Ok, recently when I served, I had problems with my shoulder/arm with a Babolat PD cortex strung @ 60lbs with KlipperUSA Syngut 16 gauge. I was demoing a racket, an Aerostorm Tour, and that problem went away. It is strung with Babolat Xcel 16 gauge @ 60lbs. As soon as i played with my PD, the problem came back. So was it the different strings or the weight of the racket that solved the problem?