Originally Posted by terry
The reason for that role is that your muscles need to be fresh and responsive on the "quality days" in your training week. If they're fresh, you'll be able to do practice repeats on those days -- no more than twice a week -- that imprint the coordination for a combination of Stroke Length and Rate that converts to very fast swimming. If your aerobic training between quality days is even a little bit too effortful, you won't be sufficiently recovered on quality day and your nervous system imprinting -- and your races -- will suffer.
Maybe I have heard this before but I never took it to heart. I will try to make this the focus of my training season.