Over extending my arm when i pitch

Well i throw about 88-90 but i tend to stay way too tight and instead of a 90 degree angle when throwing my arm is fully extended. So pretty much when i throw my arms as stiff as a tree getting cut down, TIMBER.

Um tell me how this can effect my velocity and such.(Negatives)

If you can, post some video. When is your arm fully extended? When do you believe it should be at 90 deg.?