As you probably know, when we use our muscles we don't use 100% of their power. There are neural pathways that block the ability of the muscle to pass a certain threashold (you might notice this if you're forcing someone's arm and suddenly they stop resisting).

There are situations when this is overridden. Normally it falls under the "fight or flight" - where people have lifted massive weights. Here the body (somehow - don't ask me) turns the safeties off as failing to give 100% today will mean there isn't a tomorrow. Usually severe suppression of pain goes along with this. It's the same in sport when you notice the massive bruise on yourself after the game, but barely can remember when the injury occurred as you were so hyped up.

In a dyed in the wool junkie like that, something similar is probably responsible. Her muscles are working at a level of intensity that is likely to cause major damage to tendon, bone and muscle, but her brain is so messed up it doesn't care.