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(Omega_man @ Jun. 14 2006,19:04)</div><div id="QUOTEHEAD">QUOTE</div><div id="QUOTE">How is CNS fatigue defined as related to "strength loss".
We know to stay away from failure, but in what way does its affect relate to the body?
1) Is it cumulative with in a given exercise?
(ie. Benching to failure will drain only the CNS related to that movement).
2) Is it cumulative in that it affects the entire CNS? (Benching to failure will drain the entire CNS & other unrelated muscles will suffer... ex: squats suffer too).
Then is the CNS drained at the 1) brain, 2) nerve paths to the muscles from the brain. 3) the ability for the nerves in the muscles to active the muscle fibers?
Omega</div>
First there is no way I can answer this fully as 1. It would take a lot of typing and 2. Even though I understand the principles of it I hate studying neurology and therefore I haven't.
Some defintions that Gandevia used in his outstanding look at Central Fatigue.
Central fatigue-A progressive reduction in voluntary activation of muscle during exercise.
Muscle fatigue- Any exercise-induced reduction in the ability of a muscle to generate force or power; it has peripheral and central causes.
Peripheral fatigue- Fatigue produced by changes at or distal to the neuromuscular junction.
Supraspinal fatigue- Fatigue produced by failure to generate output from the motor cortex; a subset of central fatigue.
Task failure- Cessation of a bout of exercise. This may be accompanied by peripheral fatigue, central fatigue, or both.
In most cases the peripheral mechanisms is what causes localized fatigue but continued peripheral fatigue can lead to more systemic effects. Such that your 2nd question may be representative of what is occuring.
So yes to both.
CNS is the entire system and although I don't think that the brain actually gets drained it does recieve a multiple of signals from the various body structures (Golgi, Spindles, Afferants, hormonal, chemcial and so on) which then the brains sends signals back to control what and how much the body can be taxed, a constant state of signalling through feedback and forward loops, so in essence it's all of it combined.
Think of it this way, your body is a constant running engine, and like any engine, periodic and brief increased speed or production doesn't necessarily hamper overall performance but a continue heightened production or speed thereof eventually wears the engine down and either must be stopped before damage occurs or it stops on it's own accord when it breaks. Since our body doesn't want ot break as it takes so much more energy to repair itself it has many controls built in to prevent it from breaking, reduced neural activation and drive during fatigue is one of these, along with all the other symptoms of fatigue.
Secondly it's not that we should avoid failure as with anything we do, working to failure get's us better at working to failure but it also hampers our ability to do the other things that have shown to be positively correlated to growth so it's a trade off, do you want to increase your time to failure or do you want to progressively and consistently apply a load to cause muscle growth?
an et al 
