When Does Cardio Start Burning Muscle? Science, Tips, and Hacks

When does cardio start burning muscle? A tricky question, with a somewhat complicated answer.

Read on and learn about why, how, and when your workout goes from plain ol’ cardio – to muscle-burning cardio.

And at the end, I’ve included four science-proved hacks to help you get your cardio going without losing strength! Let’s go:

FYT Personal Trainer Banner

When Does Cardio Start Burning: The Science

Like many people, you probably believe in cardio’s health benefits. Many studies find aerobic exercise great for our cardiovascular system, endurance, and mental health (1, 2).

But if your primary fitness goals revolve around building muscle and getting stronger, you might feel hesitant to do cardio. After all, there is also the risk of muscle loss when doing aerobic exercise.

The question is, why does this happen, and when do we start losing muscle mass?

The simplest answer is this:

If you overdo cardio, you will start burning muscle and losing strength. The most obvious reason for that has to do with energy availability. Cardio is demanding and pulls energy and nutrients that would otherwise go toward muscle and strength gain. It can also slow down your recovery from weight training and stop you from performing as best as you can (3).

Another reason why cardio can lead to muscle loss has to do with the processes related to the different adaptations from strength and aerobic training. Specifically, this covers the AMPK enzyme and mTOR pathway (4). 

AMPK is an enzyme that plays a vital role in making energy available for the body during periods of shortage. The enzyme also inhibits energy-demanding activities related to the synthesis of fatty tissue and more. Unsurprisingly, the enzyme also inhibits muscle protein synthesis by downregulating mTOR – the primary regulator of growth and anabolism in response to nutrients and energy availability (5).

The bottom line? Elevation of AMPK stimulates oxidation and prevents the body from anabolism. The issue is, too much aerobic exercise can lead to AMPK elevation and the cascade of adverse effects we discussed above.

In other words, cardio training can interfere with our muscle and strength-gaining progress. If we do too much cardio, it can even lead to muscle loss and overtraining. Of course, the amount of cardio that will lead to these effects will vary from person to person. 

For instance, one person with good recoverability and greater potential for muscle growth might be able to get away with more cardio. In contrast, another person might run into recovery problems early on and might have to do small amounts of cardio each week to avoid the adverse effects.

Should I Even Do Cardio If My Goal Is Muscle Growth?

After reading the last point, you might be thinking, “Well, I won’t do any cardio. I don’t like it, anyway.” Of course, this is one way to go about it, but cardio is beneficial even if you primarily care about building muscle and getting stronger.

The primary benefit of aerobic exercise is that it improves your endurance and work capacity. As a result, you can do more work, recover more quickly, and possibly achieve better results in the long run (6). Since training volume is tightly correlated with muscle growth, doing more work tends to deliver better results (7). Of course, this is up to a point, but you still need a level of aerobic capacity to do enough work and recover in time.

Some people find themselves reaching a point where their work capacity isn’t large enough for the amount of work they need to keep growing. This is paradoxical, but folks feel tired and unable to recover without necessarily doing enough training for optimal progression.

Regular cardio training is beneficial because it allows you to improve your work capacity and prevent yourself from getting stuck in such a pickle. 

The question is, how can you do cardio to improve your work capacity but without risking muscle loss? Let’s go over some valuable tactics:

How to Do Cardio Without Risking Muscle Loss

1. Do It On Rest Days

A fantastic way to offset the interference effect of cardio on your strength training is to do it on rest days. For instance, if you train Monday, Tuesday, Thursday, and Friday, you can do your cardio on Wednesdays and over the weekend. 

For one, you might be able to get some active recovery going on, provided you don’t do incredibly demanding cardio workouts. And second, you can do your cardio in a fresher and more recovered state, thus performing better and not feeling as beat up by the end.

2. Separate It By At Least Six Hours From Weight Training

If doing cardio on rest days isn’t an option, the next best thing you can do is put a few hours between your weight training and cardio. Research shows that spacing the two out by six hours seems good for offsetting the interference effect.

For instance, you can do a cardio session in the morning and hit the weights in the evening.

3. Start Small And Increase The Amount Gradually

Balancing cardio and weight training comes down to recovering well and being able to progress over time. So long as you do that, you shouldn’t worry too much about concurrent training.

In that line of thinking, it’s best to introduce cardio gradually. Start with a couple of short and less intense sessions and slowly increase the volume. For instance, start with two 20-minute sessions, gradually make them longer, introduce a third, and see how you feel.

4. Go For Less Impactful Options

Research finds cycling to be a less impactful cardio option. In contrast, intense activities like interval running and sprinting cause greater fatigue and prolong recovery time.

So, it’s best to start with a less impactful option, see how you recover from it, and only then introduce something more challenging.

Final Words

Cardio is fantastic. It brings many health benefits, elevates our mood, and can benefit our weight training efforts. But, too much of a good thing can be bad, which fully applies to cardio.

Doing too much can have the opposite effect and lead to muscle loss, issues with recovery, and fatigue. So long as you stay mindful and follow the four tactics I’ve outlined above, you will do great.

Want to learn more about cardio?

Check out my MASSIVE guide to the world of cardio workouts and exercise – LOADED with tips, hacks, and super-efficient workout routines.

See you there!

Other Helpful Resources


  1. Pinckard K, Baskin KK, Stanford KI. Effects of Exercise to Improve Cardiovascular Health. Front Cardiovasc Med. 2019;6:69. Published 2019 Jun 4. doi:10.3389/fcvm.2019.00069
  2. Sharma A, Madaan V, Petty FD. Exercise for mental health. Prim Care Companion J Clin Psychiatry. 2006;8(2):106. doi:10.4088/pcc.v08n0208a
  3. Wilson JM, Marin PJ, Rhea MR, Wilson SM, Loenneke JP, Anderson JC. Concurrent training: a meta-analysis examining interference of aerobic and resistance exercises. J Strength Cond Res. 2012 Aug;26(8):2293-307. doi: 10.1519/JSC.0b013e31823a3e2d. PMID: 22002517.
  4. Fyfe JJ, Bishop DJ, Stepto NK. Interference between concurrent resistance and endurance exercise: molecular bases and the role of individual training variables. Sports Med. 2014 Jun;44(6):743-62. doi: 10.1007/s40279-014-0162-1. PMID: 24728927.
  5. Xu J, Ji J, Yan XH. Cross-talk between AMPK and mTOR in regulating energy balance. Crit Rev Food Sci Nutr. 2012;52(5):373-81. doi: 10.1080/10408398.2010.500245. PMID: 22369257.
  6. Tomlin DL, Wenger HA. The relationship between aerobic fitness and recovery from high intensity intermittent exercise. Sports Med. 2001;31(1):1-11. doi: 10.2165/00007256-200131010-00001. PMID: 11219498.
  7. Schoenfeld BJ, Contreras B, Krieger J, et al. Resistance Training Volume Enhances Muscle Hypertrophy but Not Strength in Trained Men. Med Sci Sports Exerc. 2019;51(1):94-103. doi:10.1249/MSS.0000000000001764