for your joints and tendons.... or is the more important issue off-court strengthening and proper stretching? I realize that polys are harsh... and send 'bad frequencies' through the body (if that concept even exists)... but I can't help thinking that most players (especially recreational) don't focus on exercise off the court and stretching properly before and after play. BTW- Not a big poly guy... really hate most of them... I play/ed with mainly multis/synguts... but the polys I do enjoy are very soft ones... such as BBAce, SPPP and hybrids such as M2Pro. Really would like to try some more of the softer ones that are coming out nowadays. I used to be an exercise 'nut' (before I got back into tennis and was younger) and try to exercise regularly now (time permitting) ... but I always take 5-10min. to stretch before and after play and have never had an issue with my shoulder (aside from using a super-stiff 'tweener racquet when I got back into tennis... which was strung with a multi @ mid range)... needless to say I put that stick down immediately.