Volleyball players repetitively place the arm into extreme positions, and thus expose the athletes' shoulders to a large load and increase the risk of injuries. The shoulder complex is identified as one of the most injured joints in volleyball sports. Although many studies have revealed alterations in the function, morphology and biomechanics of the shoulder complex in volleyball players with shoulder pain, some researchers suggested that a smooth energy transfer from the axial skeleton, through the shoulder complex, to the arm and hand is crucial for overhead sports performance and injury prevention, with little evidence showing the link between trunk muscle performance and shoulder pain in volleyball players.

Therefore, the main purposes of present study are (1) to compare the trunk muscle performance (muscle strength, power and endurance) in volleyball players with and without subacromial impingement syndrome (SIS); (2) to compare trunk and shoulder muscles activity during spiking in volleyball players with and without SIS.

Volleyball players repetitively place the arm into extreme positions, and thus expose the athletes' shoulders to a large load and increase the risk of injuries. The shoulder complex is identified as one of the most injured joints in volleyball sports. Although many studies have revealed alterations in the function, morphology and biomechanics of the shoulder complex in volleyball players with shoulder pain, some researchers suggested that a smooth energy transfer from the axial skeleton, through the shoulder complex, to the arm and hand is crucial for overhead sports performance and injury prevention, with little evidence showing the link between trunk muscle performance and shoulder pain in volleyball players.

Therefore, the main purposes of present study are (1) to compare the trunk muscle performance (muscle strength, power and endurance) in volleyball players with and without subacromial impingement syndrome (SIS); (2) to compare trunk and shoulder muscles activity during spiking in volleyball players with and without SIS.

We hypothesize that volleyball players with SIS will change in trunk muscle performance and in trunk and shoulder muscles activity during spiking.

This is a cross-sectional case-control exploratory study.The investigators plan to recruit 35 volleyball players with SIS and 35 age, gender, experience matched healthy players in each group, totally 70 volleyball players in this study. Surface electromyography (sEMG) (Noraxon, USA) will be used to collect trunk and shoulder muscles activity during spiking. The kinematics were recorded using the electromagnetic motion tracking system (LIBERTY™). Isokinetic dynamometer (Biodex Multi-Joint System 4 Pro) and (BTE Primus RS) will be used to measure trunk and shoulder muscles torque and power at 60°/s and 180°/s in trunk and 90°/s, 180°/s and 240°/s in shoulder. Flexor, extensor and lateral flexor endurance tests will be used to assess trunk muscles endurance.

SPSS software (version 24.0) will be used to analyze data. The Independent t test or Chi square test will be used to compare demographic data. The Independent two-sample t test will be used to examine group differences in all outcome measures with the significance level setting at 0.05.

Inclusion Criteria:

Asymptomatic group:

• The asymptomatic group has to present no history of pain or dysfunction on the shoulder and/or neck in past 6 months.

Subacromial impingement syndrome group:

• Occurrence of shoulder pain that lasted more than 3 months and was related to a spiking movement.
• Present impingement symptoms as a combination of three or more positive signs: Neer test, Hawkins Kennedy test, Jobe test, Painful arc test, Resisted shoulder ER test

Exclusion Criteria:

• Cannot perform spiking movement due to shoulder pain.
• Acute shoulder injury at the time of recruitment or shoulder structural lesion, including hypo/hypermobility, or abnormality.
• History of surgery and fracture in the upper and lower limbs or trunk.
• History of pain in the lower limbs and trunk in the last 3 months.
• Neurological or cardio respiratory disorders.
• Receiving any intervention for the shoulder pain including medication, injection, or general physical therapy during the previous 3 months.

• Volleyball players with more than 2 years' experience on a collegiate team.
• Volleyball players have been trained by professional coach over half a year.
• Playing or training for volleyball over 8 hours per week.