Tendonitis is inflammation of a tendon, the thick fibrous cord that connects muscle to bone. It happens when a tendon is acutely overloaded by a force that’s too heavy or too sudden, causing micro-tears in the tissue. The result is localized pain that flares with movement and often comes with stiffness, swelling, and tenderness at the affected joint. Tendonitis can develop in any tendon, but it most commonly strikes the shoulders, elbows, wrists, knees, and heels.
What Happens Inside the Tendon
When a tendon sustains micro-tears, the body launches an inflammatory response that unfolds over the first three to seven days. It begins with a small amount of bleeding at the injury site, which activates platelets and draws in immune cells. Within the first 24 hours, specialized cleanup cells called macrophages move in to clear out damaged tissue. After that initial wave, the body shifts into repair mode: cells called fibroblasts multiply rapidly and begin producing new collagen fibers to patch the tears.
This repair process works well when the tendon gets adequate rest. Problems start when the tendon is loaded again before healing is complete, which can restart the cycle of damage and inflammation or push the injury into a chronic state.
Tendonitis vs. Tendinosis
These two terms sound similar but describe different problems. Tendonitis involves active inflammation from acute overload. Tendinosis, by contrast, is a degenerative breakdown of the tendon’s collagen that develops from chronic, repetitive overuse, often without significant inflammation present at all. When researchers examine tendinosis tissue under a microscope, they find immature collagen fibers that have lost their orderly alignment, increased blood vessel growth in disorganized patterns, and a tendon surface that has shifted from white and firm to dull, brownish, and soft.
The distinction matters because it changes both the treatment approach and the timeline. Tendonitis typically heals within several days to six weeks. Tendinosis caught early may take 6 to 10 weeks, but chronic tendinosis can require 3 to 9 months of rehabilitation. Treating tendinosis as if it were tendonitis (focusing only on reducing inflammation) misses the underlying structural problem and can delay recovery.
Common Types and Locations
- Achilles tendonitis: affects the tendon running down the back of the lower leg to the heel, common in runners and people who suddenly increase their activity level.
- Tennis elbow: pain on the outside of the elbow from repetitive wrist and arm motions, not limited to tennis players.
- Golfer’s elbow: similar to tennis elbow but affecting the inside of the elbow.
- Patellar tendonitis: inflammation of the tendon connecting the kneecap to the shinbone, frequent in sports involving jumping.
- Shoulder tendonitis: often involves the rotator cuff tendons, triggered by overhead motions like throwing, swimming, or painting.
What the Pain Feels Like
Tendonitis pain is typically minimal or absent at rest and ramps up with movement. Research on Achilles tendonitis shows that pain intensity climbs in a predictable pattern as you demand more from the tendon. Walking might increase pain by about 1.7 points on a 10-point scale compared to resting, while a more intense activity like heel raises can spike it by 3.4 points above resting levels.
Morning stiffness is another hallmark. Many people notice the affected area feels stiff and sore when they first get up, sometimes lasting anywhere from a few minutes to well over an hour. Longer morning stiffness tends to predict more pain throughout the day during activity. Swelling, warmth, and tenderness when you press on the tendon are also common, especially in the first few days after the initial injury.
Causes and Risk Factors
The most straightforward cause is mechanical overload: a sudden increase in exercise intensity, a repetitive motion at work, or a single awkward movement that strains the tendon beyond its capacity. But the picture is more complex than simple wear and tear.
Age plays a significant role. As tendons age, their blood supply diminishes and their collagen becomes less flexible, making them more vulnerable to injury. Obesity contributes through both increased mechanical load on weight-bearing tendons and the chronic low-grade inflammation that excess body fat promotes throughout the body.
Diabetes is one of the strongest metabolic risk factors. People with diabetes face roughly triple the overall risk of tendon problems. For specific conditions, the numbers are even more striking: diabetes is associated with a sevenfold increase in the odds of developing Achilles tendonitis and an elevenfold increase for golfer’s elbow. The connection likely involves damage to the tiny blood vessels that supply tendons, impairing their ability to repair everyday micro-damage.
A Surprising Medication Link
A class of antibiotics called fluoroquinolones (commonly prescribed for urinary tract and respiratory infections) can damage tendons directly. These drugs interfere with collagen production and promote collagen breakdown in tendon tissue. While the overall risk is low, affecting roughly 0.14 to 0.4 percent of people who take them, the odds of Achilles tendonitis or rupture jump nearly fourfold compared to other antibiotics. For people over 60 who take these antibiotics while also using corticosteroids, the risk compounds dramatically, up to 46 times greater than baseline. An estimated 2 to 6 percent of all Achilles tendon ruptures in people over 60 may be attributable to these drugs.
How Tendonitis Is Diagnosed
Most tendonitis is diagnosed through a physical exam. Your doctor will press on the tendon to locate the point of tenderness, test your range of motion, and may ask you to perform specific movements that stress the suspected tendon. These clinical tests are often enough to confirm the diagnosis.
Imaging comes into play when the diagnosis is uncertain or the injury isn’t responding to treatment. Ultrasound is particularly useful because it can evaluate the tendon in real time as you move the joint, helping distinguish between a partial tear and a complete rupture. MRI provides a broader view of the entire area and can reveal other structural problems that might be contributing to your pain, like bone spurs or cartilage damage.
Treatment and Recovery
Mild tendonitis often improves within two to three weeks with basic self-care: reducing the activity that triggered it, applying ice to the area, and using over-the-counter anti-inflammatory pain relievers. The key in the early phase is giving the inflammatory process time to do its job without re-injuring the tissue.
When pain persists or limits your movement, physical therapy becomes the primary treatment. A supervised exercise program is the most common approach, and it typically centers on progressive loading, gradually increasing the demands on the tendon to stimulate organized collagen repair. Eccentric exercises (where the muscle lengthens under tension, like slowly lowering your heel off a step) have shown strong clinical results, particularly for Achilles and elbow tendonitis. For many people, combining eccentric work with other loading patterns and stretching produces better outcomes than eccentric exercise alone.
Your therapist may also use manual techniques, soft tissue work, or external supports like braces during the rehabilitation period. The overall timeline depends on how early you start treatment and how long the problem has been developing. Catching it early and modifying your activity promptly gives you the best chance of a fast recovery.
What Happens if You Ignore It
Tendonitis that goes untreated or is repeatedly aggravated can progress to tendinosis, where the tendon’s structure deteriorates and its collagen fibers lose the organized alignment they need to bear load. This weakened tendon is bulkier but less strong, and it becomes increasingly vulnerable to tearing. A complete tendon rupture typically requires surgical repair and a significantly longer recovery. Addressing tendonitis early, even if the pain seems manageable, prevents this progression and keeps a relatively simple problem from becoming a complex one.