Muscle testing is a simple, non-invasive assessment tool used by some health practitioners during evaluation to assess many different things such as the general health status, specific supplement dosage, or allergies. During testing, the patient is exposed to the substances being tested or certain key words are used to check the autonomic nervous response. The theory is that things that are good for you trigger a positive autonomic response, thus causing the muscle group being tested to stay strong. Things that have a negative effect on the body trigger instant muscle weakness.