There is a scientific answer . . . With intense exercise, testosterone rises with plasma volume, and cortisol (the stress hormone) falls. So, when the testosterone/cortisol ratio falls below a certain level, catabolysis and official overtraining has begun. In studies with soccer players and rugby players (the two main study groups) if you stick to a traditional 3-4x/week weight routine and 3-4 cardio sessions a week you should be ok. One little study showed a benefit to cardio in building muscle mass even . . . What's probably far, far more important than catabolysis is that most people don't eat enough calories ever to build lean muscle mass. Most people take 2000cal, then train, and eat 2500 or so. The thing is, after a resistance training bout, machinery is activated for muscle hypertrophy, and if the caloric excess isn't there, like 3500+/day, then it shuts off the process. So a little extra makes you fat, with no growth. You need to grossly overeat, with very high protein or you might as well not weight train and just do cardio. . .
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