a marathon is not just the athletic practice but also having a healthy diet to boot! Ask a marathon runner what they eat, and you’ll hear a list of healthy, unprocessed foods, which tend to be less calorically dense than artificial or processed foods. 

    Eating cleaner plays a huge role in getting runners’ bodies ready for the big race day–basically providing better quality fuel for the ol’ 26-miler! Runners tend to load up on clean carbs, whole grains, clean protein, such as chicken or fish, and fruits and vegetables. If weight loss is your goal as you prep for your marathon, this type of diet will come in clutch for you. 

    6. Genetics

    There are many reasons why people who participate in marathons tend to have smaller muscles.

    It’s all in the DNA, baby! 

    Ever take one of those DNA tests–where it tells you all the different cultures that have contributed to making you–you?!! They’re totally fascinating–not just to see where you come from but also to tell you some genetic propensities you might have. 

    For example, some DNA tests will let you know if you are prone to getting certain diseases or even whether your earlobes are most likely attached or detached! 

    When it comes to why people who participate in marathons tend to have smaller muscles, much of it just comes down to nothing more than genetics. 

    Destined to be a Runner

    Especially when it comes to professional athletes, a ton of them are genetically predisposed to have major success at their sport purely just from how their bodies are built. 

    Many professional marathon runners are born with a low body mass index, meaning that they already had high chances of success with long-distance runs. In this way, some of the reason why people who participate in marathons tend to have smaller muscles is not only from the way their bodies adjusted to running for so long–but truly just from how their bodies always have been. 

    If this description does not fit you, no worries! That doesn’t mean you can’t run a marathon. It’s important to remember that, to participate in a marathon, you don’t need to be at a professional level to get fit and have fun. 

    So Wait, Will Regular Running Make Me Lose Muscle?

    Running doesn't have to make you lose muscle! 

    So, let’s say you’re like me, and the idea of running a marathon is just a not-yet in your life. No shame. 

    I like to focus on weight lifting and use running as a supplementary exercise. Adding running to your workout routine is still a great idea. But you might be wondering–after reading about how marathon runners tend to lose muscle: “Will running make me lose muscle?” We’re talking just regular running here–like a 3-mile run, for example. 

    The simple answer: No. 

    But let’s get a bit more complicated just for fun. 

    Benefits of Adding Running to Your Workout Routine 

    Lose the extra weight: Adding runs into your workout routine is a great way to lose extra weight without having to lose tons of muscle mass. 

    Cardiovascular benefit: Adding cardio into your regular workout routine is just a healthy practice–for your heart and also for the rest of your body. 

    Experts recommend getting in 30 minutes of moderate-intensity cardio activity around 5 days a week–or 25 minutes of intense cardio at least three days per week.  

    You get to use your math skills: I know I said let’s make it more complicated, but calorie tracking actually doesn’t have to be complicated. Even if math scares you, I promise this one’s easy. 

    If you expend more calories than you take in, you’ll lose weight. There you have it. 

    With a higher calorie deficit, your body will start to develop those leaner slow-twitch muscles, but if you keep your calorie deficit at a lower number, there is less likelihood of this happening. I recommend talking to a nutritionist or personal trainer to help you develop a dietary goal that works for you. 

    How Can I Keep Gaining Muscle and Run at the Same Time? 

    Since keeping cardio exercise in your routine has tons of benefits, it’s great to know that there are clear steps you can take to ensure continued muscle gain while still adding running into your routine. 

    Eat the right amount of calories: Consuming a diet with caloric surplus will help assist you grow muscle mass while still allowing running to play a crucial role in your routine. Adding a good balance of protein, specifically, can help you build muscle. I also recommend protein shakes or supplements, which can help assist with muscle growth and recovery. A personal trainer or nutritionist can help you add the right protein to your diet. 

    Lift Some Weights: When you add weightlifting into your routine, especially compound movements–where you’re working multiple muscle groups at once–you, of course, increase the chances of building muscle–even with the presence of cardio. To get muscle gain, you’ll want to progressively lift heavier weights each time you work out with lower repetitions. This will promote muscle hypertrophy–meaning bulky muscles. 

    Cardio + Weight Lifting = the Perfect Combo!: When you combine cardio with strength training, you give your body the best chances of a healthy BMI and balance of muscle, fat, strength, and endurance. 

    Strength training activates your fast-twitch muscle cells, giving them higher aerobic capacity to assist your runs with strength and power. 

    Conversely, running indirectly supports your strength training by providing overall endurance and stamina to tap into during a harder weight-lifting session. 

    Add Some Self-Care To Your Running Routine

    The QL Claw helps after a good run!


    Nothing beats the runner’s high you get after a good run. But sometimes, sore muscles after a hard run can detract from that feeling. 

    Whether training for a marathon or running a few miles for fun–I highly recommend giving your body some love back for all the work it’s doing. 

    Add some self-care into your routine with the QL Claw, our at-home massage device for your lower back. Trust me, you’ll feel instant relief when the QL Claw hits all your trigger points. It’s the perfect at-home recovery tool to help you take a chill pill after a good run. 






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