Appears there is no such thing as muscle memory. There's myelination. Your muscles can't remember, but by building these sheaths around specific neural pathways, it makes it easier for your brain to send directions that can be quickly and efficiently processed. Musicians are practicing the most intricate of finger movements very specifically over and over. It's also useful in learning languages though I haven't read much on that. It's a better understanding of "muscle memory", A more focused approach to building it. Kevin