How do I filter lookup field's values based on another value

How do I filter lookup field's values based on another value

I have two forms.

Here is my layout:

FormA:
Exercise -> Single Line
Muscle_Group -> Drop Down List

FormB:
Workout_Muscle_Group -> Drop Down List with same values as FormA.Muscle_Group
Exercise_One -> Lookup based on FormA.Exercise values
Exercise_Two -> Lookup based on FormA.Exercise values

Now, how can I filter the lookup values in FormB.Exercise_One and FormB.Exercise_Two so that FormA.Muscle_Group equals FormB.Workout_Muscle_Group?

Does that make sense?
Thanks!
Alan