Shoulder pain is the third most common musculoskeletal disorder, with estimated lifetime prevalence around 10%. The prevalence for daily non-specific shoulder pain in the working population is around 12%, and up to 23% of the workforce seeking professional help with a shoulder related disorder have been sick-listed for at least one week. People with shoulder pain are often disabled to the point where they cannot live a normal life, suffering from sickness absence and poor quality of life. Clinically, subacromial pain is the most common form of complaint and causes a great loss of shoulder function, which accounts for 33% of all shoulder-related healthcare contacts. Previous studies have clearly shown that exercises are the preferred first choice of treatment for patients with subacromial pain. However, there are some studies supporting the benefits of using targeted strength training in patients with subacromial pain, but it is not clear what the best type of exercise combined with the patient response a health care provider should prescribe: should the patient feel pain during the performance of an exercise, or should pain be avoided? The present project aims to answer the following research questions: Which type of intervention (pain acceptance/pain avoidance) is superior to self-reported improved patient reported shoulder pain, function and quality of life? And which type of intervention (pain acceptance/pain avoidance) is superior to improved changes in the tendon structure, measured by Ultrasound? An RCT study will be conducted as a randomized, controlled, blinded multi-center trial in both public and private practice settings in Denmark, Belgium, Spain and The Netherlands with inclusion of patients from National Health Service musculoskeletal orthopedic outpatient departments and/or general practitioners. Both intervention groups will receive eccentric, concentric and isometric exercises of the same load, frequency, progression etc. However, the difference between both intervention groups will be based on the specific movement, which is either pain aggravation or not. The project is organized in cooperation between University of Antwerp (Belgium), University of Malaga (Spain), University College Rotterdam (The Netherlands) and University of Southern Denmark (Denmark). With this, the current project will have the assistance of the national and European shoulder networks in order to be sure of sufficient shoulder pain patients with different cultural backgrounds. This project will contribute to clarify the best treatment option in the management of subacromial pain. Clinicians and care providers will be benefited of this increase in the body of knowledge, in order to steer subacromial pain treatment and to improve clinical decision-making, and reducing the chronification of SPS. With this, the present project also attempt to reduce the massive economic burden of subacromial shoulder pain on society.