Shoulder pain is a common musculoskeletal system complaint, accounting for 7-34% of patients in the clinic. The most common shoulder problem is subacromial impingement syndrome (SIS). Up to 45% of individuals with SIS may have unsuccessful treatment and still complain of symptoms after 2 years. This chronicity of pain may not be fully explained by structural injuries or damage, but may be related to sensorimotor changes. Decreased corticospinal excitability and increase inhibition have been found in individuals with SIS. These central motor changes may link to alteration in pain and nociception processing and the somatosensory system, which has been found in individuals with low back pain. Hyperalgesia has been found over both affected and unaffected shoulders in patients with SIS, indicating central and peripheral sensitization. However, no study has investigated whether there are changes in the central somatosensory system. Therefore, the objectives of this proposal are (1) to investigate the corticomotor and somatosensory system in patients with SIS (2) to investigate the relationship between the corticomotor and somatosensory alterations in patients with SIS. Subjects with chronic SIS and healthy subjects were recruited, with 32 people in each group. Electroencephalography (EEG) will be used to collect somatosensory activity, including somatosensory evoked potentials, spectral analysis of EEG oscillations and event-related spectral perturbation (ERSP) of the shoulder movement. Electromyography will be used to record muscle activity. Transcranial magnetic stimulation will be used to test corticomotor excitability, including active motor threshold, motor evoked potentials, cortical silent period, and intracortical inhibition and facilitation. The pressure pain threshold will be collected by a pressure algometer on the muscles of bilateral arms and legs. Pain intensity will be assessed with the Numeric Rating Scale. Shoulder function will be evaluated with the Disability of Arm, Shoulder and Hand questionnaire. Depression will be evaluated with Center for Epidemiologic Studies Depression Scale (CES-D).

Inclusion Criteria (chronic impingement syndrome group):

1. individuals have shoulder pain localized at the anterior or lateral aspect of shoulder more than six months
2. are aged 20 to 65 years old
3. shoulder impingement syndrome, which is confirmed by having at least three of the following: (a) positive Neer's test, (b) positive Hawkins-Kennedy test, (c) positive empty can test, (d) positive resisted external rotation test, and (e) presenting painful arc during arm elevation

Inclusion Criteria (acute/subacute impingement syndrome group):

1. individuals have shoulder pain localized at the anterior or lateral aspect of shoulder less than six months
2. are aged 20 to 65 years old
3. shoulder impingement syndrome, which is confirmed by having at least three of the following: (a) positive Neer's test, (b) positive Hawkins-Kennedy test, (c) positive empty can test, (d) positive resisted external rotation test, and (e) presenting painful arc during arm elevation

Inclusion Criteria (health control group)

1. individuals without any shoulder and neck problems
2. sex, age, and hand dominance match to impingement group

Exclusion Criteria:

1. have a history of dislocation, fracture, adhesive capsulities, or surgery of upper extremity
2. arm elevation angle less than 150 degrees
3. a history of direct contact injury to the neck or upper extremities within the past 12 months
4. brain injury and neurological impairment
5. inflammatory cause of the pain (e.g., rheumatoid arthritis)
6. neck pain
7. psychosis and symptom of headache or dizziness
8. allergy to plaster
9. contraindications to the use of TMS, assessed with a safety screening questionnaire

Patients with chronic impingement syndrome, acute/subacute impingement syndrome and the matched healthy control subjects.