I use muscle testing a lot! This post is intended to explain the basics of it. Obviously a muscle test does not take the place of a medical diagnosis, and if you need medical attention you should seek it. You are responsible for your own health.
That being said--let's talk about muscle testing!
The theory
The theory behind muscle testing is that your body is actually physically stronger when something is true or good for you, than when something is false or bad for you. This means that when your subconscious mind agrees with an idea, your muscles will be physically stronger than when your subconscious mind disagrees with an idea. So you can tell how your own subconscious mind reacts to an idea by testing your own muscular strength while asking yourself questions.
Getting started
Hydration is crucial for accurate results when it comes to muscle testing. So drink up! In my understanding, this is so important because water is a conductor, and muscle testing uses the body's electromagnetic field. Even slight dehydration, can tamper with accuracy here. Drink up!
Testing: The Body Sway
In this form of muscle testing, you stand and let your body sway forward or backward to indicate a yes or no response. I like this one best for accuracy because when you test with your fingers (as will be explained below), you can accidentally tighten your fingers such that it tampers with the accuracy of the test. With the body sway, though, you can be certain that if you're not consciously interfering with your body's motions, you're getting a good answer. To do the sway test:
Stand up. Close your eyes and say, out loud or in your head, "I am [your name]." You should feel your body almost independently move forward after a moment. Whichever direction you sway in indicates "yes." Then try thinking, "I am [a name that is obviously not your actual name]." Your body should lean the opposite way to indicate "no." For most people, the body will sway forward for "yes" and backward for "no."
If for some reason your body sways backwards for yes and forwards for no, or even from side to side during the control questions, it means that you are not properly polarized, and that is something I discuss in my post on muscle test troubleshooting.
Testing: Linked Fingers
I like this test because it can be done while sitting. It takes some practice to determine how strongly to hold your fingers; you want your fingers just loose enough that when something is a no, they easily pull apart. I recommend practicing on control questions (like your name) until you know exactly how tightly to hold your fingers when you test.
Here's how to do it: make the AOK symbol with both hands and link the loops of your index fingers and thumbs, so you're kind of making a Figure 8 with your fingers. Say "I am [your name]" (or think it) and pull your fingers away from each other. They should be strong; the loops should not break. Strong = yes.
Then try "I am [a name that is not your name]." Then pull your fingers apart again.
They should be noticeably weaker. Practice with this until you know how strong to keep your fingers so that when something is weak, the one hand that breaks lets the other hand go completely (so it's easy to know the answer right away!). Play with it!
Testing: Prying fingers
This one has the same rule as the last one: practice on control questions until you easily know the difference between a yes and a no answer.
Make an AOK symbol with one hand, using the ring finger instead of the index finger. Place your thumb and index finger from the other hand inside the loop of the AOK. When asking your control questions, spread your thumb and index finger to test the AOK loop. A yes should leave the loop intact, while a no should break the loop apart.
Questions beyond controls
After you've established that all is as it should be with your control questions, you can get started. I always start my testing by stating my name and getting an answer, and then asking "Am I sufficiently hydrated to get accurate results?" and "Am I grounded?"
If the answers to either of those is no, then I either drink up, or ground, depending. An easy way to ground is to imagine yourself growing roots down to the center of the Earth, and pushing all negative energy out of your body and into the Earth for cleansing via these roots. Another easy way to ground is to go on a walk outside. Keep grounding and testing until your body says it is grounded. Once it is, you can ask the real questions.
Also note that for some reason, leather interferes with accuracy and in some cases prevents muscle testing altogether. Always remove leather paraphernalia before testing.
Limitations of muscle testing
Muscle testing only works to uncover the truth as it is believed by your subconscious mind. In other words, your body does NOT know all truth. It will give you its opinions on whatever you ask it about, but that does not make it automatically correct.
The subconscious mind is not always right! For example, one thing I work with a lot is subconscious programming. You can test your body for any sort of belief, such as "I need to be fat" or "It is bad to have enough money to provide for my family." It is doubtful that anyone NEEDS to be fat, and I suspect many of us will agree that it is not wrong to have enough money to provide for one's family. But the subconscious can still believe this is the case (in which case, the program can be changed, if you work with someone trained to do that. I am trained to do that).
So if you're thinking you can muscle test to discover Absolute Truths, such as the veracity of a book of scripture or if there's a storm going on in Mexico or something, you will get an answer when you muscle test, but there are ZERO guarantees as to actual accuracy of the answer you get.
Your body is most accurate--you could say, only accurate--at providing answers to clearly stated yes-no questions that directly relate to your own body and your own subconscious mind (because the subconscious mind runs and regulates the body). Testing to know the outcome of future events will not work because of how many factors there are involved with the future, and the fact that the future is not yet set in stone. Even testing on how your own body will react to something in the future will lead to most likely inaccurate results, because your body will calculate based on how things currently are, which will change the next time you take a drink, go to sleep, or use a restroom.
You cannot accurately test for information that relates to other people (at least, not while you are plugged into your own energy). As a brief example, I was once muscle testing to discover if an issue was affecting one of my loved ones, and I kept getting conflicting answers. As soon as the person I was testing about gave me permission to muscle test on her behalf, when I plugged into her energy and muscle tested on myself as a proxy for her, I got an entirely different (and more accurate) answer to the question.
Basically, your subconscious will give you the answer it believes to be true, without any regard to actual truth. So you need to be careful of that.
Uses for muscle testing
One popular way to use muscle testing is to touch or think of particular foods you like to eat and muscle test on them. Strong or forward means they are good for you; weak or backwards means they are bad for you.
I often use muscle testing to uncover bad subconscious programming, like "I need to be poor" or "I don't deserve to be happy." Your subconscious does its best to make sure your programs come true, so if you have things like that running, your life will be sub-optimal! I like to identify these things with muscle testing and then test again after I've made changes to be sure they came through.
You can use muscle testing to identify trapped emotions, and any number of body-related issues.
You can test your character with muscle testing (for example, I recently started creating scales of attributes and testing myself to see where I stand with them--one scale is a faith scale, with 0 being no faith and 1000 being the Brother of Jared seeing the hand of God). You make your scale and then test, "I have more that 500 units of measure on this scale," "I have more than X units on this scale," until you find out how many units you have. One very faithy day, I realized that by the end of the day my faith measure had jumped by 5 units! I was so happy.
Conclusion
Muscle testing is a great way to identify subconscious beliefs about your body and mind. It is not accurate outside that field, but inside that field, it's great. It requires hydration, grounding, and practice, but it can be a skill worth developing.