Does aerobic exercise cause muscle wasting?

This is a statement that you hear frequently, especially among bodybuilders, but also among some personal trainers. The most extreme version is that aerobics make you fat, the reasoning is that it leads to muscle loss, which slows your metabolism and makes you gain fat because now you are eating too much for your slow metabolism. These types of statements are based on a certain truth, as we will see, but they are greatly exaggerated. Your muscles are not going to lose because you run half an hour a day.

There are some mechanisms by which aerobic exercise can interfere with muscle growth or cause actual muscle loss. The first is that concurrent aerobic exercise and strength training lead to competitive adaptations in the muscles. For example, steady-state aerobic exercises lead to endurance adaptations, such as increased mitochondria (aerobic energy factories) and aerobic enzymes in muscle cells, while strength training can lead to hypertrophy or growth of muscle fibers. . The bottom line is that doing both activities has been shown to cut muscle growth by about half compared to just strength training. [Docherty, 2001; Gordon, 1967].

For those of us who are doing strength training to get in shape, this isn’t a big deal, it just means that it will take longer to build muscle mass. But for bodybuilders it is interfering with the mastery of their specialty. Many bodybuilders will minimize aerobic exercises or take steps to reduce interference. I think this is where the seeds of this “muscle wasting” idea were first planted. But keep in mind that aerobics have been shown in these studies to slow the rate of muscle growth, which is a long way from causing muscle loss. Interestingly, the opposite interference doesn’t seem to occur: adding strength training doesn’t interfere with cardiovascular improvements. Many of us who practice aerobic training will complement it with strength work in the upper body, and in that case there is no problem with simultaneous training.

There is another way that cardiovascular exercise can interfere with strength training, and it may be by simply taking too long. I remember a time when I was trying to lose weight, so I was doing about 90 minutes of cardio a day. I tried to do a token amount of resistance training, but I was short on time and wasted anyway. I was much more successful when I cut back to a more reasonable hour per day and allowed more time to lift. I talk about the proper balance of different types of training in another article.

However, there are a couple of mechanisms by which excessive aerobic exercise can lead to real muscle loss due to overtraining and / or poor nutrition. Too much aerobic exercise can increase the production of catabolic hormones such as cortisol (often referred to as the “stress hormone”), which can subsequently lead to the breakdown of muscle tissue. But aerobics in moderate amounts is a relaxing activity that leads to a net decrease in cortisol. Only excessive amounts of aerobic activity lead to elevated cortisol levels in the bloodstream after the activity is complete. A study that specifically examined how much aerobic exercise is needed found that cortisol elevations did not occur when running for 40 to 80 minutes, but only occurred in two-hour runs. [Tremblay, 2005]. Ironically, high-volume resistance training can cause the same effect. [Stone, 1998]But I never heard anyone warn you not to lift weight because it makes your muscles wear out!

The other mechanism is that if your body doesn’t have enough glucose in your blood, it can make it by breaking down proteins. If there is not enough protein available from food, you will get it from muscle tissue. [Berning, 1998]. Again, this is only likely if you are overtraining or undernourished. The most obvious example of this is “hitting the wall” in the marathon or cyclists “hitting” on long rides. You can become irritated and have impaired judgment because your brain, which only runs on glucose, doesn’t get enough fuel. I have experienced both and they are not fun. I don’t know if my body broke muscles for fuel, but then it felt like my muscles had been broken, or at least hit. But both times this occurred to me after about 3 hours of exercise without fuel. On the other hand, many people who practice aerobics, thinking of carbohydrates as fuel, will increase their consumption of bad carbohydrates like white flour products or sugary drinks or “energy bars” that are basically glorified candy bars. This can lead to

unhealthy lifestyle of poor nutrition justified by overtraining.

There’s a way that people who do a lot of cardio can end up protein deficient: resistance exercise increases the demand for protein. It actually provides a small but not negligible amount of fuel (you’ve probably heard that cardio is fueled by a mix of carbs and fat, depending on intensity level, but there’s a bit of protein in the mix, too) and protein. necessary to repair any tissue damage caused by exercise [Noakes, 2004]. Strength trainers are well aware that they need more protein, but people who do cardio often aren’t. Also, since many of those who do cardio are trying to lose weight, they are probably cutting calories at the same time, which if you do so simply by reducing your portion size can decrease your protein intake. The typical recommendation for protein is 0.25 to 0.45 grams per pound of body weight, but endurance athletes may require more than 0.55 to 0.65 grams per pound. [Sharkey, 2001].

So don’t train too much and don’t eat less, and don’t eat junk. Follow common sense procedures like easy day / hard day, don’t do hours of cardio a day, and don’t try to lose more than a pound of weight per week. Make sure to balance cardio and resistance training, and your muscles will be fine.

References:

· Berning, J, “Energy intake, diet and muscle wasting”, in Overtraining in sport, Kreider, R, Fry, A and O’Toole, M, eds, Human Kinetics, 1998.

Noakes, T, Lore of Running, Human Kinetics, 2002.

Sharkey, B, Fitness and Health, Human Kinetics, 2001.

· Stone, M, and Fry, A, “Increasing training volume in strength / power athletes”, in Overtraining in sport, Kreider, R, Fry, A, and O’Toole, M, eds, Human Kinetics, 1998.

Tremblay, M, Copeland J and Van Helder, W, “Influence of exercise duration on post-exercise steroid hormone responses in trained men”, Eur J Appl Physiol, 94 (5-6): 505-13, 2005 .