Applied Kinesiology (AK) is a diagnostic of which it's focus is to identify sources of interference to the bodily structure, biochemistry or emotion, and to direct the most specific change as possible.
Your nervous system monitors and coordinates all of the bodies interactions. AK uses muscle testing to asses the nervous system and locate where the source of a problem may be. The body is always speaking to us with balance or imbalance within the nervous system and 'tapping' into that gives direction for the chiropractic adjustic thrust to be as specific and as effective as it can be.
AK was developed by George Goodheart, a chiropractor who began to write and lecture about his ideas in 1964. He associated weak muscle tests with specific changes in the body and chiropractic techniques that brought those muscles back to strength. From then, the use of the muscle testing has spread to being used with changes in the body wether it physical, chemical or emotional.