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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 weight lifting, sprinting and high intensity interval training.

Let’s talk about sprinting

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.

legs getting bigger from running

Sprinter VS Marathon Runner

It depends on your body type

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.legs getting bigger from running

It also depends on your muscle fibers

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

Other factors that can cause your legs to get bigger from running

Running on an incline will also use your muscles more (especially your quads and glutes) and will build them. This is due to the push down and jump 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 weight lifting;
  • 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.

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

  • natalia upchurch says:


    I am a mesomorph and I do the elliptical every day for 30-45 minutes with no resistance. I try to keep my heart rate in the fat burning zone but not going crazy. Could this make my legs bulkier? I do not like running as much as I like the elliptical :(

    • Rachael Attard says:

      Hi Natalia, if your legs are bulky then yes this could be a cause. The elliptical does use your quad muscles so will build them a little, but not as much as lifting weights and doing squats, lunges, etc. You don’t necessarily need to run to slim down your legs – walking is actually better! xx

  • J. says:

    Great, great, great article!

    • Rachael Attard says:

      Thanks! :)

  • Eve says:

    Hi Rachael,

    I’m looking at getting your Ebook too and but not too sure what body type is mine. I’m about 54kg and 159cm. I’m a size small on the top but M to L on the bottom. Pear shape obviously! I was on Kayla Itsine program and done it for 6 weeks and realise I got bulky which isnt what i want. I want to still lose weight and tone up that elephant legs! Could you kindly advise which one should i go for?

    • Rachael Attard says:

      Hi Eve! I’m sorry to hear that happened to you. I think you sound most like the mesomorph body type. However if you do buy the eBook, please send me an email and we can chat about it more and I will help you pick the best one :) xx

  • Katie says:

    Hi Rachael!
    I have run track for the past 6 years, and my leg muscles have built up significantly from all the running. I’ve read your article on losing muscle, but should I just be walking and eating low carb and low protein to reduce it? Also, I feel like all of my muscle is just turning to fat and my legs are not getting any skinnier. Is there anything I should be doing differently? Thank you and I love your blog so much!

    • Rachael Attard says:

      Hi Katie, thank you! :) Yes I would suggest walking and a good diet as you mentioned. Muscle doesn’t turn to fat so please don’t worry about that. You can also add in some light circuits like the workouts on my blog. Also please know that reducing muscle takes time, especially after 6 years of track. When I bulked up my legs, it took 6-12 months to get them back to slim. Keep on doing what you’re doing, be consistent and give it time! xx

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