Local causes: Excessive pressure or strain on the back may result in strains or sprains. A ligament stretched beyond its limit will send pain signals. A muscle given either too difficult a job or one that body is not accustomed to will respond with severe tearing pain. It and nearby muscles may go into a temporary, protective contraction (aka spasm) - to discourage continued use. The pain may either make you tense, which will tighten your muscles and cause more pain or if the strain does persist, it may order the muscles to stay in spasm so that your movement will continue to be restricted.
Discs also may be damaged, they can either degenerate and collapse, or rupture and often compress a nerve (aka slipped disc).
Back pain due to other illness
Backache may be a symptom of other serious illness such as
You must inform your doctor about any backache that limits your activity and lasts for more than two or three days.
Emotional problems often contribute to the development of backache. This seems particularly true of a type of low back pain that is accompanied by stiffness, difficulty in bending and moving, and shooting pains in the buttocks and legs.
Backache is widely considered as a symptom of overburdened people. A muscle spasm may be the result of depression or anxiety. Suspect depression as an underlying cause if your backache is accompanied by constipation, sleeping difficulties, loss of appetite or weight and loss of interest in sex. Other clues to depression may also be present. If emotional problems are contributing to backache, psychotherapy and medication are often beneficial.