I do recall having a gymnast years ago who had had a stress fracture in her back at 6. i can easily say that it had nothing to do with gymnastics. Her mother told us about it when she started gymnastics. Hindsight says that the gym probably wasn't the best place for her. However, she was only doing rec gym and there was very little that we did at that time to cause excessive extension of the spine. I would tend to agree though that this type of injury is very uncommon. In over 15 years of coaching she is the only one that I have seen.