Normalization to Maximal Voluntary Contraction is Influenced by Subacromial Pain

Area of Science:

• Biomechanics
• Musculoskeletal Disorders
• Rehabilitation Science

Background:

• Subacromial impingement syndrome is a common cause of shoulder pain.
• Electromyography (EMG) is frequently used to assess shoulder muscle function.
• Normalization of EMG data to maximal voluntary isometric contractions (MVIC) is a standard procedure.

Purpose of the Study:

• To investigate the influence of subacromial pain on EMG normalization to MVIC.
• To assess changes in shoulder muscle strength and EMG activity following pain reduction.

Main Methods:

• Patients with subacromial impingement syndrome underwent EMG and MVIC testing before and after a subacromial anesthetic injection.
• Muscle strength (MVIC) and EMG during arm elevation were recorded.
• Pain levels were assessed using a visual analog scale.

Main Results:

• A 64% reduction in pain was observed post-injection.
• Significant increases in MVICs were found in the anterior, middle, and posterior deltoid, and lower trapezius muscles.
• EMG normalization to MVIC was significantly affected by pain in the anterior deltoid and lower trapezius.

Conclusions:

• Subacromial pain influences shoulder muscle activity and strength.
• Normalization of EMG data to MVIC in the presence of pain can yield unpredictable and potentially inaccurate results.
• Clinicians should exercise caution when interpreting EMG data normalized to MVIC in painful shoulder conditions.