The book, The Power of Infinite Love & Gratitude by Dr. Darren Weissman, describes muscle testing this way: "The body has an innate intelligence that can be assessed with this method to determine whether it's maintaining balance. Just like the autonomic nervous system, muscle testing is a polarity-dependent mechanism where a muscle will maintain its strength as long as there's a congruency with the function, adaptablility, and the survival of the entire organism. When there's incongruency, an indicator muscle will test weak. When a muscle locks out, it's an indication that whatever is being evaluated is congruent with the overall balance of the body. When the muscle gives way, it's a sign that what's being tested is incongruent with this balance." , is not scientific, Muscle Testing can be a great tool to help you gauge whether something weakens or strengthens you, and can give you an indication as to how well tolerated it will be by your system. This technique is based on the assumption that everything has an energy, or vibration, and that when holding it within your field of energy, Muscle Testing helps you to gauge your body’s reaction to it.
The key to successful muscle testing is to have an open mind. It is also important to try to do the testing in a neutral environment with no distractions such as music, scents, etc. Drinking water beforehand may be helpful in optimizing brain function. Diaphramatic breathing can help to oxygenate the system. For additional information and tips check out The Heart & Soul of EFT and Beyond by Phillip and Jane Mountrose. Although the why's and how's are not completely understood or known, the energy system of our body is a useful guide that can enable a better understanding of our compatibility with other energies. Other energies are either compatible (make you stronger) or not compatible (make you weaker). Muscle testing is a useful tool that can help you identify that which weakens you, and therefore what you should avoid, so you will be able to limit exposure to things that may be creating undue burden on your body.