I'll have to dig.
But the gist is, at Capital Speedway WI, he was involved in a crash where the left front suspension/tire was pushed back into driver compartment and crushed his lower legs. He got pretty messed up. I think this was in the early- mid 80s.
There is now a bar that must be there to prevent this, called Mark Martin bar.
I have seen pics of it on FB, but a quick google search didn't show anything.