The role of shoulder muscles is task specific

Area of Science:

• Biomechanics
• Human Movement Science
• Orthopedics

Background:

• Understanding the functional roles of shoulder muscles is crucial for diagnosing and treating shoulder pathologies.
• The distinction between 'stabilizer' and 'mover' roles in shoulder muscles during rotation tasks requires further investigation.

Purpose of the Study:

• To compare shoulder muscle activity during unsupported internal and external rotation.
• To determine if shoulder muscles maintain their 'stabilizer' or 'mover' role irrespective of the specific task.
• To examine the impact of increasing rotational load on muscle activation patterns.

Main Methods:

• Electromyography (EMG) was used to record activity from 13 shoulder muscle sites in 15 healthy subjects.
• Recordings were taken during isometric internal and external shoulder rotation in an unsupported abducted position.
• Muscle activity was analyzed under conditions of progressively increasing rotational load.

Main Results:

• Subscapularis showed significantly higher activity during internal rotation (p<0.001).
• Supraspinatus, infraspinatus, trapezius, and serratus anterior exhibited significantly higher activity during external rotation (p<0.05).
• Deltoid, pectoralis major, and latissimus dorsi showed no significant difference in activity between internal and external rotation. All activated muscles increased activity with increasing torque.

Conclusions:

• Shoulder muscle function is task-specific, with rotator cuff muscles primarily responsible for rotation torque.
• The deltoid muscle likely contributes to dynamic shoulder stability during the examined tasks.
• Increased scapulothoracic muscle activity during external rotation suggests a need for greater dynamic scapular stability.