Overtraining?

I've been thinking about the notion of over-training lately, and I am thoroughly confused about the topic. I can understand how a weight lifter can over-train, because he or she would be creating micro-tears in the same muscle fibers without allowing the previous tears to fully heal. What I don't understand is cardio over-training. I've seen some of the workout schedules for marathoners and iron-man competitors, and they are absolutely insane. It's like these guys have never even heard of the phrase "over-training." Is cardio over-training a myth?

I think it's possible to have cardio over-training, but it takes awhile to get there.  I think part of it is what sort of base you are starting from, physical endurance, and the sort of training.  I've read personal accounts from distance cyclists where they've literally had to be peeled off their bikes because everything just sort of shut down.  Afterward, all thoughts of continuing to train sort of go out the window.  They become apathetic and lose all ability to get any gains from further training.  At this point, the only thing to be done is to rest and reset.  Now, these accounts are from people who ride like 300+ miles a week on average and compete in endurance centuries and double centuries.

I did experience some over-training myself early in my training, but that was a combo of weight training and cardio.  Just a case of going nuts and doing WAY too much too fast!

 Overtraining is actually a hormonal crisis state brought on by stress and accumulated CNS fatigue, not neccesarily a function of what exercises you've been doing.

 Though lifting weights is a CNS-intensive activity and you get the first warning signs there - your maximal strenght is a function of coordinated motor unit recruitment. Maximal strength going down means your nervous system is starting to misfire and lose coordination.

 That's technically called 'metabolic overreach' if it's not a chronic condition - taking a week off or working at a lower intensity for a while will deload your system and allowing you to return to intense training. We mostly still call it overtraining because - well, it's the same thing only it hasn't been going on that long.

 Actual metabolic overtraining is a prologed crisis state where this condition has become chronic - this can happen because of general life stress from work, for eample. Or because one keeps on training through metabolic overreach, thus putting further stress on already-fragile systems and pushing them beyond their limits.

 That's usually when you have to start peeling the cyclists off the bikes.