Nerves travel from the spinal cord, through openings in the vertebre of the spine, and then out to muscles and organs. When a nerve passes through a muscle it can become impinged as the muscle goes into spasm. This will cause you to have pain, numbness, tingling &/or weakness in any of the muscles that are innervated by this nerve.
For example, a spasm in the scalenes can cause pain to be felt in the upper back, chest, across the shoulders, down the arm and into the forearm (see image below). A common burning pain that is felt in the center of your back, along the shoulder blade, may actually be caused by a spasm in your neck. You can rub your back all day, but until you treat the spasm in your neck you will never get rid of the pain.
Nerves also pass alongside muscles. For example, the median nerve, which gives feeling to the hand, runs between strong muscles in the forearm. If one of these muscles shortens from repetitive strain, you will feel pain and numbness in your hand and wrist. You will rub your hand, shake your hand, and put all your focus of attention onto your hand and wrist, but the source of the numbness is in your forearm – or even as far away as your neck.
This same situation happens all over the body, it’s called reflective pain, which means that pressure or damage to a nerve will refer pain to a different area, usually where the nerve ends.