Adduction and Infraspinatus Activity in Shoulder Pain

Shoulder external rotation exercises are commonly used during shoulder rehabilitation to improve rotator cuff muscle function. Adding isometric shoulder adduction during external rotation may modify the activation of the muscles involved in the exercise, but the effect of different levels of adduction pressure has not been fully established in adults with shoulder pain.

This study was designed to examine whether progressively increasing shoulder adduction pressure during external rotation changes shoulder muscle activation in adults with chronic shoulder pain and adults without shoulder pain. The study used a randomized crossover design, in which each participant performed the same external rotation exercise under three different levels of adduction pressure. The order of the experimental conditions was randomized and balanced to reduce potential effects related to the sequence of conditions.

The study focused primarily on the activation of the infraspinatus muscle, while activity of the deltoid, latissimus dorsi, and middle trapezius muscles was also assessed. Muscle activity was recorded using surface electromyography during the experimental exercise. The study was conducted during a single laboratory session.

Inclusion criteria

Shoulder Pain Group (SPG):

• Adults aged 18 years or older.
• Medical diagnosis of chronic non-traumatic shoulder pain lasting more than 3 months.
• Resting pain intensity of at least 2 cm on a visual analogue scale (VAS).
• Ability to perform at least 130° of shoulder flexion, 90° of shoulder abduction, and 45° of shoulder external rotation.

Pain-Free Group (PFG):

• Adults aged 18 years or older.
• No current shoulder or neck symptoms.
• No shoulder or neck symptoms during the previous 2 years.
• No previous medical consultation for shoulder or neck discomfort.
• Full shoulder abduction and external rotation range of motion.
• Ability to perform pain-free maximal isometric contractions during shoulder abduction and external rotation.

Exclusion criteria

Shoulder Pain Group (SPG):

• History of neck or upper limb surgery.
• History of glenohumeral instability or upper limb fracture.
• Neck pain or pain in another segment of the upper limb.
• Systemic diseases that could influence shoulder movement or muscle activation, including rheumatoid arthritis or neuromuscular diseases.
• Physical therapy treatment during the previous year for pain in the affected upper limb.