I've read somewhere that the peviously trained cells, while after injury and long rest period smaller and fewer are still built with a large number of mitochondria (that was needed for the previous performance levels). This seems to be one of the muscle memory mechanisms.
For muscles, at least, which are not single cells but actually syncytia with multiple nucleuses, weight-bearing exercise produces an increase in the number of nucleuses and thus a relatively permanent increase in the ability of the syncytia to synthesize protein.
So retraining tends to be about double or even triple the speed of the original adaptation. This is one additional reason you should be extremely skeptical about "before and after" photos
