This is a short description from Dr. Bradley Nelson on muscle testing.
Muscle testing is a noninvasive tool we can use to identify imbalances in the body. The science behind it is similar to a lie detector test or a polygraph. In a polygraph, a person is hooked up to sensitive electrodes and asked certain questions. The person's answers generate a certain electrical response in the body, which is read and graphed by the machine, showing if the answers are true or false. This is done on a very delicate, minute scale. The muscles of the body are also affected by this electrical state- so we can use them to show this same response, just on a more pronounced, larger scale. Any muscle will become weakened in a state of incongruence or falsehood, making resistance difficult. Conversely, muscles will remain strong and able to resist when in a state of congruency or truth. What's nice is that muscle testing allows us to access important information about health, we just have to know what questions to ask, then apply the resistance to a muscle and get our answers!