A question that’s been asked over and over in the fitness world is: Does cardio burn muscle? When comparing an ultra-distance runner to a competitive sprinter, it is evident that excessive aerobic exercise can waste muscle and regular sprinting can create a muscular physique.
This is because sprinting causes small microtears and resistance training increases muscle protein synthesis leading to muscle hypertrophy. But what about aerobic exercise? Is this form of exercise the antithesis of building a muscular physique? Does aerobic exercise eat up muscle?
The only way to answer this question is to understand the myths behind cardio—and the many truths to keep in mind when training your clients.
Keep Reading: How to Structure an Upper Body Workout for Beginners
Myth: Aerobic Exercise Eats Muscle
The ability for a muscle to generate force by contracting is known as muscle quality. A negative consequence of aging is muscle quality, especially in type II muscle fibers. Independent of cross-sectional muscle area, the inability of contraction of a muscle is poor muscle quality.
Cardio and aerobic exercise improves muscle quality and safeguards against losing muscle innervation capability. It also protects and maintains strength in both younger and older populations.
A long-term study compared sedentary to aerobically trained participants in three different age categories: 20-39, 40-64, and 65-86 years. They found that isometric grip and knee extensor strength increased significantly more in the trained group over time.
What this tells us is that aerobic exercise actually reduces strength losses. What’s more, in addition to neural and strength benefits, aerobic exercise increases muscle protein synthesis and can hypertrophy muscle.
One study analyzed triathletes and discovered that four months of training improved muscle protein synthesis (MPS) rates by 22 percent, on average. Cycling training, a traditional aerobic activity, has been found to hypertrophy quadriceps muscle by 10 percent in middle-aged men and a 2014 review concluded that aerobic exercise acutely and chronically increases protein metabolism and induces skeletal muscle hypertrophy.
These increases in MPS during aerobic exercise may be due to improved mitochondrial respiratory capacity (MRC) as declines in MRC have been linked to decreased rates in MPS. A six-month intervention consisting of middle-aged men engaging in jogging found that mitochondrial density in type II muscles increased as much as 20 percent.
All of this tells us one thing. The answer to “does cardio burn muscle” is clear: no. It is incorrect to say that all aerobic exercise reduces muscle mass due to both hypertrophic responses and repeated findings of increased MPS—”all” being the key word.
Truth: Long Duration Aerobic Exercise Reduces Muscle
Aerobic exercise lasting longer than 75 minutes will lessen both liver and intramuscular glycogen stores. At this point, fatty acids contribute to a greater degree, but unfortunately, amino acids also contribute to the metabolic mix.
This means, if one’s goal is to maintain strength and muscle, high-volume aerobic exercise, particularly running, may need to be limited. If you’re an ectomorph like me, anything over 45 minutes results in adverse outcomes.
However, other body types can go longer and not experience negative consequences. As a trainer, it’s important to have a clear understanding of your clients’ metabolic capabilities. Too much of a good thing is dangerous and aerobic exercise is no exception.
Truth: A Negative Energy Balance and Exercise Can “Eat” Muscle
Notice I used the word exercise instead of aerobic exercise. Many people combine aerobic exercise with a low-calorie diet. In the context of a period of caloric restriction, all exercise can cause a reduction in lean muscle mass.
However, to say that aerobic exercise is the cause is incorrect and not supported in scientific literature. In fact, when combined with resistance training, aerobic exercise is more effective than resistance training and aerobic exercise by themselves to improve body composition. This explains why bodybuilders hit the treadmill during pre-competition cutting periods.
Truth: Cycling is Better Than Running to Maintain Strength
Research indicates that when it comes to concurrent training, the bicycle reduces strength losses more than running. The reason being that running uses more muscle and is physically more demanding than cycling. In addition, cycling is a concentric dominant sport, whereas running has far more eccentric contractions, the latter of which causes more significant muscle damage.
Truth: Aerobic Exercise Lowers Power Levels
As a college professor, I regularly invite successful seasoned professionals to speak to my graduate students. Last year, two revered Division I college football strength and conditioning coaches independently stated they engage in very little endurance activity. The "aerobic" training they engage in is power endurance combining resistance training and velocity-based training.
Their rationale to support their methods is simple; a faster team will still be faster in the fourth quarter, though more fatigued. Anecdotally, these coaches know that too much aerobic exercise can lower peak power, perhaps the greatest correlator to maximal speed. In other words, if peak power goes down, so does maximal speed.
Compelling evidence supports the philosophy of these coaches. A 2012 meta-analysis examining the interference effect of aerobic exercise and resistance training, co-authored by current strength and conditioning sport scientist of Alabama football, Dr. Matt Rhea. This research found that power was the most negatively affected physiological variable by endurance exercise.
With this finding in mind, trainers who work with athletes of anaerobic sports should spend most of their time training anaerobically.
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Does Cardio Burn Muscle?
Context Is Key
When looking at the data and science, the answer to “does cardio burn muscle?” is clear: not necessarily. Ultimately, it’s important to bring this question into context.
Aerobic exercise in moderation does not eat muscle or reduce strength levels. It can improve strength and contribute to muscle growth, especially in middle age and older adults. Surprisingly, aerobic exercise can cause muscle hypertrophy due to increases in MPS. Regular aerobic exercise is also heart-friendly, which will help maintain strength over a lifetime, and should not be viewed as a "muscle-eating" activity.
However, use your better judgment when programming this mode of exercise for clients. Recognize that too much endurance activity, and bouts of exercise that are too long, can cause loss of lean muscle mass, especially during periods of caloric restriction.
Monitoring successful and unsuccessful outcomes is needed to ensure that you’re not crossing the client’s threshold to use a more significant percentage of amino acids as fuel. This exhibits the importance of scientifically monitoring body composition, but I digress; that is another topic altogether.
Finally, if the client is an anaerobic athlete, a similar approach is recommended. Power is the most critical physiological variable that must be maintained and improved for anaerobic athletes. Too much endurance training can negatively impact power and performance.
With all of this in mind, you can program cardio into your clients’ workouts without worrying about reducing their results. In fact, by doing so properly, you're encouraging their bodies to build strength now and in the future.
Crane, J., MacNeil, L., & Tarnopolsky, M. (2012). Long-term aerobic exercise is associated with greater muscle strength throughout the life span. The Journals of Gerontology, 68(6), 631 – 638.
Bylund, A., Bjuro, T., & Cederblad, G. (1977). Physical training in man. Skeletal muscle metabolism in relation to muscle morphology and running ability. Eu J Appl Physiol Occup Physiol, 36(3), 208-223.
Doering, T., Jenkins, D., Reaburn, R., Borges, N., Hohmann, E., & Phillips, S. (2016). Lower integrated muscle protein synthesis in masters compared with younger athletes. Med Sci Sports Exerc, 48(8), 1613-1618.
Izquierdo, M., Hakkinen, K., Ibanex, J., Kraemer, W., & Gorostiaga, E. (2005). Effects of combined resistance and cardiovascular training on strength, power, muscle cross-sectional area, and endurance markers in middle aged men. Eur J Appl Physiol, 94(1-2).
Konopka, A., & Harber, M. (2014). Skeletal muscle hypertrophy after aerobic exercise training. Exerc Sport Sci Rev, 42(2), 53 – 61.
Wilson, J. M., Marin, P. J., Rhea, M. R., Wilson, S. M., Loenneke, J. P., & Anderson, J. C. (2012). Concurrent training: A meta-analysis examining interference of aerobic and resistance exercises. Journal of Strength and Conditioning Research, 26(8), 2293-2307.
Yarizadeh, H., Eftekhar, R., Anjom-Shoae, J., Speakman, J., & Djafarian, K. (2021). The effect of aerobic and resistance training and combined exercises modalities on subcutaneous abdominal fat: A systematic review and meta-analysis of randomized clinical trials. Advances in Nutrition, 12(1), 179 – 196