Every time that you lift your arm, the rotator cuff tendon will pass through the subacromial space (this is the narrow gap that is found at the top of your shoulder). You will develop Shoulder Impingement Syndrome if the tendon catches on the bone that is located at the top of the subacromial space. Touching or rubbing against this bone is more likely if:
You have a tendon injury that has resulted in inflammation or tearing. This is usually caused by overuse, sporting activities, or sometimes simply due to age and general wear and tear.
There is irritation or inflammation of the sac between the bone and tendon (often caused by a specific sporting injury or overuse of the arm)
You have pre-existing conditions from birth that have resulted in a flattened or curved acromion
You are vulnerable to bone spurs that often develop in older age