Background: Subacromial impingement syndrome (SIS) is a common shoulder disorder characterized by pain, altered movement patterns, and functional impairment. Forty-four to sixty-five percents of the patients with SIS would develop chronic symptoms. Chronic subacromial impingement syndrome (CSIS) is involved in the alteration of central nervous system, such as somatosensory dysfunction and corticomotor changes. The treatments for these patients usually focus on shoulder control and strengthening exercise but no treatment has been found to be targeted on the alterations in somatosensory dysfunction. Focal muscle vibration (FMV) is a useful tool to provide proprioceptive stimulation and has showed short-term effects of improving sensorimotor performance in both healthy and diseased individuals when combined with exercise. To our knowledge, no study has used FMV combined with shoulder exercise for individuals with shoulder problems.

Purpose: To investigate the effects of FMV combined with exercise on somatosensory activity, corticomotor excitability, pain and function in individuals with CSIS.

Methods: This is a randomized control trial. We will recruit ninety subjects with CSIS to receive a 12-session treatment protocol of FMV combined with shoulder exercises. The subjects will be randomized into a treatment group or control group. While the treatment group will receive 20-minute FMV when doing strength training, the control group will have the vibrators attached on but without being turned on during strength training. Both groups will have a 10-20 minutes of shoulder control training following the FMV protocol. The outcomes will be measured before and after the 12-session treatment protocol and include measures of somatosensory activity (using Electroencephalography, EEG), corticomotor excitability (using transcranial magnetic stimulation, TMS), pressure pain threshold, depression scale, pain intensity in a numerical rating scale, and shoulder function. The somatosensory measures include somatosensory-evoked potentials and event-related potentials during arm elevation. Corticomotor measures include motor evoked potentials, active and rest motor threshold and cortical silent period as well as intracortical inhibitory and facilitatory mechanisms. Shoulder function will be measured with the disabilities of the arm, shoulder and hand scale, the patient-specific functional scale, and the global rating of change scale.

Statistical analysis: The continuous and categorical variables of basic data will be analyzed by and independent t test and a Chi-square test, respectively. A two-way mixed ANOVA will be used to test between-group and within-group intervention effects. If there is a significant interaction effect, post hoc pairwise comparisons with Bonferroni correction will be used. The alpha level is set at 0.05.

Inclusion Criteria:

1. Anterior or lateral shoulder pain ≥ 3 months
2. 18 to 65 years old
3. The most pain intensity ≥ 3/10 Pain-Numeric Scale (NRS)
4. SIS, at least 3 positive findings of the following tests:

(1) Neer's test (2) Hawkins-Kennedy test (3) Empty can test (4) Resisted external rotation test (5) Presenting painful arc during arm elevation (6) Tenderness of the rotator cuff tendons

Exclusion Criteria:

1. History of dislocation, fracture, adhesive capsulitis or surgery of upper extremity
2. Neck pain
3. Shoulder flexion or abduction less than 150 degrees
4. History of direct contact injury to the neck or upper extremities within the past 12 months
5. Brain injury and neurological impairment
6. Inflammatory cause of the pain (e.g., rheumatoid arthritis)
7. Psychosis and symptom of headache or dizziness
8. Taking centrally acting medication
9. Contraindications to the use of transcranial magnetic stimulation (TMS)
10. Received shoulder-related physical therapy or corticosteroid injections in the past three months.
11. Received high-concentration platelet-rich plasma (PRP) injections in the shoulder joint in the past year.