A common question of fitness clients is, does cardio burn muscle? When comparing competitive athletes in endurance training to those of a competitive sprinter, it is visibly so. One can see that long distance cardio training can waste muscle and regular sprinting can build muscle mass. This is because sprinting causes small microtears and resistance training increases muscle protein synthesis, leading to muscle hypertrophy. The question becomes, is the reverse true for aerobic exercise and does it eat muscle mass? The simple answer is no. The scientific research on long, steady state cardio training shows it does not have a negative impact on muscle mass. We’ll explore the science that answers the ‘does cardio burn muscle’ question. Then, we’ll address recommendations you can give clients for their cardio workout.
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Myth: Aerobic Exercise Eats Muscle
The ability for a muscle to generate force by contracting is “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 activity improve 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 participants 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. This suggests that aerobic activity can actually prevent strength loss. It also can increase muscle protein synthesis and can increase muscle hypertrophy.
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 activity 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.
The science is clear. Cardio alone does NOT burn muscle tissue.
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, 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 long duration cardio is no exception.
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Truth: A Negative Energy Balance and Exercise Can “Eat” Muscle
Many people combine aerobic activity 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 strength training and aerobic exercise by themselves to reduce body fat percentages. 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 prevents 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
It’s common for professional athletes to engage in power endurance training and strength training as a form of cardio training (instead of long, steady state cardio). Anecdotally, 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 training 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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Context Is Key
The data and science suggest that general cardio training does NOT burn muscle. However, one must keep things in 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.
Monitor client outcomes to ensure 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, and conducting ongoing fitness assessments with clients.
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.
These are the types of workout programming nuances a fitness expert with a degree in exercise science will learn. And, a degree in exercise science from Lionel University, includes the exercise physiology that will help you debunk myths such as cardio training leads to decreased muscle mass. Lionel has a variety of programs to choose from, including associates degree, bachelor’s degree, or master’s degree,
As you go through your degree program at Lionel, you’ll also earn your personal training certification and Master Trainer certificate in the first few months. This means you’ll be able to train others and take your personal fitness to new levels before graduation day. And, with the help of financial aid, earning your exercise science degree is even more of a possibility.
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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).
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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