I'll bet you £1000 those 9k dumbells are too heavy for your rear delt flyes.
They are, definitely. I'm using limited range of motion on the first set. This is courtesy of John Meadows's Rear delt destroyer protocol. I do them more of a swing style and with limited range of motion, then the lower weights are with proper ROM.
Check it out, I really like the idea. I get lots of reps, a good burn and it only takes one short drop set.http://www.t-nation.com/free_online_art ... r_sets&cr=
4-5kg would be most optimal for proper reps.