Why Are My Legs Getting Bigger From Running?

By Rachael Attard, Updated Jun 9, 2022
legs getting bigger from running

I get a lot of complaints from women who have found their legs getting bigger from running, which isn’t the desired results for most of us. So let me explain why this is happening.


Running uses your glutes, quadriceps, hamstring and calves constantly, meaning that your leg muscles are working and this will cause them to develop and get bigger in size.

Any form of exercise that engages your muscles will cause them to grow in size. However, some exercises cause more muscle growth than others.

Running will not build as much muscle as weightlifting, sprinting and high-intensity interval training.


We all know that sprinters have very muscular physiques, especially compared to distance runners. Sprinting engages your muscles more and focuses on muscular power. Therefore, sprinting will build more muscle than just steady state running.

Doing a lot of sprint training and plyometric type exercises will develop a more muscular look, but will also help you get lean. If you don’t mind this look, then this is probably a great style of workout for you. If not, it might be best to avoid these style of workouts.

When I was training for a triathlon, I used fast past interval runs (not a full on sprint) to help me develop my cardio fitness quicker. It definitely did this, but what I found was that it actually helped me lean out my legs quicker too.

If you still want to include some cardio interval training, have a read of this blog post which explains how to do the fast past runs.

why are my legs getting bigger from running


Your ability to get bulky muscle from running depends on your body type. If you don’t know your body type, click here to find out.

Ectomorphs find it very difficult to gain muscle and will probably never get bulky even if they do lots of weight lifting. Mesomorphs build muscle easily but can also lose muscle and fat quickly too. Some mesomorphs may get bulky legs whereas others might not.

And finally, endomorphs probably already have naturally muscular legs so find it easy to bulk up.

Running does burn a lot of calories and will help you lose weight all over your body. It may help you lose weight in your legs too. But at the end of the day, it will still build muscle in your legs. I have mentioned this before, but if you are a shorter endomorph build, I would avoid running as it can make your legs bigger.


There are 2 basic types of muscle fibers. Slow twitch fibers are those used for longer endurance style workouts; and fast twitch fibers are those used for short bursts of energy such as sprinting, HIIT and plyometric workouts.

Everyone has different amounts of these fibers, and the amount and type you have will determine whether or not your legs get bigger from running. Someone with lots of fast twitch fibers might have muscular legs, even though they don’t do any type of weight lifting or sprinting. They may even build muscle just from walking.

On the other end of the scale, someone with lots of slow twitch fibers might do lots of sprinting and HIIT and find it difficult to gain muscle.

As you can see, genetics play a huge factor in whether or not your legs will get bigger from running. Some people will run all their life and their legs will stay slim and toned, while others will get bigger legs from running after just a month or so.

You need to adjust your running (and exercise program) to suit your body type and your goals.

legs getting bigger from running
Both women are long-distance runners. The difference is their genetics


Running on an incline will also use your muscles more (especially your quads and glutes) and will build them. This is due to the pushdown and jumps motion. If you want a leaner look, always try to run on a flat surface.

Another factor that might cause your legs to get bigger from running is your exercise experience. If you haven’t done much exercise and have very little muscle in your legs, then you start running, you are going to build muscle and this is going to be a significant difference.

But if you already exercise and have muscle in your legs, the change caused by running might not appear as significant. What is considered significantly bigger depends on your preference.

Another thing to keep in mind is that your legs can also swell after a run. This is short-term and the swelling will reduce. So try not to judge your progress based on what your legs look like immediately after a run.


The takeaway points from this are:

  • Running builds muscle in your legs, but doesn’t build as much as sprinting, HIIT and weightlifting;
  • Your body type and muscle fibers (i.e. genetics) will determine whether your legs will get significantly bigger from running. But “significantly bigger” is determined by YOU and what you think is too big;
  • You need to adjust your running (and exercise program) to suit your body type and your goals; and
  • Run on a flat surface and at a steady pace to avoid building more muscle from running.


If running has made your legs bulkier and you would like to lean them out, I should also mention my 3 Steps to Lean Legs Program which is designed to help you get lean legs without the bulkiness :)

There are 3 different versions fo my program, one for each body type. Each version combines the right type of cardio and resistance training for one particular body type so you can be sure you’re going to get the best possible results. 

And of course, since you cannot train and be on a crappy diet, I included a complete 8-week meal plan in my program.

Also, the resistance training part of my program now has FULL-LENGTH videos that you can follow from warm up to cool down.

To find out more about my 3 Steps to Lean Legs Program click the link below:


To find out more about my FULL-LENGTH VIDEOS, follow this link: 


And as always, if you have any questions, please feel free to ask! 

Love Rachael xx

Tags: cardio q&a, do squats make your legs bigger or smaller, how to get skinny legs, how to slim calves, how to slim muscular thighs
Picture of Rachael

Rachael is an Australian born certified personal trainer and nutritionist who holds a Bachelor degree in Science.

After struggling for years to find an exercise and diet program that is tailored to women striving for lean and toned body with no bulk she designed her Lean Legs Program. This program is tailored to each body type and focused on helping women get toned but feminine bodies, without getting bulky.

Her mission is to empower women and help them stay in shape in a healthy and balanced way.

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47 comments on “Why Are My Legs Getting Bigger From Running?”

    Eddie says:

    Hi, i am 23 year old and i believe i have endomorph body type. I have been struggling to reduce the size of my calf, they make me feel so insecure which is why i dnt wear skirts and skinny jeans. I have tried running without inclined mode, squats and most of the exercises but i have not succeeded to reduce the size of my calf muscles it just makes me leg look so manly and only gets bigger and bigger. Its definitely not the genes, i do not know what is the reason behind. Please if you could give me some tips on it. Thank you.

      Lean Legs Support says:

      Hi lovely,
      Calves are a bit tricky because genetics is a big factor. I often get these questions so I wrote a very detailed blog post about it.

      I think you’re going to find it very helpful :)

      And if your calves got bigger from training or you’ve gained weight, I’m happy to tell you that my Lean Legs Program could help you with that :)

      You can find out more about it here. :)

    Joss says:

    Hello! I am a 15 year old girl who likes to run on the treadmill, and I noticed that my legs are getting bigger. I want to lose weight, so should I give up running?

      Ana - Lean Legs Support says:

      Hey lovely,

      Thanks for reaching out!

      Since you are still very young it is best to stay focused on simply staying physically active.
      However, don’t include diets until you’re fully developed and exit the teenage phase of your life because it can seriously damage your metabolism and lead to gut issues that will make it difficult for you to slim down in the future.

      Make sure that the treadmill doesn’t have an incline installed – that you’re using the one with a 0% incline option.

      When you work out, there is an increase in blood that is being pumped into your system and the muscles becomes temporarily engorged which will make them appear bigger than normal but again, this is only temporary. Keep this in mind in case you’re already using a 0% incline treadmill.

      You can also practice walking on a treadmill instead of running.

      Hope this helped!


    Peggy says:

    Hello, so I am 31 years old female, From reading your article my body type is definitely an ectomorphs I find it very difficult to gain muscles will probably never get bulky legs which I love, but I also noticed I loose muscles quicker than gain muscles from any little cardio that I’ve done, i have been doing cardio like spinning, stair masters, gym classes, alone with my weightlifting. My eating pretty clean I have discipline, I don’t really run so I was thinking about adding some sprinting in my routine to see if I can get some sorts of bulking in my legs

      Ana - Lean Legs Support says:

      Hey lovely,

      You may also include some HIIT and resistance trainings to your routine in order to get more definition. :)


    Mathilde says:

    Hi, so i am 13 years Old Girl, and i have Seem to notice that my legs are big. There is some fat in MY thights but not that much so i am scared that if i try to do cardio or running they just get bigget:(
    I relly want to slim Down the muscle in MY thight but i just startes growning so i dont know if i sjould do a deficit or what
    Please help me i relly need it

      Tijana - Lean Legs Support says:

      Hi lovely,

      Thank you for reaching out! :)

      ​Since you are 13 years old, your body is still going through a lot of transformations so ​if you want to lose weight, it’s best you talk to your parents so they can get you an appointment with a specialist. A nutritionist can advise you on your nutrition and workouts so that you can achieve the results you wish in a healthy way that suits your age.

      Wishing you the best of luck! xx