When you are using the Muscle Spline Deformer you can create custom muscle shapes. These extra shapes will be triggered when the joint moves since they are triggered by the length of the spline.
We can also use these customs shapes differently by trigerring them manually or with a set driven key here is how.
Bring up a file that has this set up (if you don't have any file you can download the tutorial files from here (DragonLeg_Sliding_Start.mb in Lesson 5 of the Simple Muscle)
Select the muscle and create a few custom shapes (check the manual for in-depth workflow )
Create a locator, this will be our driver.
Open the Connection Editor and load the locator in the Outputs side and the cMuscleSplineDeformer in the Inputs side
We connect the locator.translateX to the cMuscleSplineDeformer.shapeData.shapeData.shapeLength
and Voila! When moving the locator in X the first shape will come on and off. We can also do the same set up for set driven keys.