Well, I don't think I would call it "so-called" muscle memory, because this stuff is real. I'm not really that fascinated by the subject, but many people seem to be amazed by the "mystery" of muscle memory. Even as a young kid, I realized that the mind also had a lot to do with strength training and results. Hell, even thinking about building muscle or gaining strength while exercising, can help you achieve greater gains. The facts undoubtedly come to the surface, when you have someone who has strength trained for a few years and then takes several years off and allows their body become softer & weaker. BUT, when they restart their training after the long hiatus, they quickly have results & gains that a typical person couldn't achieve in such a short period of time.
I'm so glad that I trained for years in the past, because I have currently took a couple years off from weight lifting and upper body fitness...and have lost a decent amount of strength and muscle mass. But thankfully for the mystery of my muscle memory, when I do start back, I can achieve gains and be back to where I used to be, within a few weeks. Hey, I've did it before, and this stuff is really true.
I'll provide a couple extra resources below, in case you're looking for more on this subject:
"Anyone who has lifted weights, on and off, for several years is familiar with the concept of "muscle memory." Muscle memory in this context refers to the observation that when a person begins lifting weights after a prolonged lay off, it is much easier to return to their previous levels of size and strength than it was to get there the first time around. Even when significant atrophy (muscle shrinking) has taken place during the layoff, previously hypertrophied muscle returns to its previous size more quickly than usual." Read More: http://www.thinkmuscle.com/articles/haycock/muscle-memory.htm
"Muscle memory consolidation involves the continuous evolution of neural processes after practicing a task has stopped. The exact mechanism of motor memory consolidation within the brain is controversial; however most theories assume that there is a general redistribution of information across the brain from encoding to consolidation." Read More: http://en.wikipedia.org/wiki/Muscle_memory
---End of Post "Mystery of Muscle Memory"
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