Quoted message said:(Nomenclature🙂 Why do we have just two muscle types, if Type IIA and Type IIB are seemingly as
different as Type I and Type II?
I believe historically at first only 2 types were identified, and then it was realized that there
were actually 2 different types of Type II fibers once people actually started being able to detect
differences in the myosin heavy chains (MHC) or the consequence of these differences. Now it is
known that there are 3 types of Type II MHC (a, x, and b) expressed in mammals, although humans only
express a and x, but not the fastest b isoform. But because historically, using histochemical
staining for ATPase activity (which is determined by the MHC) established the nomenclature for human
fibre types, you still see the 3 human fibre types given as Type I, Type IIA, and Type IIB (although
expressing Type IIx MHC). Given that some muscle fibres also express combinations of either Type
I/IIa or Type IIa/IIx MHC you end up with more of continuum of muscle fibre contractile
characteristics rather than 2 or 3 distinct fibre types. You can also have differences in the
mysosin light chains and calcium handling proteins that may affect contractile properties, and then
you have differences in the amount of glycolytic or oxidative enzymes that will affect energy
production and the fatigue resistance of fibre.
Quoted message said:What's a typical motor unit firing rate? How many action potentials can a sarcomere handle per
unit time? (Depending, I suppose, on what "handle" means, and if there's a "typical" to be found.)
Typical average motor unit firing rates for whole muscle are pretty low, I believe around 15-25 Hz.
A fast motor unit reaches tetani around 60 Hz, anything higher would be overkill. But you could
drive the muscle fibres at higher frequencies using electrical stimulation. I believe you have to go
pretty high (like > 150Hz) to start getting electrical fatigue, which i guess would define the
maximal rate the fibre could "handle".
