Well, a motor unit is the motor neuron and all the muscle fibers it innervates.
Here is one regarding fiber types, recruiting, and conversion: especially note slides 10-20
http://www.slideshare.net/JayCholewa/skeletal-muscle-physiology-and-applications-to-training
Check this out. The whole thing is good, but for the sake of fiber type and training you may want to start around slide 84:
http://www.slideshare.net/JayCholewa/biomechanics-for-strength-training
A few things.
1. We two muscle fiber types, and a total of 7 subtypes:
I
Iac
IIc
IIac
IIa
IIax
IIx
There is no interconversion (i.e.: you cannot convert a type I to a type II...not in humans)
There is intraconversion (type II subtypes can be converted toward IIx or IIc)
With strength training IIx are almost all converted to IIa or IIax..depending on the type. IIc's are also converted this way. The more explosive and short the sets, the more they go toward IIax. the longer the sets the more they go toward IIa.
With strength training, just about all fiber will hypertrophy. IIa seem to have to the greatest potential.
You will see on slide 18 of presentation 1 that type IIX basically runs out of energy after 4 sec of all out work, this relates to slide 16 where you will note how fast IIx fatigue. This is why the adaptation with typical strength training, where sets take 15-30 seconds, is the intraconversion of IIx to more fatigue resistance IIax and IIa.
Br