"Chronic flaws in training may lead to an adverse cumulative effect - overtraining" Science of Sports Training: how to plan and control training for peak performance - Thomas Kurz, p321.
When physical training is above the bodies ability to recovery from, symptoms of overtraining start to manifest. If you've trained seriously for any length of time, you've probably experienced this to some degree. Using a systems theory approach to training, all stresses acting upon the body have to be taken into account when training 'physically'.
Interestingly, there are actually two types of over-training: basedowic and addisonic types. Basedowic overtraining involves an over-activation of the sympathic nervous system at rest; whilst Addisonic involves and over-activation of the parasympathetic nervous system at rest (and also during activity).
The two types of overtraining have some overlap: fatigue; loss of performance; diminishing gains; etc., but there are large differences in the development, progression and treatment between the two.
Basedowic over-training usually results from an overload of excessively intense exercise; such as someone who engages in high-intensity protocols without a solid foundational conditioning. High levels of emotional and mental stress (along with the physical) are often involved. This type of over-training involves adverse affects the thyroid gland. (See HERE) People with this type normally get picked up earlier because of weight change; sleep and behavior change, and changes in cardiovascular function.
Addisonic over-training is more of the 'classic' view of overtaining; where too much volume over time leads to stagnation, platuae, and eventually loss of performance. This type of overtraining can be 'silent' for the first one or two stages, i.e not much is noticed in terms of effects, other than tiredness and/or poor progress. This type of overtraining is named after Addisons disease, of which it shares some characteristics. The HPA axis (hypothalamus; pituitary and adrenal glands) are effected during this type of overtraining, leading to adrenal fatigue.
The book Science of Sports Training - Thomas Kurz (author of the more famous 'Stretching Scientifically' - which is excellent) is one of my favorite training books! It was probably the first place I saw over-training broken down into these two types.. The chapter on over-training details about 20, or so, signs and symptoms for each types; recognizing the 3 stages of each of these types; and details the treatment of these types. It's worth picking up, if you can find a copy.
It's been suggested (and I agree) that part of over-training comes from the bodymind becoming addicted to its own endogenous pharmacopeia of endorphins and other neurotransmitters, produced via exercise - getting high on your own supply, so to speak. Though this is the adverse side of these effects, I'm personally expecting the 'nootropic' uses of exercise to become more and more popular (above people training for 'fitness') for many people in the coming years.. this is a topic I'll blog about soon.