Not sure about the latest ejector seats but back injuries used to be quite common when ejecting.
Most RAF cases get helicoptered to Nottingham for a spinal check up - there isn't much you can do about the instantaneous G-forces when leaving the aircraft. However, as more accidents happen we learn more about what causes the injuries, for instance in a recent case a pilot re-tightened his straps just before he pulled the handle - they had already been tightened prior to take off, so in fact all he did was bend his spine, leading to a serious injury. Seat design is improving (parachute deployment, stabilising deflectors, "intelligent" rocket control to alleviate tumbling etc)...
There's a good article on the MB seat testing and the company's Meteors in the current FlyPast...
