Tendonitis develops when a tendon is stressed beyond its ability to repair itself. This usually happens through repetitive motion, sudden overloading, or a combination of both, though age, certain medications, and underlying health conditions can also set the stage. Understanding the specific triggers helps explain why some people develop tendon problems while others doing similar activities don’t.
What Happens Inside the Tendon
Tendons are made mostly of tightly packed collagen fibers suspended in a gel-like matrix. When you use a tendon repeatedly or push it past its limits, small tears form in those collagen fibers. Your body responds by sending blood and immune cells to the area, producing the pain, swelling, and warmth you feel in the first few days.
If the tendon gets adequate rest, this inflammatory phase resolves within about a week, and the body starts laying down new collagen to patch things up. That repair process takes another two to three weeks before the tendon enters a longer remodeling phase, where the new tissue gradually reorganizes to match the original structure. Full remodeling can take anywhere from three months to a year.
The problem is that many people keep using the tendon through this process. When microtrauma happens faster than the tendon can heal, the collagen fibers begin to degenerate and lose their organized alignment. The tendon fills with weaker type III collagen instead of the sturdy type I collagen it normally relies on. Cells within the tendon start dying off. At this point, the condition shifts from acute tendonitis to a chronic degenerative state sometimes called tendinosis, which is harder to treat and slower to resolve.
Repetitive Motion and Overuse
The most common cause of tendonitis is simply doing the same movement too many times, too forcefully, or without enough recovery. This is why tendonitis clusters around certain activities and body parts: tennis elbow from gripping and swinging, Achilles tendonitis from running, rotator cuff tendonitis from overhead reaching.
What matters is the combination of repetition and force. Research published in The Journal of Hand Surgery found that people in jobs requiring both high repetition and high force had a risk of hand and wrist tendonitis 29 times greater than people in low-repetition, low-force jobs. Repetition alone raises your risk. Force alone raises your risk. Together, they multiply it dramatically.
You don’t have to be an athlete. Any activity that loads a tendon in the same pattern, day after day, can trigger the cycle. Gardening, playing a musical instrument, knitting, or even scrolling on your phone with your thumb are all common culprits. Weekend warriors who go from minimal activity to intense exercise are especially vulnerable because their tendons aren’t conditioned for the sudden spike in demand.
High-Risk Jobs and Movements
Certain occupations carry significantly higher rates of tendon problems because of the physical demands baked into the work. CDC data shows that agriculture, forestry, fishing, and hunting workers have the highest rates of frequent exertion on the job, at nearly 71%. Construction follows at 67%, and accommodation and food services at about 58%. These industries involve repetitive lifting, bending, gripping, and awkward body postures that stress tendons throughout the arms, shoulders, and lower legs.
Office work creates its own set of risks. Tendon force in the wrist and hand is directly related to both the force you exert and the angle of your hand while you exert it. Typing and mouse use involve thousands of small, repetitive finger and wrist movements each day. The flexor tendons in your fingers can press against the median nerve when your wrist is bent, which is why a poorly positioned keyboard doesn’t just cause tendon pain but can also contribute to nerve compression symptoms. Keeping your wrists in a neutral, straight position while typing reduces the mechanical load on these tendons considerably.
Age and Blood Supply
Tendons become more vulnerable as you get older, and the primary reason is blood flow. Tendons already have limited blood supply compared to muscles, and clinical ultrasound studies show that blood flow to uninjured tendons decreases with aging. When an older tendon is injured, the drop in blood supply is even more pronounced. Research from the University of Pennsylvania found that aged tendons showed significantly reduced blood vessel density after injury compared to younger tendons, meaning fewer pathways for delivering the oxygen and nutrients needed for repair.
This reduced healing capacity means that the same amount of repetitive stress a 25-year-old tendon shrugs off can overwhelm a 55-year-old tendon. The collagen itself also becomes less elastic with age, making fibers more prone to tearing under load. This is why tendonitis becomes increasingly common after 40, even in people who haven’t changed their activity level.
Medications That Damage Tendons
A class of antibiotics called fluoroquinolones (including ciprofloxacin and levofloxacin) carries an FDA boxed warning for tendonitis and tendon rupture. These drugs can weaken tendon structure, sometimes severely enough to cause a rupture without significant physical stress. The Achilles tendon is the most commonly affected.
The risk isn’t limited to older adults. While the FDA warning highlights increased risk in people over 60, an FDA review of adverse event reports found that tendon problems occurred at similar rates in younger and older patients. Only 17% of reported cases involved patients 60 or older. Taking corticosteroids at the same time as fluoroquinolones further increases the risk.
Long-term corticosteroid use on its own can also weaken tendons by interfering with collagen production. Statins, used to lower cholesterol, have been linked to tendon problems as well, though less commonly.
Health Conditions That Raise Your Risk
Diabetes affects tendons through several pathways. High blood sugar over time leads to thickening and scarring of connective tissue, which is why people with longstanding diabetes are more likely to develop conditions like Dupuytren’s contracture (where fingers curl toward the palm) and diabetic hand syndrome (stiffness and waxy thickening of the skin and joints). Type 2 diabetes is also associated with hardening of tendons and ligaments along the spine, possibly driven by elevated insulin or insulin-like growth factors that promote abnormal tissue growth.
Rheumatoid arthritis and other autoimmune conditions create chronic inflammation that can attack tendon tissue directly. Gout deposits uric acid crystals in and around tendons, causing irritation. Thyroid disorders, both overactive and underactive, can alter tendon metabolism and make them more susceptible to injury.
Sudden Injuries and Training Errors
Not all tendonitis develops gradually. An acute injury, like a hard landing, a sudden sprint, or an awkward twist, can inflame a tendon in a single moment. This is true tendonitis in its original sense: a short-term inflammatory response to a specific event.
More often, though, what feels like a sudden onset actually reflects weeks or months of accumulated damage that finally becomes painful. The tendon has been quietly degenerating, and one slightly harder effort pushes it past the threshold where you notice symptoms. This is why tendonitis can seem to appear “out of nowhere” even though the underlying process has been building.
Training errors are a major trigger for athletes and recreational exercisers. Increasing mileage, weight, or intensity too quickly doesn’t give tendons time to adapt. Tendons remodel more slowly than muscles, so you can feel strong enough to handle a heavier load while your tendons are still catching up. A general guideline is to increase training volume by no more than about 10% per week, though individual tolerance varies.
Structural and Biomechanical Factors
How your body is built influences which tendons take extra stress. Flat feet or high arches change the angle of pull on your Achilles tendon and the tendons around your ankle. Leg length differences shift load unevenly. Muscle imbalances, where one muscle group is significantly stronger or tighter than the opposing group, force tendons to absorb forces they weren’t designed for.
Poor movement patterns compound these structural factors. Running with excessive heel striking, lifting with your shoulders rounded forward, or gripping tools with a clenched fist all increase the mechanical load on specific tendons. Sometimes the fix is surprisingly simple: a shoe insert, a different grip technique, or strengthening a weak stabilizer muscle can redistribute stress away from the overloaded tendon.