Tendonitis is the inflammation of the tendons; shoulder tendonitis is inflammation of the tendons of the muscles around the shoulder. The signs of inflammation are pain, warmth, redness, tenderness to touch/pressure and loss of function.

Shoulder tendonitis occurs when the rotator-cuff muscles are overloaded, fatigued, traumatized and/or are subjected to age-related degenerative changes. Pinching or impingement of the inflamed tendons that occurs under an overhanging bony structure (called the acromion) is known as the impingement syndrome.

Specifically, impingement happens when a loaded arm is raised repeatedly towards the side over the head or more importantly when the arm is lowered sideways from over the head.

X rays may show a hook or spur that increases the odds that pinching the rotator cuff tendons.

Rotator cuff muscles can also suffer partial and/or complete tears due to an acute, severe fall or fraying due to repeated injuries. The signs and symptoms of tears are pain in the shoulder often radiating down to the middle of the arm, especially when the arm is raised overhead, weakness and in severe cases a complete inability to lift the arm. An MRI is the most common test used for diagnosis. Clearly, complete tears are first treated using arthroscopic surgical techniques and second with movement and therapeutic modalities.