as far as i know none of strong lower freestylers have used them
very often people ask if they should go to the gym and work out legs or use a treadmill to get more power and stamina
as oLi said - "if you want to be good in freestyle - do freestyle", but not waste your time on anything besides that
weights can result in serious back injuries - so i would say - searching for easy ways is not a good idea, good thing to do it practice