One of the main things we do in physical therapy is muscle testing. Manual muscle testing (MMT), where PTs can test your muscle strength in almost every muscle you have. It is so amazing how we can apply a certain amount of force in a specific position to determine whether or not your muscle is where it is suppose to be. There are certain muscle grading charts that grade your muscle strength, like a key chart. The therapist, based on their own knowledge, determines what your muscle grade is.
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This chart is what I use when testing muscle strength, there are several charts you can find but I find this one the most helpful for myself. The most objective (most reliable) grade is less than a 3. You can easily spot a 3- if the subject cannot hold a muscle position, for example, to test the middle deltoid you have the subject abduct their arm out to the side, if you put the subject in this position and the subject cannot actively hold it, their muscle grade is is less than a 3.
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If the subject can hold the position then the therapist can apply a fair amount of force and have the subject resist, 5/5 is normal muscle strength, so in this case with the middle deltoid, the therapist applies a force down on the arm, subject resists adduction and therapist judges what the grade it is.
This is just a very brief overview of MMT, if there is any questions or concerns just let me know, I will be happy to reply.
