I think all force is coercion, but not all coercion necessarily involves force.
A lot of coercion uses deceit or manipulation. Some force is justifiable, such as in self defence (it is not "initiated force" in this case), but what I call "coercion" is never justified.
I don't include self-defensive actions as coercion (by definition), even if you are able to "talk" a thug out of attacking you. He initiated the force- he is the aggressor- so your actions are strictly self-defense.
For me, coercion is purely the initiatory act of forcing or causing someone to act in a way that is against their legitimate will.