Understanding the causes of back pain requires understanding how the spine works. Most of us think of our spine as a rigid, curved structure that supports the weight of our body - like a coat rack, which is entirely wrong.
The spine is made of individual bones called vertebrae, seven in the neck, twelve in the thoracic, and five in the lumbar region. These twenty-four vertebrae are linked together by ligaments and muscles. If the spine were removed from the body, it would flop over like a wet noodle. Surprised?
The strength required to hold the body upright when standing results from muscles acting on and the ligaments limiting motion between vertebral bones. The body's spine muscles perform a delicate balancing act of keeping the spine adequately positioned automatically as we lift boxes off the floor, bend to touch our toes, or run down the street. Each activity requires very different spine configurations, so the spine constantly changes shape. The spine is not like a coat rack, it is a dynamic structure that changes shape as you move.
The muscles pull one vertebra against another to turn and tilt the spine. When this balancing act is disturbed by injury, tumors, or infection, muscles are forced to work "overtime" to configure the backbone to not only be ready for your next move but to do it in a way that minimizes pain from its damaged parts, Overworked muscles can spasm which is a significant source of back pain (Pain Source #1 - Muscle Spasm).
The other primary source of back pain is the joints connecting each vertebra to the next. Five joints connect each pair of vertebrae, 24 x 5 = 120+ joints - damage in any of these joints can cause pain. The vertebral disc is a particular type of joint called a shock absorber joint" because instead of containing only fluid, it has a soft gelatinous material called the intervertebral disc. Arthritis is a wastebasket term that means "inflammation of a joint." It can occur without injury due to aging or problems with the immune system, which causes the body to attack its tissues. Arthritis is present in the spines of most people over sixty, even if they have never had trauma, infection, or cancer of their spine. (Pain Source #2 - Arthritic joints).
Ligaments are bands of muscular tissue that connect the vertebral bones and limit their motion relative to each other. For example, some ligaments help form a joint capsule and keep the two bones of the joint connected. The ligaments are richly innervated with pain nerve fibers and are a third potential source of pain when injured (Pain Source #3 - Ligamentous sprain and strain; joint capsule damage).
The fourth source of back pain is from the vertebral bones themselves. The bone itself has little or no sensation. But all bones, including vertebrae, are covered by a thin layer of tissue called periosteum which is highly innervated. Damage to the periosteum when a bone is fractured, contused, infected, or invaded by cancer is excruciating (Pain Source #4 - Injury to bone).
The fifth source of pain is Inflammation. The NIH describes inflammation as "Inflammation is a normal part of the body's defense to injury or infection, and, in this way, it is beneficial. But Inflammation is damaging when it occurs in healthy tissues or lasts too long. Known as chronic Inflammation, it may persist for months or years. When spine anatomy is damaged, Inflammation can follow, and Inflammation causes pain. This is complex and involves special cells and chemical mediators." (Pain Source #5 - Inflammation). Motrin, aka Advil, Aka ibuprofen, is a medication that reduces Inflammation. It is one of a class of drugs referred to as NSAIDS (Non-steroidal Anti-inflammatory Drugs).
The sixth source of pain is pain itself. Pain causes muscles to spasm as part of a protective mechanism that can cause muscles to contract maximally to stabilize the spine. With the onset of severe back pain from whatever cause, pain fibers are connected in the spinal cord to circuits that activate muscles in that region of the spine. This automatic activation of para-spinal muscles can result in muscle spasms, causing a vicious cycle of increasing pain. For example, patients with back pain often have palpable back spasms - like a "Charlie Horse" of the spine. Disrupting this spasm with deep massage can reduce the spasm and minimize back pain. (Pain Source #6 - Pain-induced muscle spasm).
The seventh source of pain is compression of nerve fibers by disc herniations, abnormal bony growth associated with Arthritis, infection, and tumors. Sciatica is sharp shooting pain that begins in the back and shoots down one or both legs. For example, a patient with a disc herniation compressing a nerve root in their spine can have sharp shooting pain down their leg resulting from nerve root compression. This is commonly known, and so it's last on our list. (Pain Source #7 - Nerve root compression).
For example. Suppose Maria slips and falls on a wet floor onto her buttocks. The fall causes one vertebra to fracture, multiple vertebrae to become bruised, causes spine joints to inflame, and causes ligaments and muscle tissue to sprain and strain. When Maria tries to walk after her fall, she finds unbearable back and left leg pain. Her back becomes palpably hard with spasms - in other words, if you were to push your fingers into her low back, you would encounter rock-like muscles in spasm. She stops to rest, and pain slowly decreases as muscle spasms subside. She starts physical therapy and Motrin, but the pain persists month after month. Within six months, all seven pain generators are active in this patient: #1 - Muscle Spasm, #2 - Arthritic joints, #3 - Ligamentous sprain and strain, aka joint capsule damage, #4 - Injury to the bone, #5 - Inflammation, #6 - Pain-induced muscle spasm, #7 - Nerve root compression.