Although it's generally accepted that stiff rackets = arm pain, I don't buy into it completely. In my experience I have only ever experienced arm pain when using a flexy racket (Storm series, nBlades, PK Ki5, and demo-ed many, many oether so called arm freindly rackets). I know this bucks the belief trend, but I have always been comfortable using the pure drive standard (over the past 10 years). Me-I'm 48, and play 4 times a week.
It really comes down to a personal preference, and what feels right, is usually right......sure I do occasionally get the odd shoulder twimge, but thankfully it's on rare occasions, and probably from over use, cramming in too many sets, in a short period of time, and perhaps trying to "heave" ace's too much for free points.
I've sold my so called arm freindly rackets, which I really felt played havoc with my arm, especially when you keep mixing up the rackets you play with. All that to-ing and Fro-ing is recipe for arm trouble.