Structure and Function of the Upper Trapezius and Its Role in Chronic Shoulder Pain: an Investigation of Kinematics, Morphology, Muscle Quality and Activation Distribution

Inclusion Criteria: * Chronic shoulder pain group * 18-50 years old18-50 years old * unilateral and non-traumatic shoulder pain * pain duration longer than 3 months * pain intensity more than VAS=2 during movement * presence 3 of 5 positive pain provocation tests: Hawkins-Kennedy test, Jobe test, or Neer's test Painful Arc, and Resistance Test against External Rotation. * Healthy control group * presenting no shoulder pain at rest or during daily activities * negative results of the pain provocation test: Hawkins-Kennedy test, Jobe test, Neer's test, Painful Arc, Resistance Test against External Rotation and Spurling's neck compression test. Exclusion Criteria: \- Chronic shoulder pain group * history of significant shoulder trauma, such as fracture or ultrasonography clinically suspected full thickness cuff tear * recent shoulder dislocation in the last 2 years * diagnosis of adhesive capsulitis characterized by a gradual and painful loss of both active and passive ROM in all shoulder planes, primarily external rotation * shoulder pain originating from the cervical spine by using Spurling's neck compression test * shoulder pain owing to the following disorders: arthritis (e.g., rheumatoid arthritis, osteoarthritis of shoulder complex), neurologic (e.g., stroke), neoplastic (e.g., breast cancer). and/or referred pain (e.g., visceral referred pain) * corticoid injections over the shoulder during 6 months prior to the study * history of shoulder and neck surgery