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(Peak_Power @ Nov. 19 2006,19:45)</div><div id="QUOTEHEAD">QUOTE</div><div id="QUOTE">Perhaps the body also "remembers" the condition it was in, and is able to quickly make the adaptions necessary to get back there, but pushing into new territory requires the body to adapt in a way that it never has before, which is why its harder...this is more speculative, but I find it occurs with cardio/aerobic fitness as well (after not running/cycling for a while, I can quickly regain fitness, but find it hard to push into new fitness territory).</div>
It's been a long time since I read about it, but my recollection is that increasing your cardiovascular fitness causes the body to actually develop new capillaries. I presume that increased vascularity is not reversed if you stop your cardio work, so that may be at least part of the explanation for a quick rebound in aerobic conditioning.
As for the body "remembering" I've read my muscles lots of books and they still can't remember a damn thing.
But the stuff about fascia stretching, neural changes, satellite cell changes, etc. seem plausible as possible reasons for the so-called muscle memory. I don't know enough to say which ones might be responsible, but the explanation must be in some physiological changes that don't get reversed even when the muscles get detrained and lose size.