For a long while I have struggled using the correct grips for badminton. Using the correct grip my arm used to lock out, caused by what I thought was a shoulder injury/ problem. This locking out puts huge strain on my hand and shoulder and causes injuries so I use an incorrect grip to avoid this. I always assumed it was due to me having wrong technique somewhere else, other than a problem in my forearm. Today though one of my friends was watching me trying to use the correct grip and pronate and said it looked wrong. Basically I had my shoulder rotating in a very awkward manner to get the racket face to pronate as you see in say Jimmy Lin's video on youtube, Gollum's badminton bible videos or any pro player in slow mo also on youtube. He told me to try and just pronate using only my forearm and we discovered that compared to everyone else present the range of motion in my forearm through rotation was only about 140 degrees maximum and about 90 degrees comfortably. He seemed to be able to move his through a lot greater range. Does anyone know what could be causing this or a way of increasing the range of rotation in my forearm?