In the manual therapy video below, NAIOMT faculty member Stacy Soappman explains why it's useful to have three different muscle testing approaches for the lumbar spine. Essentially, each has a different intent:
For neurological conductivity, fatiguing versus consistent weakness.
Looking at the power position, and giving it a manual muscle test grade to document in chart to use an objective measure to show how they've improved and gained strength over the course of physical therapy.
To pick up minor tissue damage, if looking for grade one strain.