If you visit a practitioner who does muscle testing to determine if dysfunction exists in your body you may have noticed they use some form of "II" (parallel lines) or "X" to assess the state of your muscles. This is because they are trying to determine if your body or specific muscles are Normotonic, Hypertonic or Inhibited.
Normotonic simple means your muscles are strong when they need to be strong and weak when they should be weak. Hypertonic refers to muscles that are always strong regardless of whether they should be relaxed or not. Finally inhibited is like a muscle that had the power turned off and can’t be found, which comes in the form of the muscle testing weak.
The "ll" and "X" can be used to determine the state of the muscle by having the client look at the shape/cards while testing. When not looking at the cards your muscle action should facilitate (be strong), when looking at the "X" you should also facilitate, and when looking at the "II" your muscle being tested should inhibit (go weak). Anything other than this means a muscle is not normotonic and has some kind of dysfunction being fed into it.
This was originally discovered by Dr. Jose Palomar in 1995 and a paper was published in an applied kinesiology article in 2000. Dr Palomar found an article related to car accidents in the USA being related to parallel lines on the road. Police reports showed that 2 solid parallel lines almost doubled the amount of accidents. As soon as the road changed to broken lines the number of accidents severely decreased. Dr. Palomar thought about how that could be related to the Central Nervous System and started testing his theory on patients. He discovered "ll" would cause muscle inhibition and an "X" should create a state of facilitation.
This theory is summarized as the following. When we look at the parallel lines it utilizes our visual system along with the vestibular and proprioception systems. Unlike the "X", the "II" do not have a point of fixation, this leads to the central nervous system receiving visual input that there is “some kind of a movement”. At the same time the vestibular system reports there is no movement and the body is remaining static. As a result the brain receives contradictory information and to protect itself initiations a global stretch reflex to inhibit muscles.
When a patient looks at an X there is a point of fixation for the eyes the central nervous system perceives this as normal and does not cause a global stretch reflex. This is why the proper response for the "X" should be muscular facilitation.
The "ll" can also be observed by looking at an optical illusion below. The brain activates a stretch reflex.
Fun Fact: Try using this optical illusion instead of the "ll" and you'll see it works just as well!