I've often thought how great muscle memory is as a musician. When learning a new track you will listen to it over and over - in some cases hundreds of times - before you feel fluent in the performance, not needing to think your way from one note to the next. There's a real sense of freedom when you can breeze through a once difficult part of a performance, later reflecting on how much repetition went into getting it to sound so "easy".
Sometimes I'll sit down at the keyboard and without any effort play through a piece I haven't thought of in years. No sheet music, no accompaniment, no recent listening - just the action stored in the brain... and it can sound as good as it ever did.
Earlier this week a friend of mine, learning piano, commented on wanting to learn the track "Army" by Ben Folds. In particular he mentioned the left hand arpeggios playing so quickly at times (check for example 1 minute 15 seconds into the video below).
Just today I was playing Dark Knight (below), a composition of mine well over 10 years old. The piece starts with a fairly complex left hand part, which continues regularly to keep the tempo well after the right hand melody joins in. Having not played the track in quite some time, this once tricky left hand line came out again with relative ease.