I wanted to write this post because I know a lot of women use cardio to burn muscle, and on the other end of the spectrum, I know of people avoid cardio because they don’t want to burn muscle.
So I wanted to set the record straight as to whether or not cardio really does burn muscle.
WHEN DOES CARDIO BURN MUSCLE?
Cardio can in fact burn muscle, but it is not common and is quite difficult to do. For cardio to burn muscle, the following situations need to happen:
1. MUSCLE (I. E. PROTEIN) NEEDS TO BE THE ONLY FUEL SOURCE LEFT FOR YOUR BODY TO BURN
This is really hard to achieve. When you do cardio, your body will burn carbohydrates first, then fats, then protein. Protein will only be burned only as a resort.
If you are eating super low calorie (i.e. going down the starvation path), and if you don’t have much body fat to begin with and you start doing a lot of cardio, you will start to burn muscle when you exercise.
This is something that marathon runners need to be careful about, but for most people who exercise and go for runs, is not usually a concern.
2. YOU EAT VERY LOW PROTEIN
Protein (i.e. amino acids) are the building blocks of muscle. But protein also provides structure to internal organs, forms red blood cells, is important for your hormones, plus lots more.
If you don’t eat enough protein, your body will take protein from your muscles for the other bodily functions that require amino acids. By eating a very low protein diet, your body will use up your muscles.
SUMMARY
Yes cardio can burn muscle, but only if you eat very low calorie, have a very low body fat percentage and eat very low protein.
For the average person who eats at a slight calorie deficit (for weight loss) or at maintenance level and exercises regularly, losing muscle from cardio alone is very difficult.
HOW DO YOU LOSE MUSCLE?
Most women I talk to about losing muscle have gained too much muscle in their thighs from lifting and want to know how to reduce the muscle. I have written a blog post on how to slim down muscular thighs so have a read of that as it has lots of great information and tips.
But essentially, losing muscle is hard and will take a long time. I built up a lot of muscle in my legs from too much crossfit and HIIT, and it took me almost a year for my leg muscles to return to their normal size. It is a slow process.
If you want to speed up the muscle loss, you can eat low calorie and low protein, but please don’t increase your calorie deficit by more than 500 calories or you may cause more problems with metabolic damage.
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CAN I STILL BUILD MUSCLE WHEN I DO CARDIO AND EAT AT A CALORIE DEFICIT?
On the other end of the spectrum, I know a lot of people are worried about losing muscle if they do cardio and eat at a calorie deficit. A lot of people want to lose body fat but keep their muscle.
You CAN still build (and maintain) muscle if you do cardio and eat at a calorie deficit – I’ve done it personally! I’ve read of articles that say you can only do build muscle in this situation if you are a newbie to exercise and are overweight, which I am neither.
When you lift weights or do any form of resistance training, your muscles tear and then repair themselves. When they repair themselves, they become bigger and stronger.
Protein helps with this repair process. Your muscles will repair themselves regardless, but if you eat low protein, the repair process will just take longer.
It is harder and slower to build muscle when you eat at a calorie deficit, but it can still happen.
High intensity cardio such as running builds muscle too (because your muscles tear and repair) so as long as you don’t go too crazy with lots and lots of intense cardio (and you don’t eat super low calorie and low protein), you won’t lose muscle.
I know some of this is confusing, so if you have any questions, ask away! :) xx
3 STEPS TO LEAN LEGS PROGRAM
If you built too much muscle, especially in your lower body and you want to slim down your legs, you might like my 3 Steps to Lean Legs Program.
It’s designed to help you get lean and toned legs without building unwanted muscle. My program combines cardio with lower intensity resistance training and it also has a complete 8-week meal plan. That way you can be sure your diet is on point while you train :)
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It’s FREE
Is TRX similar to Pilates in not bulking up? In your experience would it be similar to bodyweight training? Luv your blog, so informative and encouraging!
Thanks lovely! I haven’t actually done a TRX class.. But some TRX exercises are fine for not bulking, and others (such as squats, lunges, etc) might cause bulk. It’s more about what type of exercises they do in the class! xx
I have been following your skinny legs guide for 3 weeks now and am really enjoying it. I also love reading your blog and find it helps me learn more about health and fitness, however, I can never find anything on the internet anywhere about why my legs are “fat,” “loose,” or “jello-like” whenever I sit or lie down. When I’m standing my legs do appear slimmer than what they were once before by a little bit but when I sit down with my thighs flat against a chair or lie down, it’s like I have no muscle and it just larges like jello. Do you have any reason for why this could be happening and any workouts or advice that could prevent it?
Prior to doing your workouts, I did cardio and strength training which is why I’m confused as to why I seem like I barely have any muscle or that it’s not thin when I sit down.
As far as my diet is concerned, I eat healthy and control what goes into my body. I try to get enough greens and proteins within my days.
Thank you.
Hi lovely! I have the same thing. My legs look slim when I am standing but sitting down they look bigger if they’re against a chair or something. I think this is actually quite normal and the reason is because of both body fat and muscle, but probably mostly body fat. When you reduce the fat on your legs, you should notice an improvement in this! Hope you’re still enjoying my eBook :) xx
Hi Rachael,
This is a great blog! I have “the bulky legs” problem. I have been doing in the last couple of months 3-4times week strenght training – squats, lunges… No cardio at all. As a result my leg muscles got bigger – and i do not like that look. ( fyi i used to be a figure skater for 10years-so my legs were anyhow not very skinny). So now i am trying to make them thinner and appear leaner. How much walking should i do and how about jogging – not at all? Thank you so much!
Hi lovely! I’m sorry to hear that :( I would do as much walking as you can – perhaps an hour 5-6 days per week. And yes you can definitely add in some jogging. Some women do get bigger legs when they do too much running, but usually only if you are short or an endomorph body type. If not, jogging can help. I would aim for about 20-30 minutes 2-3 times per week. Good luck! xx
So what types of exercise should I do if I want to decrease muscle mass and lean out? Especially the legs and arms. Is cardio good for this? MY body is very quick to gain muscle so I am always hesitant, however, I also want to lose fat so don’t know what to do!
Hi Mikay, have a read of this blog post on how to reduce muscle. If you want to reduce the muscle size, you really need to either stop using it or use it less. You can still do resistance training but try the lighter style of resistance training (such as this and this). And yes cardio will help! Good luck xx
This post is very useful! I love your topics ? What is the best way to lose muscle or to avoid getting more muscle in our legs from strength training, doing 2 HIIT sessions and 1 Pilates session per week or 1 HIIT session and 2 Pilates sessions per week instead? Or it depends on how many cardio sessions I do every week besides the strength training?
Hi Mika, to reduce muscle in your legs you really need to stop using the muscle or use it less. So I would do 1 HIIT and 2 Pilates, but just be careful with HIIT, especially if there’s a lot of leg focused exercises. Good luck! xx
If I need to stop using the muscle legs or use it less, then why walking or running helps in losing leg muscle? I am so confused
I have become addicted to your blog, haha! Losing muscle bulk is definitely a challenge. How do we lose muscle while simultaneously maintaining enough muscle to avoid becoming “skinny fat” and to avoid causing metabolic damage? Also, what is considered low intensity when riding a stationary bike? Is it about 9-11 mph? I could really use your expertise!
Hi lovely, haha that is awesome :) To avoid becoming skinny fat, I would still recommend doing resistance training but just avoid using the muscles that you want to lose (quads for most women). You can still do upper body and core workouts, and then try to just do lower body workouts that focus on your glutes. I’m not sure if you’ve seen them, but I’ve posted lots of workouts you can try on my blog :) Also, cycling will build muscle but it just depends on your goal. That sounds like a good speed though xx
This is very important information, because the wrong type of cardio will also build muscle and work against your goals. So, would you always recommend walking and non-impact style cardio while trying to get rid of muscle bulk, and also not adding muscle bulk from cardio? Great blog! Love you!
Thanks lovely! Yes walking and non-impact exercises are best for getting rid of muscle bulk, but unfortunately it will take a lot of time.. xx