[shemp]

I don't know what the "literature" answer is, and am not going to search-- but this is what I've come to understand: HIIT doesn't mean sprints. It's INTERVALS of ANYTHING at an effort level that can't be sustained for more than say, a minute. The work-to-rest ratio you should typically aim for is 1:1 or 1:2-- although, "Tabata Intervals", which are apparently magic, have a ratio at 2:1.

edit: And another thing I want to add. I don't know about sprinting more than once a week-- even if that is the only HIIT you do.