I had a tennis clinic today and one of my coaches said that I will probably get back problems from my toss because I begin tossing the ball while my weight is on my heels and and then I release the ball as my weight is moving forward. He also doesn't like me throwing the ball at around 11 o'clock for the kick serve since one of my school's captains has back pains from throwing the ball at around 11 o'clock. He says that the captain started to do this at around 15 (my age) and is 18 now with that back problem. He recommends I stop these habits. I know that kick serves do have a tendency to cause back problems, but I don't know how to change the service motion because it feels natural and another coach does say it looks graceful and smooth.