I wonder how common are back or spine problems amongst dedicated high jumpers these days? E.g. chronic issues after the end of the career.

As compared to Fosbury flop, the older "straddle technique" must have been much easier on the body, or not?

Having "high jumper's genes", I nearly crossed 1.90 m in high school by simply mimicking the Fosbury style as seen on TV -- or, rather, simply by using brute force. I do, however, still remember that this jumping session caused serious back pain afterwards. I was also completely exhausted. An interesting experience, turns out you do use a surprising, immense amount of explosive power while jumping. In particular when you don't know how to jump properly. :)