MJ said in one of the interviews that whenever the music started playing, you couldn't tie him down. He had to move. He couldn't help but dance. Even with arthritis and inflamed lungs, he had to move when he heard music. You saw him in the film say "I'm not gonna go full out here" but you can tell he's getting so into the moment that he just does it, then afterwards he says "I shouldn't be singing out, I'm trying to warm up my voice for this moment". He couldn't help himself. It just happens when he hears the music. It's like the music takes over his body, and Michael Jackson the man, turns into Michael Jackson the performer, and then after the song's over, Michael Jackson the man realises that Michael Jackson the performer has just gone and sung full out when Michael Jackson the man knows he shouldn't be singing full out.
It's the same with dancing. Michael Jackson the man could say, "well my back is killing me, I'm tired, I'm not going to go crazy dancing today". Fine. Up on the stage, music starts and BAM! Michael Jackson the performer is back. Popping and locking, side glides, running around, kicking his legs up in the air... Michael Jackson the man comes back to his body and says "I thought I told myself I wasn't going to dance. What was that?! What WAS THAT?!" Sorry to be so confusing lol, but that's just how I have always seen it. Michael Jackson the man can say he's going to hold back, he can say he's in pain, he can say he's not going to sing full out, but Michael Jackson the performer doesn't care. Michael Jackson the performer is going to do what he wants and Michael Jackson the man is going to have to deal with it. You've heard people say Michael was a different person on stage? It's the truth.
Even though Michael was in pain, and I'm fairly sure he was, when he was on stage, something came over him. None of it mattered anymore. It's after the show is over, and Michael Jackson the man lands back in his body, that's when he'd feel it. That's when he'd say "I shouldn't have done that. Now I'm in pain". And he'd say to himself, "Next time, I'm going to take it easy". Gets back on that stage, and does the same thing as before. He couldn't help it. It just happened. He naturally moved to music. It was instinctive.
Arthritis or not, once Michael got on that stage, you couldn't stop him. I know it doesn't make any logical sense. I know what you're saying, technically, with arthritis in his back, he shouldn't be able to move like that. The coroner must be wrong. He probably did have arthritis, but like I said before, he couldn't help but move when he heard music. It just happened.
^ Straight from the horse's mouth.