What causes tendinitis?
Tendinitis is irritation and micro-damage in a tendon from more load than it can currently handle, usually from repetitive motion rather than a single injury. It shows up in the shoulder, elbow, knee, Achilles, and wrist most often, wherever a tendon is asked to do the same motion again and again without enough recovery time between sessions. Rest alone rarely fixes it, because the tendon needs progressive loading to actually rebuild its capacity.
Tendons adapt to load slowly, over weeks and months, compared to muscle. Doing the same throwing motion, keyboard movement, or running stride thousands of times without enough recovery between sessions outpaces the tendon's ability to keep up, causing microscopic damage that accumulates into pain and stiffness. The fix is not necessarily doing less. It is often restructuring how the load is applied so the tendon has time to adapt between exposures.
Jumping into a new training program, suddenly increasing mileage, or ramping up intensity too quickly asks a tendon to handle more force than it is currently conditioned for. Tendons need a gradual buildup to strengthen safely. When that buildup is skipped, whether from enthusiasm or a tight schedule, the tendon takes the brunt of the mismatch between ambition and current capacity.
Poor form during a lift, an inefficient running stride, or throwing mechanics that put extra strain on one part of the tendon all increase the load it absorbs with every repetition. Small mechanical inefficiencies do not matter much for one rep, but they add up dramatically over thousands of repetitions. Correcting the underlying movement pattern often resolves tendon pain that keeps returning despite rest.
Tendon tissue naturally loses some elasticity and blood supply as we age, making it somewhat more prone to irritation from the same activities that never used to cause a problem. This does not mean tendinitis becomes inevitable or unmanageable. It means the tendon may need a slightly more gradual loading approach and more consistent strengthening to stay resilient against the same demands.
A tendon connects muscle to bone, and if the muscle attached to it is weak, the tendon absorbs force the muscle should have handled instead. Weakness in the shoulder blade muscles, for example, often shows up as tendinitis in the rotator cuff. Strengthening the muscles around the affected tendon, not just the tendon itself, is frequently what resolves pain that keeps recurring.
