Line of action is independent of the excitation attribute on the fiber.
Setting pos sensitivity to 2 should do what you want. You can test this yourself with a scene like this:
If you change the pos sensitivity to 2 on the line of action, you'll at least see the fiber color change quicker. If you're not seeing any shape change as a result of the line of action auto firing, you may need to increase the fiber strength, or decrease your muscle stiffness.