I do two exercises to stretch the tendons in my arms. First, stick your arm straight out in front of you and let your hand fall at the wrist. With the other hand, pull backwards slowly on your fingers. Your palm should be facing back at this point and you should feel the stretch on top of your arm between the elbow and wrist.
Second, again stick your arm straight out in front of you and pull your hand back. Your palm should be facing straight out like you are making a "stop" sign. Then with the other hand, pull grip your fingers and pull back. You will feel the stretch in the bottom part of your arm. Repeat each exercise for the other arm and you will feel some relief. it works great for me.