I have tennis elbow and I have to say the only thing that cured it for me, was using lighter equipment.
I bought the 22 Xtreme pole and Extreme brushes. I also use 40m of Gardiners reinforced pole hose on the end of my reel, making everything lighter. I think I can honestly say that within a week it made a difference. Now (years in) I don't feel it at all.
I have to say that not everyone is a fan of Xtreme poles (they need a lot more TLC than the SLX) and some people feel that Extreme brushes are not up to the job.
But for me the whole setup works perfectly. Yes i've had a few spinning sections on the pole where the carbon has worn, but i've built it back up with 2 part Araldite and it works perfectly again. Yes, brushes don't last as long, but it's a business expense, it's not terrible!
I'm not sure if I would be cleaning windows at all without that equipment; and that makes me sad...I'd miss not meeting all the 'interesting' customers!