Does Cardio Burn Muscle? It Doesn’t (If Done Right)

Table of Contents

• Does Cardio Cause Muscle Loss?
• 6 Mistakes That Lead to Muscle Loss When Doing Cardio
• How to Find Balance
• Does Cardio Benefit Muscle Mass?
• FAQ
• Bottom Line

Like many training enthusiasts, you’ve probably asked yourself, “Does cardio burn muscle?”

There are many opinions on the topic, with some experts claiming that cardio is the worst for muscle gain and others saying that endurance exercise may support long-term growth.

So, who is wrong? Let’s discuss in this review of cardio and muscle loss.

Does Cardio Cause Muscle Loss?

The idea that cardio will cause you to lose your hard-earned muscle is misguided. 

Asking a question like, “Does cardio make you lose muscle?” implies that endurance exercise alone is the issue and that, no matter what, if you run, cycle, or swim, you will lose size.

In reality, several things need to happen for the body to burn muscle for energy. 

Most notably, you must be in a calorie deficit (eating fewer calories than you burn), not get enough protein (less than 0.8 grams per pound of body weight), and avoid resistance training. [1] [2] [3]

What’s Behind the Belief?

The idea that doing cardio burns muscle primarily stems from a lack of understanding of how the body functions and responds to different exercise types. 

People often assume that since cardio is energy-demanding and doesn’t cause that big of a muscle stimulus, it must lead to muscle loss.

Plus, there is also the observational evidence to keep in mind. 

It’s evident that many endurance athletes, such as marathon runners, have slimmer and less muscular builds compared to more explosive athletes, such as weightlifters.

It’s true that cardiovascular exercise primarily uses fats and carbs for fuel. It could use the available protein (circulating amino acids and muscle tissue) for energy if it runs low on these. [4] [5] 

However, a simple fix would be getting more carbs before and during cardio to prevent that.

6 Mistakes That Lead to Muscle Loss When Doing Cardio

Here are the 6 worst mistakes that could lead to you losing muscle while doing cardio workouts:

Your Cardio Workouts Are Too Long

Doing too much cardio is perhaps the biggest mistake that could lead to muscle loss or at least hinder your ability to build new muscle and get stronger. 

There are two primary reasons for that.

First, cardio, even less demanding options like cycling, generates fatigue, impacting your recovery and performance in the weight room. [6] 

As a result, you might not be able to do the same intense, high-volume workouts that stimulate muscle growth.

Second, cardio burns calories, making it more difficult to maintain a surplus to gain weight and build muscle optimally. 

Even worse, too much cardio can put you in a calorie deficit, leading to weight loss and drastically increasing the risk of losing muscle.

So, too much cardio causes muscle loss or at least limits muscle and strength gain. [6]

You’re Doing Cardio Before Weights

Another common mistake that could lead to muscle loss is doing endurance exercise before lifting weights. Even if the workout isn’t as long, doing it shortly before training can tire you and impact your resistance training. 

As a result, you may not be able to train as hard and create the same stimulus. Over time, that can hinder new growth and even cause you to lose some muscle.

The Type of Cardio You Do is Too Demanding

Some cardio activities are simply more demanding than others. They lead to more joint stress, involve more muscles, and are generally more taxing for the mind. 

In addition to affecting recovery, such activities can also affect your performance in the weight room and limit growth. [6]

One popular cardio activity that can be quite stressful is running. While highly beneficial and fantastic for improving your mood, it can fatigue your legs and lead to some ankle and knee issues in overweight individuals and those who run with poor form.

As such, you may be unable to train your legs as hard, especially if you run less than 48 hours before training your lower body at the gym.

You’re Not Lifting Weights

Solely focusing on cardio is a surefire way to lose muscle and strength simply because you’re not providing the necessary stimulus. 

As the specific adaptations to imposed demands (SAID) principle states, the human body can adapt to various stressors. So, if you only do cardio, the primary adaptation will be endurance, not strength or muscle gain.

Your Diet is Too Low In Calories

This is something we mentioned above, but it bears repeating: caloric restriction leads to weight loss and increases the risk of burning muscle for energy. [1]

Your body doesn’t want to hold onto too much muscle if it doesn’t need it because this is metabolically costly tissue. [7] 

The more muscle you have, the more calories you need to sustain yourself, which is not ideal from a survival standpoint. 

So, while dieting or starving (your body can’t tell the difference), you’ll likely lose muscle. 

The risk is even greater if you lose weight too quickly (beyond one percent of your body weight weekly), don’t get enough protein, and don’t do resistance training. [8]

You Don’t Eat Enough Protein

Protein provides the building blocks (amino acids) your body needs to hold onto muscle and carry out many processes necessary for good health. 

Not eating enough protein would make it difficult for your body to maintain protein turnover (replacing old and worn-out proteins with new ones), leading to muscle loss. [9]

The effect is exacerbated when you also do demanding cardio activities that involve multiple major muscle groups (e.g., swimming).

How to Find Balance

It’s possible to lose muscle from cardio, but the risk is significantly lower when you follow these rules:

• Duration – Keep your cardio workouts shorter (20 to 30 minutes) to burn calories and enjoy the health benefits without overworking yourself and impairing your recovery.
  
• Frequency – Do cardio two to four times per week to reduce the risk of it interfering with your resistance training. [6]
  
• Type – Go for less challenging cardio activities, such as cycling on a stationary bike or using an elliptical trainer, to limit fatigue and perform better in the weight room. [6]
  
• Intensity – Maintain a moderate intensity of 65 to 75 percent of your maximum heart rate (you can use a heart rate monitor, preferably a chest-strapped one).
  
• Timing – Don’t do cardio before weights. If possible, do it on rest days from the gym or at least six hours before the gym. If that’s also impossible, lift weights first and then do some cardio. [6]

Does Cardio Benefit Muscle Mass?

Though it may sound counterintuitive, regular cardio alongside resistance workouts (concurrent training) can lead to better growth in the long run. 

Cardio can be beneficial because it can improve your aerobic and work capacity and support muscle recovery. [10] 

As a result, you can do more productive work and recover adequately, which is crucial for muscle gain. [11]

Here is a quick list of cardio benefits for muscle mass:

• Improved endurance for high-rep training
• A boost in work capacity
• Potentially better recovery (between sets and workouts)

FAQ

Let’s check out the most commonly asked questions about cardio and muscle loss:

Will Cardio Cause Muscle Loss?

Cardio alone won’t necessarily lead to muscle loss. However, long and demanding training sessions can burn energy and generate fatigue, impacting your performance in the weight room and making it more difficult to ingest enough calories for healthy weight gain (bulking). 

When done right (timing, intensity, duration, and modality), it won’t lead to muscle loss, and the impact on your resistance training program will be negligible.

Does Cardio Burn or Build Muscle?

As the name suggests, cardio is a type of physical activity that primarily strengthens the cardiovascular system and improves endurance. 

Less intense cardio activities don’t provide as big of a stimulus to promote muscle growth and might even contribute to breakdown under certain conditions (the ones we discussed above).

That said, certain types of cardio, such as HIIT (which is done at a higher intensity for a shorter period), can provide a training stimulus and result in modest muscle growth. [12]

Does Cardio Burn Muscle or Fat First?

Cardiovascular exercise, particularly at lower intensities, primarily burns through the available carbohydrates (glucose) and triglycerides (fats). 

Body fat is typically the next source of energy. The lower intensity of training gives the body enough time to generate the necessary energy for you to continue. 

To put it a bit more scientifically, a fatty acid gets oxidized (broken down) into two carbon acetyl CoA molecules. These enter the citric acid (Krebs) cycle to generate GTP molecules, which are equivalent to ATP (the primary energy currency in the body).

For cardio to break down muscle, the duration would need to be incredibly long, with no nutritional support.

How To Do Cardio Without Losing Muscle?

Limit the duration of each session to 20 to 30 minutes, train at a moderate intensity (65-75% of your maximum heart rate), go for less demanding cardio activities (e.g., cycling), and don’t do cardio shortly before weight training. 

If you must do cardio and weights together, lift first and finish off with cardio.

Bottom Line

Here is a brief recap of the main ideas in the article:

• Does cardio reduce muscle mass? It doesn’t typically lead to muscle loss (unless overdone significantly), but too much can impair recovery and performance, hindering your ability to gain muscle and strength.
  
• How much cardio can I do without losing muscle? This is highly individual, but most people should be able to do between two to four weekly sessions of 30 minutes at a moderate intensity and hold onto muscle just fine.
  
• Does cardio build muscle? Given the limited muscle stimulus, cardio isn’t generally the best way to build muscle.
  
• What other things lead to muscle loss? Losing weight too quickly, not eating enough protein (0.7-0.8 grams per pound), and not doing resistance training.