My son has been playing since 9 years old and I have been using MSV Focus Hex 1.18 in a full poly string job. He has been fine all the way until I changed over to Kirchbaum Pro Line 2 with 1.25 guage at 56 lbs. Then again, I also switch him over to a different racquet at the same time.
The racquet that he was using and is currently using is the Babolat Pure Control Team. I bought him the Babolat Pure Storm Team GT to try out and that is when he felt pain in his shoulder and arm. I switched him back to the Pure Control Team and now he is using the MSV Focus Hex 1.18 in the mains at 50 lbs and the MSV Co-Focus 1.18 in the cross at 48 lbs.
He is a good player and is currently around the top 20 in Boy's 14 in NorCal and is ranked around 290 in the Nation.