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

• stable load shoulder press (Behavioral): Participants performed a shoulder press using a barbell loaded at their individual 15-repetition maximum
• Unstable load shoulder press (Behavioral): Participants performed a shoulder press using a unstable loaded at their individual 15-repetition maximum

Primary outcomes

• Scapular kinematics (degrees) during shoulder elevation measured by VIPER™ system [Immediately after the single session]: Scapular upward rotation, posterior tilt, and external rotation angles (in degrees) were recorded during shoulder press using the VIPER™ electromagnetic tracking system. Data were collected at 30°, 60°, 90°, and 120° of humeral elevation, and reported as mean values for each angle.
• Surface electromyography (TeleMyo2400T G2, Noraxon USA Inc., Scottsdale, AZ, USA) [Immediately after the single session]: Muscle activation of upper trapezius, lower trapezius, serratus anterior, anterior deltoid, biceps brachii, triceps brachii, rectus abdominis, erector spinae during shoulder press
• Barbell trajectory (measured with camera) [Immediately after the single session]: The trajectory was captured by attaching reflective markers to the end of the barbell and recording kinematic data at a sampling frequency of 60 Hz using a camera.

Secondary outcomes

• Shoulder average pain intensity [Immediately after the single session]: Pain is measured with Visual Analogue scale (VAS) Score range from 0 to 10. Scoring of 0 represents no pain and scoring of 10 represents pain as bad as it could possibly be.

Sex: All, ages: 18 Years to 40 Years, accepts healthy volunteers

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.