Introduction: Growing evidence shows that non-specific shoulder pain is a common musculoskeletal condition. It often involves a severe reduction in the functionality of the upper limb with a remarkable impact on daily life activities and social participation. Like back pain disorders, it is known that it has a high risk of chronification and is associated with central and peripheral sensitization phenomena. These processes are clinically detectable as hyperalgesia, allodynia, and wind-up phenomenon, but it is not yet explored if those processes are reported in shoulder pain patients. Moreover, modern clinical neuroscience had revealed that behavioral factors such as anxiety, depression, poor self-efficacy, kinesiphobia, and catastrophization contributes to pain chronification but a clear involvement of these factors has not been defined yet for shoulder pain. Considering that the relation between anatomical alterations detectable with imaging techniques is very poor when assessing chronic pain patients it is useful to explore this condition from a biopsychosocial point of view that nowadays is lacking.

To better define and treat non-specific shoulder pain patients, exploring central and peripheral sensitization, chronic pain contributing factors, and their health status is of paramount importance. Methods: A case-control study will be performed. Based on the best evidence the sample size was calculated. It was estimated 68 people with non-specific shoulder pain and 68 healthy controls are needed to be evaluated. All participants will fulfill the following validated rating scales exploring central sensitization, pain chronification factors, disability and health status. The disability will be assessed using The Shoulder Pain and Disability Index, central sensitization using the Central Sensitization Inventory (CSI), catastrophization using the Pain Catastrophizing Scale (PCS), kinesiophobia using the Tampa Scale for Kinesiophobia (TSK), anxiety and depression using the Hospital Anxiety Depression Scale (HADS) and self-efficacy using the Pain Self-efficacy Questionnaire (PSEQ). A trained physiotherapist will complete the ICF Generic Core Set, and after sharing it with the patient, a patient-clinician evaluation of the health status will be defined. Moreover, a clinical examination will be carried out to detect tactile and mechanical allodynia, wind-up phenomenon, and mechanosensitivity changes in the shoulder. A multivariate model will be considered for the statistical analysis to check for any confounding, and tests for estimating variance differences will be used.

Expected results: The study is expected to describe the relevance of processes related to central and peripheral sensitization in chronic shoulder pain patients, with a plausible difference between healthy controls. It will be possible to define the relevance of pain chronification factors and health status considering the complexity of pain from a biopsychosocial point of view. The investigators expect to identify which factors are related to disability and loss of participation in shoulder pain. Conclusion: The study is aimed to offer a multifactorial overview of patients with non-specific shoulder pain to provide tailored and precise rehabilitation programs, considering the clinical relevant factors and reducing the resource waste in less effective interventions.