Muscle Testing:

Muscle testing is an essential part of a thorough neurological examination.

Muscle strength in a particular area of the body (called a myotome) may help identify the level a nerve root is compromised. This gives us information about the level of the spine where a problem may be present.

During muscle testing, we are looking for muscle weakness of a particular muscle or group of muscles. Results may indicate problems with the spinal cord, nerve root, or inter-vertebral disc herniation pressing on the spinal nerve roots.

The way I explain this is to imagine a light bulb (the muscle)  a wire (the nerve) and a dimmer switch (your spine). The more the dimmer switch is turned down, the less electricity down the nerve (wire) and dimmer the light or the weaker the muscle. Most places focus on the light bulb never addressing the switch or the wire.