More About Applied Kinesiology...
Muscle-testing comes out of the larger field of Applied Kinesiology and is widely misused. In New Age circles it is often touted as the way to have the answers to everything. Literally. Of course, that's ridiculous. It is simply a way to allow the body and the subconscious mind to respond to ideas.
The theory goes like this: Any major muscle that is well isolated (meaning there are not too many other compound muscles simultaneously engaged), will respond to subtle energetic shifts that go off in the body. If a client holds an arm out forward, strengthening their deltoid muscle and a practitioner pushes down on the arm, the muscle will bounce back, resisting the pressure. But if an idea is presented that the client is incongruent with (such as having a male say he is a female) the muscle will respond differently. The incongruence of the idea causes a slight energetic shift in the body and instead of bouncing back the muscle will give way or go "weak." Because of this simple body response, muscle-testing can be used as a rudimentary tool for accessing stored trauma content in the client's system that is fueling psychological symptoms.
But here is the common New Age error: A Muscle-Test Is Not A Fact. It is a response of the subconscious mind. And the subconscious is quite fallible.
It is the subconscious mind that is responding when a muscle goes weak and the subconscious, while being a remarkable data center, is not a good judgment center. It does not remotely have "all divine wisdom." Data and wisdom are not the same thing. The subconscious operates out of very fear-based, almost infantile judgment and cannot be looked to as the source of all the answers. Its strength is that it catalogues one's life experiences like a hard-drive. Its data is exhaustive about one's own experiences, the emotions that got stored in the body from trauma and how they are fueling symptoms. But one must get through quite a bit of fear and resistance on the way.
With all of its weaknesses understood, it remains a useful tool in accessing subconscious trauma and relieving emotional symptoms. But the proof is always in the fruit of the work not in the muscle-test.