Typically I'd have an authoritative & playful tone if it was something like that,
"Hey, just a little tip, you'll be less prone to injury if you keep your back still; I'd hate to see your hard work go to nothing!"
Typically it's hard to correct people out of the blue if you don't really know them. It also causes them to lose face, as you are pointing out a flaw/something wrong and telling them to do it better. So, you may want to rebound the negative with a positive, like in the example above. You're giving her the tip/correction, but not completely laying into her.
For pick-up reasons, I suggest not opening her with this. Do it later on, when you're going to the gym the morning after an amazing night in bed

. Then she'll listen to you and probably be a lot more grateful.