Basically muscle memory is the same as explaining the lift on air plane wing with the Bernoulli theorem.
It is a nice shortcut but really has not that much to do with what actually happens
Haha, agreed. I've always gotten a little annoyed when people incompletely explain aircraft lift using Bernoulli's theorem... "the air above the curved wing moves faster, so it has lower pressure than the air below it... " Sure, but who says that air moving over a curved wing moves faster? There's no such thing as "equal transit time" for air; the air above the wing will take its own damn pace traveling, thank you. BUT Bernoulli's theorem is an easy intuitive way to explain the phenomena than a messier explanation that calls on all of Newton's three laws and the Coanda effect.
Here we're getting into semantics, the difference between "motor memory' and "muscle memory", though semantics are definitely important.
Those are some interesting papers, but for people who don't feel like slogging through the molecular biology, they basically are about the phenomena and underlying mechanisms that govern how a given muscle has the capability to fairly rapidly re-grow in size and strength in response to exercise after a period of inactivity and atrophy IF it had exercised before; this is a truer "muscle memory" in the sense that the muscle "remembered" its previous state of conditioning and was able to better re-attain that state.
This is different from "motor memory" which is what most people use to describe the phenomena of "doing something repeatedly makes you better at doing it," since this is a function of the motor neurons and your brain, not your muscles.
It is certain that repetitive exercise has an influence on which and when motor units are fired but that depends on motor neurons not muscular fibres.
As well motor leaning have lasting effect on neuron of the motor cortex which dictates what motor neurons are activated in what order. it seems that "memory cells " has been recently put in evidence by Dr. Emilio Bizzi and his collaborators.
(Li C-SR, Padoa-Schioppa C, Bizzi E: Neuronal correlates of motor performance and motor learning in the primary motor cortex of monkeys adapting to an external force field. Neuron 2001, 30:593-607.)
If you are interested there was an interesting thread on the flinch where reflexes vs cognition was mentioned.
Oops, I got ahead of myself, Phillippe said it a lot more succinctly than I.
So anyway, what we're talking about really is "motor memory," the ability of our motor neurons and brains to form more efficient, faster synaptic pathways, our motor units to become increasingly more sensitive to a given neurochemical response, etc.
"Muscle memory" is something else, though for the sake of simplicity of language among laymen, I use them interchangeably too.