Why are you trying to use ISO 50 for fast movement? As modern films go, 50 is a rather low speed. If you're shooting slide film, you could always switch to Provia 400x and keep much of the same look while allowing for much lower power out of your strobes.
I am assuming here that you're using monolights. If so, shut off the modeling lamps and set your shutter to its max speed as the RZ is a leaf-shutter camera and syncs at all speeds.
I hope you sprung for the high power monolights, as that's what is going to provide the shortest duration. Since it's a woman's arm that's blurring, and not a bullet, you should be able to get away with ordinary equipment and still stop the movement.
Posting a photo is more valuable to a forum than 1000 words.