Actually, the advice in this article is dangerous. While technically, you will burn muscle if you exercise at a high level for an extended period, this is because you aren't getting enough calories to support your regimen. People are so afraid of calories nowadays, they don't realize that your body needs carbs in order to perform work. The answer isn't to eliminate cardio, which can boost your immune system and make you feel better overall (unlike what the article states). The answer is to change your diet to eliminate things like simple carbs and saturated fats. I'm under 10% body fat and I do both cardio and weight training and I feel better than I ever did in my 20s. Instead of simply taking my advice (or the idiot who wrote this article), read up on how your body's metabolism works and how the different sources of calories are used for energy and repair. And, damnit, don't be afraid of calories or cardio, focus on what you're eating.