Muscle testing works by a simple principle.
Your see, our bodies work by little electrical currents flowing through them. These currents are made by sodium and potassium ions swapping sides of our membranes. When these currents are strong, our muscles are strong; when these currents are weak, our muscles are weak. Fancy machines have measured and found that there is a difference in the electrical currents in strong muscles, weak muscles, and tired muscles.
Muscle testing uses this principle and explains why we can touch a reflex point of an organ system and test to see how well it is working. Also, each different organ has one specific muscle that relates to it from when you were being formed in your mother’s womb, when that organ and muscle pulled its nerve supply off of the same place in your developing nervous system.
And since everything unique thing is made up of its own unique atoms, everything has its own electromagnetic field. This is because the electrons that spin around the nucleus of each unique thing, those spinning electrons make a unique electromagnetic field.
If the electromagnetic field of something harmonizes with you, it makes you stronger. If that electromagnetic field doesn’t harmonize with you, it makes you weaker. This allows us to test people on a large variety of things to see how, and where they react to it.
This is the basis of our testing.
Research Validates Muscle Testing
There is “a close clinical association has been observed between specific muscle dysfunction and related organ or gland dysfunction.” 
Over 100 studies show there is “evidence for good reliability and validity” that is “not dependent upon examiner bias.”
“Studies show that the difference between weak muscles and strong muscles can be measured by their “neurologic electrical characteristics.” 89
There are double-blind studies where manual muscle testing has been found to be 95% accurate.,
12 randomized controlled trials (RCTs) that were reviewed show that MMT findings were not dependent upon examiner bias. 
 ICAK-USA, http://www.icakusa.com/content/faqs. Accessed 4/10/12.
Dr. Scott Cuthbert D.C., Dr. George J. Goodheart Jr D.C, Index Medicus Journal article on MMT and AK (.pdf / 316 kb). Accessed 4/10/12
Anne M. Jensen,1,2Richard J. Stevens,1,2 and Amanda J. Burls, BMC Complement Altern Med. 2016; 16: 492., Estimating the accuracy of muscle response testing: two randomised-order blinded studies, BMC Complement Altern Med. 2016; 16: 492, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5131520/, 11/30/16. Accessed 12/28/18.
Eddy Fan, Nancy D. Ciesla, et al, Inter-rater reliability of manual muscle strength testing in ICU survivors and simulated patients
Intensive Care Med. 2010 Jun; 36(6): 1038–1043.
Published online 2010 Mar 6. Accessed 3/13/19. doi: 10.1007/s00134-010-1796-6
 Research Supporting the Reliability of The Manual Muscle Test On the reliability and validity of manual muscle testing: a literature review, Cuthbert SC, Goodheart GJ Jr Chiropr Osteopat. 2007 Mar 6;15(1):4.