Students and therapists often get a fixed idea of a
muscle's functional joint action(s). We look up the muscle
we want to learn about in our muscle atlas textbook,
and we read that it either does or does not do a certain
joint action. And we're done, right? Not necessarily.
Muscle function is actually much more fluid than this. After
all, ultimately, a muscle's joint action is based on its line of pull
relative to the joint it crosses. And this relative line of pull can
change if the position of the bones of the joint changes. To
understand this better, let's look at a few examples in the body,
beginning with a muscle that could be called a de-rotator.
