This is a statement you often hear, especially among bodybuilders, but also some personal trainers. The most extreme version is that aerobics makes you fat, the reasoning being that it leads to muscle loss, which slows your metabolism and causes you to gain fat because you are now eating too much for your slow metabolism. These types of claims are based on some truth, as we will see, but they are greatly exaggerated. Your muscles are not going to wear out because you run for half an hour a day.
There are a few mechanisms by which aerobic exercise can interfere with muscle growth or cause actual muscle loss. The first is that simultaneous aerobic exercise and strength training lead to competitive adaptations in the muscles. For example, steady-state aerobic exercise leads to endurance adaptations, such as increases in mitochondria (aerobic energy factories) and aerobic enzymes in muscle cells, while strength training can lead to hypertrophy, or growth of muscle fibers. muscular. The bottom line is that doing both activities has been shown to cut muscle growth in half compared to strength training alone. [Docherty, 2001; Gordon, 1967].
For those of us who are strength training to get fit, this isn’t a huge problem, it just means it will take longer to build muscle mass. But for bodybuilders it is interfering with competition in their specialty. Many bodybuilders will minimize aerobic exercise or take steps to reduce interference. I think this is where the seed for this “muscle wasting” idea was first planted. But keep in mind that aerobics in these studies has been shown to reduce the rate of muscle growth, which is a far cry from causing muscle loss. Interestingly, the opposite interference does not seem to occur: adding strength training does not interfere with cardiovascular improvements. Many of us who are into aerobic training will supplement it with upper body strength work, and there’s no problem with concurrent training in that case.
There’s another way cardio can interfere with strength training, and that may simply be by 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 was short on time and exhausted anyway. I had much more success when I cut back to a more reasonable hour per day and allowed more time to lift weights. 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 actual muscle loss due to overtraining and/or poor nutrition. Too much aerobic exercise can lead to increased production of catabolic hormones like cortisol (often referred to as a “stress hormone”), which can subsequently cause muscle tissue to break down. But aerobics in moderate amounts is a relaxing activity, leading 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. One study that specifically examined the amount of aerobic exercise needed found that cortisol elevations did not occur from running for 40 or 80 minutes, but only occurred during two-hour runs. [Tremblay, 2005]. Ironically, high volume resistance training can cause the same effect. [Stone, 1998]But I’ve never heard anyone warn you not to lift weights because it wears your muscles down!
The other mechanism is that if your body doesn’t have enough glucose in the blood, it can make it by breaking down protein. If there isn’t enough protein available in food, you’ll 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 “thumping” cyclists on long rides. You may become irritable and have poor judgment because your brain, which can only run on glucose, isn’t getting enough fuel. I have experienced both and they are not fun. I don’t know if my body broke down the muscles for fuel, but afterwards I felt like my muscles had been broken down, or at least beaten up. But both times this happened to me after about 3 hours of exercise without consuming fuel. On the other hand, many aerobic exercisers, thinking of carbohydrates as fuel, will increase their intake of bad carbohydrates like white flour products or sugary drinks or “energy bars,” which are basically glorified chocolate bars. This can lead to a
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 inconsiderable 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 as well), and protein is necessary to repair any tissue damage caused by exercise [Noakes, 2004]. Strength trainers are well aware that they need more protein, but cardio people often aren’t. Also, since many cardio exercisers are trying to lose weight, they’re probably cutting calories at the same time, which if you do by reducing portion sizes can lower 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 overtrain and don’t underestimate, and don’t eat crap. 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 you do a balance of 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 & 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, “Increased 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 .