Tendinitis is inflammation of a tendon, the thick, flexible cord that connects muscle to bone. It causes pain, swelling, and stiffness near a joint, most often in the shoulder, elbow, wrist, knee, or heel. The condition is common among athletes (affecting roughly 39% of athletic populations to some degree) and people whose jobs involve repetitive motions, though it can happen to anyone.
You may also see it called “tendinopathy,” which is now the preferred medical term for persistent tendon pain related to overuse. The shift in language reflects a better understanding of what’s actually happening inside the tendon, which isn’t always pure inflammation.
What Happens Inside the Tendon
Tendons are made of tightly organized collagen fibers designed to handle tension. When you overload a tendon through sudden increases in activity, repetitive motion, or a single awkward movement, those fibers become irritated and the surrounding tissue swells. In true tendinitis, this is an inflammatory response: the tendon is painful and swollen but structurally intact. There’s no microscopic damage to the collagen itself, and with proper rest, the inflammation resolves.
The problem arises when the irritation doesn’t get a chance to heal. Over time, the collagen fibers begin to break down, a condition called tendinosis. The tendon becomes thickened, stiff, and scarred. It loses its elasticity. Some of this damage is only visible under a microscope, which is why many people don’t realize their “tendinitis” has progressed to something more stubborn. This distinction matters because the two conditions respond to different treatments: inflammation benefits from rest and ice, while degenerated tissue needs targeted loading exercises to rebuild.
Where Tendinitis Occurs Most Often
Tendinitis tends to show up wherever a tendon is repeatedly stressed. The most common locations map neatly to the activities that trigger them:
- Shoulder (rotator cuff tendinitis): common in swimmers, overhead athletes, and people who work with their arms raised. Rotator cuff tendinopathy accounts for about 6% of all tendon injuries in athletes.
- Elbow: tennis elbow affects the outer elbow, golfer’s elbow the inner side. Both result from repetitive gripping or wrist movements.
- Knee (patellar tendinitis): often called jumper’s knee, this affects the tendon just below the kneecap. It’s equally as common as Achilles tendinitis in athletic populations, each around 6%.
- Achilles tendon: the large tendon at the back of the ankle is vulnerable to running, jumping, and sudden changes in training volume.
- Wrist and thumb: repetitive hand and wrist motions, from typing to assembly work, can inflame tendons along the wrist.
Lower limb tendons are affected more often than upper body tendons overall, likely because they bear more load during movement and sport.
Symptoms to Recognize
The hallmark of tendinitis is pain at or near a joint that worsens with movement. It typically feels like a dull ache that sharpens when you use the affected area. You might notice stiffness first thing in the morning or after sitting still for a while, which loosens up with gentle movement but returns with activity.
Swelling and mild warmth around the tendon are common in the early inflammatory stage. Some people feel a gritty or crackling sensation when they move the joint. The pain usually builds gradually rather than appearing after a single obvious injury, though acute flare-ups can follow a sudden increase in activity, like a weekend of heavy gardening or a new exercise routine.
If the pain persists beyond a few weeks despite rest, that’s a signal the tendon may be moving beyond simple inflammation into degeneration. Pain that wakes you at night or doesn’t improve at all with rest warrants a closer look.
What Increases Your Risk
Repetition is the biggest driver. Any activity that forces a tendon through the same motion hundreds or thousands of times, whether that’s throwing a ball, typing, or using a screwdriver, creates cumulative stress. Poor technique amplifies the problem, as does ramping up intensity too quickly (the classic “too much, too soon” pattern in runners and gym-goers).
Age plays a significant role. As you get older, tendons lose flexibility and become easier to injure. The blood supply to tendons is already limited compared to muscles, and it diminishes further with age, slowing repair.
Several medical conditions and medications raise the risk beyond what activity alone would explain. Diabetes is a known risk factor. Certain antibiotics in the fluoroquinolone class, corticosteroids like cortisone, and aromatase inhibitors used in breast cancer treatment can all weaken tendon tissue. If you’re taking any of these and develop tendon pain, that connection is worth flagging to your provider.
How It’s Diagnosed
Most tendinitis is diagnosed through a physical exam. Your doctor will press on the tendon, ask you to move the joint in specific directions, and note where the pain is sharpest. For shoulder tendinitis, one common test involves holding your arm out to the side with your thumb pointed down while the examiner pushes down on your arm. Pain or weakness during this maneuver points to the supraspinatus tendon in the rotator cuff.
Imaging usually isn’t needed for a straightforward case. Ultrasound or MRI may be ordered if the pain isn’t responding to treatment, if there’s concern about a partial tear, or if the diagnosis is unclear. These tools help distinguish between acute inflammation and the structural changes of tendinosis.
Treatment and Recovery
In the early inflammatory phase, the basics work: rest the tendon, apply ice, and reduce the load on the affected area. This doesn’t mean complete immobilization. Tendons actually heal better with controlled, pain-free movement than with total rest. The goal is to remove the aggravating activity while keeping the tendon gently active.
For tendinitis that lingers beyond a few weeks, structured exercise becomes the most effective treatment. Progressive loading programs gradually increase the demand on the tendon through a series of stages. You start with static holds (contracting the muscle without moving the joint), progress to slow, controlled movements with increasing resistance, then advance to explosive and sport-specific exercises. Each stage is guided by pain: mild discomfort is acceptable, but sharp pain signals you’re moving too fast.
Eccentric exercises, where you slowly lower a weight rather than lift it, have strong evidence behind them for conditions like Achilles and patellar tendinitis. For patellar tendinitis specifically, a decline squat performed on a sloped surface is a staple rehabilitation exercise. The principle behind all these approaches is the same: controlled stress stimulates the tendon to rebuild and reorganize its collagen fibers.
Tendon healing follows three overlapping phases. First comes the inflammatory response (the first few days to a week). Then the repair phase, where new collagen is laid down. Finally, a remodeling phase where the collagen fibers align and strengthen. This entire process is slow because tendons receive less blood flow than muscles. Mild cases may resolve in two to three weeks, but tendinitis that has progressed to tendinosis often takes three to six months of consistent rehabilitation.
The Problem With Steroid Injections
Cortisone injections are commonly offered for tendon pain because they reduce inflammation quickly and can provide significant short-term relief. However, the trade-off is real. A cohort study of shoulder patients found that those who received steroid injections had a 9.8% incidence of rotator cuff tendon tears, with an adjusted risk roughly 7 times higher than patients who didn’t receive injections. This elevated risk was present even after a single injection.
The concern is that corticosteroids, while quieting pain and swelling, may weaken the collagen structure of the tendon over time. This doesn’t mean injections are never appropriate, but they’re increasingly viewed as a short-term bridge rather than a solution, especially for tendons that are already showing signs of degeneration.
Preventing Tendinitis
Most tendinitis is preventable with attention to load management and body mechanics. If your work or sport involves repetitive motion, take breaks every 20 to 30 minutes to stretch and change position. Rotate between heavy or repetitive tasks and lighter ones throughout the day. When working with your hands, keep your wrists straight, your elbows at roughly 100 degrees, and your upper arms close to your body.
In athletics, the most protective strategy is gradual progression. Increase training volume by no more than about 10% per week. Warm up before activity. Prioritize technique over intensity, particularly in overhead sports and weightlifting. Strengthening the muscles around a joint distributes load more evenly across the tendon, which is why general strength training is one of the best long-term defenses against tendon problems.