I was thinking more pain in the lungs and the burn in the legs and buttocks, not joint paint or an impending heart attack. One would hope that anyone would just the tiniest bit of gumption would recognise the difference.
Joint pain is obvious, yes. Cardio, not so much.
I won't go into it too much (he says, going into it too much) but I had a wakeup call on one of my first rides back after a year off with injury.
A two hour slightly hilly ride, nothing too extreme. Felt slightly winded and tight-chested but otherwise as good as you'd expect for one coming back from injury and significantly heavier than pre-injury.
None of the normal sensations telling me to back off. No lactic acid, burning muscles or gasping, except the usual sensations on steeper climbs. I put being winded and tight-chested down to being unfit and my clothes being too tight.
Looking at the numbers after the ride showed I was in Z5/Z6 for the entire 2 hours. My average HR was 181, max was 196. (For comparison, my pre-injury numbers were ~145 avg/160 max, if fresh, and lower if tired, and my theoretical max HR was 182).
Cardiac stress test in the hospital showed that my heart was ok, but that I was redlining very easily and pushing my heart far harder than I realised.
I now watch my HR like a hawk when out on the bike rather than relying on feel. (And I'm never taking health for granted again.)