Many people start running to lose weight. Running burns more calories than practically any other exercise. Running a ten-minute mile for an hour will burn 600 to 800 calories for most people!
But does running make your legs bigger? Or, more specifically, does running make your thighs bigger?
That’s the question we set out to answer in this article. After all, running is a strengthening exercise, and as muscles get stronger, they grow bigger. However, when you think of a “runner’s body,” you likely picture long, lean, muscular legs. Or, you may picture strong, big legs.
In fact, there are a lot of factors that influence whether running makes your legs bigger or your thighs bigger. In this article, we are going to examine:
- How Do Muscles Develop With Exercise?
- Does Running Make Your Legs Bigger?
- Factors That Influence Whether Running Makes Your Legs Bigger
- What To Expect When You Start Running
- Tips For Thinner Legs From Running
So, let’s go!
How Do Muscles Develop With Exercise?
Muscles grow after they heal from the damage caused by exercise. Every time our muscles contract when we run, we create microtears in the fibers. (This is when your muscles feel sore.)
When we are done running, our bodies repair the microtears and build the muscle back stronger. They also become larger in what’s called hypertrophy, explains exercise physiologist Todd Buckingham.
Hypertrophy is triggered when the muscles are pushed to their limits, break down, rebuild, and come back stronger and bigger to repeat the same exercise with more ease next time.
Related: Heavy Legs from Running? Here’s How to Fix It
But does running make your thighs bigger? Let’s see!
Does Running Make Your Legs Bigger?
The type of running you do is the biggest factor that influences the size of your muscles, according to Buckingham.
We have different types of muscle fibers in our bodies. And, just like people come in all different shapes and sizes, so do our muscles! The type of running you do recruits different muscle fibers and, thus, has a different result in your leg size.
Long-distance running will not make your legs bigger because it uses Type I muscle fibers.
These small fibers are great at enduring fatigue, but they do not have the power that larger muscles such as the Type II muscles have.
So, while you’re building muscle and getting stronger by running longer distances, you may not see an increase in leg size.
If you are sprinting or striding, that’s another story.
Sprinting, or short bursts of fast running uses Type II muscle fibers which are larger and can more forcefully contract for big power output.
“Sprinting will build muscle more closely to the traditional way we think of (like a bodybuilder). Those Type II muscle fibers will hypertrophy (i.e., get bigger) and cause an increase in muscle size,” explains Buckingham.
This is why you see sprinters with such large thighs and glutes compared to distance runners who have very thin and lean legs.
Related: Is There an Ideal Running Weight for Peak Performance?
Factors That Influence Whether Running Makes Your Legs Bigger
There are additional factors that influence whether or not running makes your legs bigger. Let’s take a look at them:
#1: Running Intensity
As noted, if you are doing distance running at a slower pace, your leg size will not increase. You will build Type I muscle fibers which are smaller. If you are sprinting, you will build Type II muscle fibers which are larger (and thus more powerful). This will make your legs bigger.
Trail runners often have bigger legs due to the uneven terrain and hills they run on.
Hill running is like lifting weights which requires forceful contractions of your muscles, including your Type II muscles.
Therefore, if you run a lot of hills in your training, it’s possible running will make your legs bigger, says Buckingham.
Related: Sore Quads After Running Hills? Here’s How to Fix It
#3: Amount Of Running
Furthermore, if you’re a distance runner who is running a lot of miles, your leg size will not increase.
This is because long-distance running increases hormones that impair muscle protein synthesis and hypertrophy (gaining size).
“The more aerobically conditioned you are, the less likely it is that you will also be big and bulky. The two just don’t mix,” says Buckingham.
#4: Your Diet
Running torches calories. On average, running burns about 100 calories a mile. So, if you run 6 miles in an hour, you’ll burn 600 calories!
The more you run, the more weight you will likely lose unless you eat more than the calories you burn, which is a common mistake new runners make.
Many new runners overestimate the number of calories they burn, or they don’t fuel properly, leading to binging later on. Eating a small carb-rich snack and refueling after runs longer than an hour will help you avoid over-eating.
However, if you intake more calories than you burn, it’s possible your thighs will get bigger from running.
Related: The Complete Runner’s Diet: What to Eat for Top Performance
#5: Your DNA
Some people build muscle faster than others. It’s in their DNA, and therefore it’s tough to rewrite your genetic code.
“Genetics plays a large role in the make-up of your muscle fibers; about 50 percent,” explains Buckingham. “If you are born with a high density of Type II muscle fibers, your legs will probably be bigger at baseline than a runner with a high density of type I muscle fibers.”
However, if you’re running to get leaner legs, don’t despair:
Distance running is still predominantly focused on the Type I fibers, so those type II fibers won’t grow any bigger.
Plus, it can actually change the characteristics of some of your muscle fiber types, Buckingham explains.
“Type IIa fibers (there are also Type IIx fibers) are intermediate fibers and can take on the characteristics of Type I or Type IIx fibers, depending on your training. So, if you are a distance runner and have a lot of Type II fibers, the Type IIa fibers will become more like Type I.”
So, just because you have a lot of Type II fibers doesn’t mean that your legs will gain more size than a runner with more Type I fibers.
Related: Should You Eat Protein Before Bed?
What To Expect When You Start Running
If you are new to running or returning to running after a long break, you should expect changes in your legs and your body—some immediate and some not.
- You will feel sore. You will likely notice that your legs feel sore after the first few running sessions. It is not uncommon to have aches and pains as your body adjusts to the new stress.
Take care in allowing for enough rest in between sessions (usually 48 hours for new runners), eating well, and sleeping. Also, do not increase mileage too much (usually about 10 percent of the volume week to week).
- You may feel tired. After a couple of weeks of running, your legs may be fatigued—like they lost their pop. This is another symptom of your body adjusting to the training load.
Don’t worry, though. Your cardiovascular system and your musculoskeletal system will begin to adapt and strengthen. Running will then feel easier. Your pace will naturally quicken. And it will be fun!
- You’ll be hungry. After a couple of weeks of running, it is also not uncommon to feel hungry. Answer by eating more but intelligently. Eat a healthy meal or snack every few hours that includes carbs, proteins, and healthy fats.
Runners who do not fuel before their runs and wait to refuel after their runs often find themselves downing bags of chips or boxes of cereal after suppertime. Do not let this happen to you! Eat frequently and well!
- You’ll feel glad. Over time as your body adapts, your legs will become stronger and more defined. Walking, going up steps, and climbing will feel easier. You’ll feel more powerful in your stance overall.
And be glad you became a runner!
Tips For Thinner Legs From Running
There are steps you can take to ensure your legs don’t get bigger from running.
#1: Start Slow
Don’t run too much or too fast early on. Run every other day. Take walk breaks, and go at an easy, conversational pace. Increase running volume by 10 percent each week. This will help you avoid overtraining and injury.
#2: Run Far
As you ease into your new running habit, stick to the long slow distance runs. These strengthen your Type I muscle fibers—the ones that don’t get bulky.
#3: Run Flat
Avoid strenuous hills or rocky terrain on your runs. These can build your Type II muscle fibers. If you want thinner legs, you do not want to grow these fibers.
#4: Run Slow
Fast running recruits those Type II muscle fibers, so aim to run slow to use more Type I. Also, slow running can burn more fat—leading to weight loss.
#5: Eat Smart
Make healthy choices by eating whole foods and eating often. Do not starve yourself before or after your runs. This will lead to overeating and weight gain!
Marathon training is a great way to get fitter, thinner legs. Check out our marathon training resources to help you with your goal!