J.R. -
Mr. Orange is correct. Force, not speed, is the primary factor that determines voluntary motor unit recruitment. The more force required the more motor units are involved, and likewise more force activates higher threshold motor units. It's the cascade (progression of force) effect, which has been known since Henneman's research report ws printed in 1957. (Hundreds, if no thousands of experiment have tested the principle in an attempt to either deny or confirm its truth, but the nearly every result demonstrates that most of the Size Principle, and thus the order of recruitment, is true.)
You can lift a pencil fast, but you aren't recruiting many fast fibers (more accurately motor units). The force required is to low for fast fibers to be recruited. Just the same is you can blink your eyes fast, but there is no fast twitch involvement because the force requirement is super low. Conversely, if you have to lift the back end of a Buick car, you will obtain maximum recruitment of fibers, including Type I (slow), Type II (a) (fast oxidative), and Type IIx (they used to be called Type IIa), which are the one that are more powerful but have low endurance potential.
I've said this before, but this is nothing new to anybody who has studied exercise physiology in the last 50 years:
"Force, not speed, determines how many motor unit fibers are activated. Even though you turn your legs over faster on the flat, speed doesn't determine activation of fibers. Force does. That's why hill sprints are great." (In Pete Magill's excellent article in Running Times: (http://runningtimes.com/Print.aspx?articleID=20476)
To clarify, if you choose to take small steps while running on a track, which is flat, and you turn over your legs fast, you aren't necessarily going to recruit many of your fast fibers (motor units). However, if you voluntarily demand maximum force from your legs, as you sprint down a track, you recruit far more fibers at the high end of the spectrum - the Type IIa and IIx (fast twitch). So, though speed is apparent to the eye and to the stopwatch, it is the requirement of high-force that causes that speed, along with minimum friction.
To grasp the force concept more, go ahead and try to sprint all-out up a very steep hill and you'll notice that your speed is quite low, compared to sprinting on a flat track; but you are recruiting as many, or possibly more, of your Type II (a and x) (fast twitch)fibers because a very high force requirement is demanded by your brain (i.e. your choice to run all-out).
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The "Size Principle" by Henneman:
http://jn.physiology.org/content/93/6/3024.full
and
http://books.google.com/books?id=YAAT1-hebMgC&pg=PA88&lpg=PA88&dq=muscle+fiber+recruitment+force+%2B++not+speed+%2B+determines+recruitment+%2B+Henneman&source=bl&ots=IQC5kMYHvX&sig=e_R8jFHQbj_wIwl4UhIkbGjuNxA&hl=en&ei=4a3MTavkJZK2sAOpnezoCA&sa=X&oi=book_result&ct=result&resnum=9&sqi=2&ved=0CFUQ6AEwCA#v=onepage&q&f=false
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A basic, but clear, explanation of the size principle in action:
http://www.t-nation.com/article/bodybuilding/maximum_recruitment_training_i&cr=