The Effect of Unstable Loads on Scapular Kinematics and Muscle Activation During Overhead Press in Overhead Athletes With Subacromial Pain Syndrome

The inclusion criteria for both the experimental and control groups were as follows: (1) amateur or recreational overhead athletes aged 18-40 years (e.g., volleyball, tennis, baseball); (2) sport participation history of more than 3 years; (3) weekly overhead sport participation of more than 4 hours; (4) ability to meet the minimum loading requirements of the equipment for both stable and unstable conditions, defined as a 15-repetition maximum (15RM) for stable shoulder press of ≥10 kg and unstable shoulder press of ≥8 kg. The individual inclusion criteria for the experimental group were: (1) diagnosis of subacromial pain syndrome, defined as at least 3 positive findings out of 5 clinical tests (Painful Arc test, Neer impingement test, Empty Can test, Hawkins-Kennedy impingement test, and Resisted External Rotation test); (2) Visual Analogue Scale (VAS) score of ≤3 during shoulder press performance; (3) symptoms present on the dominant side. The individual inclusion criteria for the control group were: absence of shoulder pain within the past 6 months, including no pain at the glenohumeral joint, acromioclavicular joint, sternoclavicular joint, scapulothoracic joint, or cervical spine. The exclusion criteria were as follows: (1) history of surgical intervention for fracture or dislocation of the shoulder or elbow joint within the past 6 months; (2) direct trauma to the cervical spine or upper extremity within the past 1 month; (3) shoulder pain or neurological symptoms of cervical origin; (4) any condition preventing the completion of the experimental protocol; (5) bilateral shoulder pain.