How can stiff and tight muscles result in back pain?

Due to strain and sprain on nerves of back

Explanation:

Stiffness and tightness leads to sprains and strains which are caused when the muscle remains inactive for a long period of time. Muscle at the back are tight and hence they constrict the nerves and its impulses. Nerve constriction leads to pain sensation. Also a muscle is required to be flexible enough to cause movement. Since the back muscle is tight , it put lot of strain on the spinal nerve which further causes back pain