Having seen Dook's post 11 above I realise that my post 5 was somewhat misleading in that it may have implied that ejection injuries are common. They are not and I too know of many colleagues and friends who have had injury free ejections. What I should have said is that if an injury is sustained during a pre-meditated and other than high speed ejection it is more likely to be a compression fracture. Dook is absolutely right - posture is the key. However, age is also a factor. As the vertebrae discs lose their flexibilty, their ability to absorb a rapid acceleration reduces.
We members of the Martin-Baker club are eternally grateful for their life-saving seats. My apologies for my potentially misleading post. It would be interesting to know though who the oldest successful ejectee was and how he fared (I am assuming that the oldest will be a male).