I have seen several references to situps being harmful due to "high compressive lumbar load" and recommending crunches. When researching where the quote came from, it looks like every reference quotes the same paper by a Dr. Stuart M. McGill named Stability: from biomechanical concept to chiropractic practice. The author being a chiropractor pricked my ears since this is a profession which had sponsored beliefs ranging from "vertical subluxations" that can harm everything from backs to your brain to your liver to your chi (and, in the lab, any given pair of chiropractors will identify completely different "vertical subluxations" for the same patient). I don't know if Dr. McGill has a degree in any form of science-based medicine — his various biography pages are coy as to what he has other than a "PhD" — but I'm willing to give him the benefit of the doubt.
That said, has there been an supporting or opposing research on his claims? Is there any proof other than a 1999 paper?