Muscle Activation and Coordination in Patients with Subacromial Impingement Syndrome: An EMG Study

David H Hawkes, Omid Khaiyat, Ahmed T Makki, Graham J Kemp, Jo Gibson, Simon P Frostick

Background: Shoulder pain from impingement syndrome is common and can have a negative impact on patients’ quality of life. A recent multi-centre randomised control trial has questioned the role of surgery in favour of a conservative approach. However, current rehabilitation strategies are poorly evidence based and hampered by an inadequate understanding of the pathophysiology of the condition. The aim of this work is to evaluate aberrant muscle activation and coordination in patients with subacromial impingement using electromyography (EMG).

Methods: Thirty nine patients with subacromial impingement syndrome (SISG) and 34 healthy controls (CG) were recruited. Subjects completed self reported outcome measures and the Functional Impairment Test-Hand Neck Shoulder and Arm (FIT-HaNSA). EMG was recorded from 15 shoulder girdle muscles, using a telemetric system, during a task designed to replicate activities of daily living. The Pearson Correlation Coefficient (PCC) was used to study the coordination between muscles.

Results: Significant differences were demonstrated between the study groups for all patient reported outcome measures (p=<0.01) and the FIT-HaNSA test (p=<0.01). There was no difference in activation levels between any of the peri-scapula muscles (p=0.137-0.991). Aberrant coordination was seen between the upper trapezius and both the deltoid (PCC=0.25 SISG, 0.46 CG; p=0.04) and serratus anterior (PCC=0.41 SISG, 0.59 CG; p=0.35) in the patient group.

Discussion: Normal scapula motion positions the glenoid effectively, creating the basis for arm movement. The upper trapezius and serratus anterior elevate and upwardly rotate the scapula which is balanced by the middle and lower trapezius. This coordinated force couple is absent in patients with subacromial impingement syndrome. This effectively narrows the subacromial space causing painful compression of the soft tissue structures.

Implications: The findings of this study indicate that shoulder rehabilitation strategies need to focus on restoring the normal balance between of the peri-scapular muscles and deltoid.

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