Joint pain stems from dozens of different conditions, but most cases trace back to a handful of root causes: cartilage wearing down, the immune system attacking joint tissue, crystals forming inside the joint, infection, injury, or the soft tissues around a joint becoming inflamed. Nearly 19% of U.S. adults have diagnosed arthritis, and that number climbs to over 50% in people 75 and older. Understanding which mechanism is behind your pain matters because the causes behave very differently and call for very different responses.
Cartilage Breakdown in Osteoarthritis
The most common cause of chronic joint pain is osteoarthritis, where the cartilage cushioning the ends of your bones gradually breaks down. This isn’t just “wear and tear” from aging. It’s an active biological process driven by specific enzymes your body produces in excess.
The destruction happens in two stages. First, enzymes called ADAMTS-4 and ADAMTS-5 attack aggrecan, a molecule that gives cartilage its ability to absorb shock and resist compression. Losing aggrecan is like losing the padding inside a shoe. This stage is potentially reversible because the body can rebuild aggrecan if the process is caught early enough. But once aggrecan is stripped away, the collagen fibers underneath become exposed and vulnerable. A second enzyme, MMP-13, then begins degrading those collagen fibers. Collagen loss is irreversible. Once it’s gone, the cartilage cannot repair itself, and bone begins grinding against bone.
This is why osteoarthritis tends to feel mild and intermittent at first, then steadily worse over years. The early stiffness you feel in the morning or after sitting for a while reflects aggrecan loss. The deeper, more constant aching that develops later signals collagen destruction and structural damage to the joint.
When the Immune System Attacks Joints
Rheumatoid arthritis works through an entirely different mechanism. Instead of cartilage wearing down over time, the immune system mistakenly identifies the synovial membrane (the thin lining inside your joints) as a threat and launches an inflammatory attack against it.
Immune cells flood the joint lining and release a cascade of inflammatory signaling molecules, especially TNF-alpha, IL-1, IL-6, and IL-17. These signals recruit more immune cells, creating a self-reinforcing cycle of inflammation. Specialized cells in the joint lining begin to multiply aggressively, forming a thickened mass of tissue called pannus that invades and erodes both cartilage and bone.
This is why rheumatoid arthritis feels different from osteoarthritis. It tends to affect joints symmetrically (both wrists, both knees), causes prolonged morning stiffness lasting an hour or more, and can come with fatigue, low-grade fever, and a general sense of feeling unwell. It typically starts in smaller joints like the fingers and toes before progressing to larger ones. The pain reflects active inflammation, not just mechanical damage, which is why it often responds to medications that suppress the immune response.
Crystal Deposits Inside the Joint
Gout is caused by uric acid crystals forming directly inside a joint space. Your blood becomes saturated with uric acid once levels rise above 6.8 mg/dL. Beyond that threshold, uric acid crystallizes into needle-shaped deposits called monosodium urate crystals that settle into joints, most famously the base of the big toe.
When immune cells encounter these crystals, they try to engulf them, which triggers an intense inflammatory alarm system called the NLRP3 inflammasome. The result is a rapid, dramatic flare: the joint becomes swollen, red, hot, and exquisitely tender, often overnight. Many people describe gout attacks as the worst pain they’ve ever felt. The pain peaks within 12 to 24 hours and can last days to weeks. Between flares, the joint may feel completely normal, which can create a false sense that the problem has resolved. Without treatment, uric acid crystals continue accumulating and flares become more frequent.
How Body Weight Multiplies Joint Stress
Excess weight doesn’t just add a little extra pressure to your joints. Research on knee osteoarthritis found that every single pound of body weight translates to roughly four pounds of force on the knee with each step. That means losing just 10 pounds removes about 40 pounds of load from your knees every time you walk, climb stairs, or stand up from a chair. Over the course of a day, that adds up to thousands of pounds of cumulative stress removed from the joint.
This force multiplier effect explains why weight loss is one of the most effective interventions for knee and hip pain, often reducing symptoms enough to delay or avoid surgery.
Muscle Loss and Joint Instability
Muscles do more than move your joints. They stabilize them. As people age, they naturally lose muscle mass and strength, a process called sarcopenia. When the muscles surrounding a joint weaken, the joint becomes less stable, and the structures inside it absorb forces they weren’t designed to handle alone.
This creates pain through two pathways. The mechanical pathway involves stress receptors in the joint becoming more sensitive as stabilizing muscles weaken, lowering the threshold at which normal movement registers as painful. The inflammatory pathway involves a phenomenon sometimes called “inflammaging,” where aging tissues produce chronically elevated levels of inflammatory molecules like TNF-alpha and IL-6. These same molecules both accelerate muscle breakdown and sensitize pain-sensing nerves, creating a feedback loop where muscle loss worsens pain, and pain discourages the activity needed to maintain muscle.
Age-related changes in cellular energy production make this worse. As the tiny power generators inside cells (mitochondria) become less efficient with age, they produce more damaging molecules called reactive oxygen species. These damage proteins, fats, and DNA in both muscle and nerve tissue, simultaneously weakening muscles and amplifying pain signals.
Infections That Target Joints
Bacterial infection inside a joint, called septic arthritis, is a medical emergency. Unlike other causes of joint pain that develop gradually, septic arthritis typically comes on suddenly. The joint becomes intensely painful, swollen, warm, and red. Moving it, even passively, hurts significantly. Most patients develop at least a low-grade fever above 99.5°F, and up to 58% develop a higher fever above 102°F. The absence of fever does not rule it out.
Lyme disease, caused by a tick-borne bacterium, takes a different path. Joint pain from Lyme disease typically develops one to several months after the initial infection. The swelling can migrate between joints and may be subtle in deeper joints like the shoulder or hip. This delayed, shifting pattern can make it hard to connect joint symptoms to a tick bite that happened weeks earlier.
Bursitis and Tendonitis
Not all joint pain comes from inside the joint itself. Bursitis and tendonitis involve the soft tissues immediately surrounding a joint and are among the most common causes of localized joint pain in otherwise healthy people.
Bursae are small fluid-filled sacs that act as cushions between bones and the muscles, tendons, or skin that move over them. When a bursa becomes inflamed, typically from repetitive motion or prolonged pressure, you feel pain at a specific spot near the joint that worsens with movement. Tendonitis is inflammation of the thick cords connecting muscle to bone. Both conditions feel similar: pain and stiffness that gets worse when you move the affected area and may intensify at night.
The most common trigger for both is overuse, especially during activities your body isn’t conditioned for, or repetitive motions performed with poor posture or awkward positioning. Shoulders, elbows, hips, and knees are the usual targets.
Who Is Most Affected
Joint pain conditions are not distributed evenly across the population. Women are more likely to have arthritis than men, at 21.5% versus 16.1%. The relationship with age is steep: only 3.6% of adults between 18 and 34 have diagnosed arthritis, compared to 53.9% of those 75 and older.
Income and geography play a role too. Adults living below the federal poverty level have arthritis rates of 24.7%, compared to 16.6% among those with higher incomes. People in rural areas have higher rates (23.5%) than those in large cities (16.1%). These disparities likely reflect differences in occupational physical demands, access to preventive care, and rates of obesity, rather than biology alone.