[Palaaemon]

In answer to your question about the fragile title appearing out of seemingly nowhere, it did. LoL. The injury process is dynamic. I believe it goes like this. Not going too detailed.

1. The injury happens and is assigned (type and number). So say arm and value 21.
2. Possibly a part of that value is added to, or all of the new arm injury value (along with the others) overwrites the Injury Proneness (IP) value.
3. The IP value is recalculated.
4. It's checked to see if the IP value level exceeds the next threshold for the next injury title. If not nothing happens. If it does than change title.

I am not positive about how exactly they do it, but I am fairly confident in #4.

Each injury type (arm, leg, back) has a rating between 1-200 with 200 being the worst. There is also IP rating which encompasses all of the types and also has the same 1-200 range.

So the Injury Proneness rating is where the titles come from that you see.
Iron Man - around a 1 rating
Durable -
Normal -
Fragile -
Wrecked -

I used to have the level numbers memorized but I seem to have misplaced those neurons. Hope this all helps you understand this better.