Here's a great article on Applied Kinesiology, courtesy of Dr. Brian Butler in the U.K.:

How it all began

In 1964, Dr. George Goodheart, D.C., a chiropractor stumbled upon a remarkable discovery. Using some standard kinesiological muscle tests to analyse a patient's postural problems, he noted one muscle was particularly tight, and that other muscles opposing it tested weak. The man had suffered from long-term back pain which had not resolved after much treatment.

Traditionally, body workers strive to reduce tension in tight muscles with stretching exercises and massage. Also, structural manipulation can often help relieve muscular tension.

During the treatment, Dr. Goodheart massaged the side of the man's leg. Suddenly the man exclaimed that the pain had gone. Puzzled by this phenomenon, the chiropractor rechecked the muscles. He found that the weak muscle had somehow strengthened. He also noted that the excessive tension in the opposing muscles had virtually gone, which had allowed the man's postural imbalance to correct, and the pain went away.

Breakthrough in "Applied" Muscle Testing

This breakthrough was to become a vital factor in the development of a new art/science called Applied Kinesiology. Dr. Goodheart, with a genius for research, started to investigate this phenomenon. He recalled seeing previously in a journal, the work of Chapman, an osteopath who had mapped some reflexes with points on a chart of the body which he considered stimulated lymphatic flow.

With painstaking attention to detail, Dr. Goodheart checked each of Chapman's reflexes by testing the response of various muscles. He found that each muscle group was related to one or more of these points, by noting which weak muscles became stronger when the appropriate reflexes were activated with firm massage. This helped to explain what had happened when the man's pain was relieved.

He invited some of his colleagues to check his findings, and test out his theory on some of their patients. They found that they obtained similar results with weak and tight muscles when massaging Chapman's reflexes.

Eclectic Research Embraces All Realms

Over the next several years, the muscle testing procedures of kinesiology were applied to check other aspects of physiology, notably circulation, nutrition and acupuncture energies.

Some points were discovered by a naturopath called Bennett, who used a fluoroscope in his investigations. Regrettably he succumbed to cancer as a result of the excessive radiation, but not before noting that when touched, these reflexes appeared to improve blood flow to the organs.

So now, it was possible to be very specific when strengthening muscles. Chapman's reflexes, which became known as "Neuro-Lymphatic" reflexes enhanced lymph flow, and Bennett's reflexes improved blood circulation. Both were found to have a strengthening effect on specific weak muscles.

It was not long before muscle testing was being used to check some aspects of nutrition. Foods were placed in the mouth, and chewed, and any change in muscle strength was noted. Some nutrients were found to weaken some muscles, but not others. Other foods conversely were found to strengthen muscles on some subjects, but weaken them on other people. This led to some tentative conclusions about using muscle testing to "read" the reaction of the body to foods to which a person might be sensitive.

Western orthodox approaches have been reluctant to consider acupuncture theories of Meridian life force energy. Car mechanics know that no service is complete without checking the electronics and ignition of a car, however good the mechanical work! Dr. Goodheart was intrigued to find if there was a connection between the electrical organ meridians of acupuncture and the muscles.

This led to another very important new discovery of an intimate and vital link between particular organs and muscle groups. He found that those muscles that share similar nerve pathways through the vertebral foramina, were strengthened by the same organ meridians.

It has long been observed that mental and emotional feelings affect posture. This led to yet another line of research to find exactly which muscles and organs were affected by different forms of emotional stress.

Holistic treatment made simpler

This new tool of AK muscle testing in relation to function is revolutionary. It is a new way to communicate with the body/mind. It is a biofeedback "language" which the body understands, and can respond to. It makes it possible to use the client's own body as the research tool to find out what is needed. Being able to find imbalances and sub-clinical conditions through muscle testing helps remove the doubt, the guesswork and the hard work of subjective diagnostics.

So now Applied Kinesiology (AK) muscle testing analysis and balancing may be used holistically to address the whole person. AK can be used both as a therapeutic modality, and also, and even more importantly in the opinion of many, as a method of preventive medicine of the highest order.

This preventive potential of AK is due to the fact that muscle testing can pick up sub-clinical imbalances which are imposing a strain on the persona, which if ignored, may well accumulate and eventually cause serious problems if not resolved.

The body of information now known as Applied Kinesiology has been developed by physicians. The techniques have been clinically tested for reproducibility. Those who have invested the time and the money to learn the art/science of muscle testing properly, have a tool that is without peer in enabling unbiased and non-intrusive assessments to be made of almost any aspect of life's functions and dysfunctions with the exception of disease.

After hundreds of hours of practice in classroom workshops and in clinical practice, Kinesiologists have learned to avoid mentally interfering with the test. They have kept their own opinions out of the equation as much as is possible. They have refined their ability to "listen" with great attention to the response of the muscle fibres being properly and sensitively tested. They also know how to help the client keep an open mind as to the result of any test.

They study carefully the multiple tests necessary accurately to analyse the complexity of the imbalances the client may have presented with. They use patient detective work to assess cause and effect. They investigate carefully, and allow the client's muscle responses to reveal the answers to them which will show how their health problems may best be addressed. They use their understanding of the binary logic of the bodies' many computers to help them in their search for primary lesions.