A little late to chime in but what type of rash is it? I get a rash from irritating while I surf, from the constant friction, is that it?
You can try putting under armour slider's underneath your panty shorts :keke: and see if that relieves anything. Try some firming lotion and baby powder to help clear it away.
You also might be executing your lower back incorrectly. When you sweep past your shins in the movement don't scrape your quads, if anything just brush them.