Overtraining is not a myth, though I do agree with Dr. M that the term is overused without taking into acount the individuality associated. Everyone will eventually overtrain, however, the point (volume/frequency) at which they will overtrain is different for everyone. Even that kid who was blowing up training chest 5-6 times a week could eventually overtrain. His threshold for overtraining is just much higher than your average trainer. You must experiment, find your threshold and bring yourself to that point without superseding it. Just like diet (and everything in this game), training is individual and only through experimentation can you find what is optimal for you.