What is the primary shoulder muscle responsible for abduction?

From the Guidelines

The primary shoulder muscle responsible for abduction is the deltoid muscle, specifically its middle fibers. The deltoid is a large, triangular muscle that caps the shoulder joint and is primarily responsible for lifting the arm away from the body in the frontal plane. While the supraspinatus muscle initiates the first 15-30 degrees of abduction, the deltoid takes over as the main abductor beyond this point and can complete the motion up to approximately 90 degrees. Further abduction beyond 90 degrees requires rotation of the scapula, assisted by other muscles like the trapezius and serratus anterior. The deltoid's effectiveness in abduction stems from its advantageous position on the lateral aspect of the humerus, providing optimal leverage for lifting the arm outward. Weakness or injury to the deltoid significantly impairs a person's ability to raise their arm to the side, affecting many daily activities like reaching for objects on high shelves or combing hair. According to the study on rotator cuff tendinopathy 1, the supraspinatus muscle plays a role in the initial stages of abduction, but the deltoid is the primary muscle responsible for this movement.

Some key points to consider when evaluating shoulder abduction include:

• The deltoid muscle is responsible for the majority of abduction movement
• The supraspinatus muscle initiates the first 15-30 degrees of abduction
• The scapula rotates to assist in abduction beyond 90 degrees
• Weakness or injury to the deltoid can significantly impair abduction.

In the context of real-life clinical medicine, understanding the primary muscle responsible for shoulder abduction is crucial for diagnosing and treating conditions that affect this movement, such as rotator cuff tendinopathy 1.

From the Research

Shoulder Muscle Responsible for Abduction

The primary shoulder muscle responsible for abduction is a topic of interest in the field of sports medicine and orthopedics. According to the studies, the muscles responsible for shoulder abduction include:

• Supraspinatus: contributes to glenohumeral abduction, external rotation, and internal rotation 2
• Deltoids: middle deltoid contributes up to 35-65% of abduction torque, while the anterior deltoid contributes around 2% 2
• Infraspinatus and subscapularis: generate forces that are two to three times greater than supraspinatus force during scapular plane abduction (scaption) 2
• Rotator cuff muscles: provide significant abduction torque, with the subscapularis contributing around 30%, supraspinatus around 25%, and infraspinatus around 10% 2

Muscle Activation Patterns

The activation patterns of these muscles during shoulder abduction have been studied using electromyography (EMG). The results show that:

• Supraspinatus initiates the movement, but not earlier than many other shoulder muscles, including infraspinatus, deltoid, and axioscapular muscles 3
• Middle trapezius and middle deltoid are also activated before movement onset 4
• The prime movers, such as supraspinatus and middle deltoid, are among the first to reach peak amplitude or display the highest %MVC values 4

Biomechanical Comparison

A biomechanical comparison of infraspinatus muscle fiber moment arms during abduction has been conducted, showing that:

• The superior, middle, and inferior infraspinatus muscle fibers work together to produce moment arms that change throughout abduction in an intact shoulder, after a supraspinatus tear, and after superior capsular reconstruction (SCR) and reverse total shoulder arthroplasty (rTSA) 5
• SCR most closely resembles the intact shoulder, whereas rTSA transforms the infraspinatus into an adductor 5