All tissues are directed by the nervous system.
The brain controls all processes of the body through the nervous system.
The spinal nerves exit the spinal column and travel to every tissue of the body, controlling everything. Processes we think of as automatic are actively regulated, without our conscious control, by nerves. Imagine the nerve controlling your heart falling asleep like your foot. A Subluxation can put pressure on a nerve, and will cause abnormal function.
Muscles that attached to the back will stiffen up, or spasm, when the spine is injured.
The left side of the picture shows the large superficial muscles of the back. The right side shows the smaller muscles that will often spasm with a spinal injury, causing anything from a stiff neck to a sharp, constant pain. Adjusting the spine and correcting the Subluxation will cause the body to no longer feel the need to spasm, and the muscle will relax.