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Originally Posted by karky
the fact that the CNS gets more fatigued with heavy lifting and speed lifting than light lifting, like bw squats, and stuff like that.. does that have anything to do with that when you do speed and heavy work, your recruit more muscle fibers? If you do BW squats, you probobly just need to activate the slow twitch fibers, right? But if you do heavy squats or speed squats, you have to activate all the way up to fast twitch (IIB) fibers. Could that be an explaination as to WHY it is like that? The CNS has to work harder to activate more monitor units and more muscle fibers..
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Yes the CNS has to work harder to fire more motor units and fire them faster. This can be accomplished in a few different ways though. Like you said, heavy lifting and speed lifting accomplish this. But explosive bodyweight movements do as well. Anything where you are trying to put maximum
force into the movement is CNS intensive. Remember force = mass x acceleration, so only one of those needs to be maximal to get a large force production. Sprints and jumping are CNS intensive because even though they are bodyweight, acceleration is maximal so force is high. A 1RM is slow, but the mass is maximal so force is high.
I think another player in all this is inter-muscular coordination. The CNS must work harder when it needs to activate muscle fibers from several different muscles simultaneously to accomplish the task. A heavy set of squats will be more CNS intensive than the same relative intensity of curls. This gets back to the total amount of muscle recruited, it's not solely dependent on how much you hit a particular motor unit, how many motor units you hit is very important as well.
So even though I see no training benefit or reasoning behind it, you could get away with a lot more maximal work with isolation exercises than big compound lifts. You may be maximally taxing every fiber your biceps have got, but in terms of the CNS, that ain't much.