Which of the following muscles is responsible for initiating abduction of the shoulder joint?

Explanation

## Shoulder Abduction: The Supraspinatus Role **Key Point:** Supraspinatus initiates the first 15° of shoulder abduction. The deltoid takes over from 15° onwards to complete abduction up to 180°. This is a classic NEET PG fact. ### Phases of Shoulder Abduction | Phase | Range | Primary Muscle | Mechanism | | --- | --- | --- | --- | | Initiation | 0–15° | Supraspinatus | Acts as a rotator cuff stabilizer; initiates movement against gravity | | Main abduction | 15–90° | Deltoid (middle fibers) | Primary abductor; deltoid takes over | | Continued abduction | 90–180° | Deltoid + scapular rotation | Requires scapulohumeral rhythm | **High-Yield:** The supraspinatus is essential for the first 15° of abduction. Without it (e.g., in rotator cuff tear), patients cannot initiate abduction smoothly and may show a "shrug sign" (compensatory trapezius contraction). **Clinical Pearl:** In supraspinatus pathology (tear, tendinopathy), patients lose the ability to initiate abduction smoothly. They may be able to abduct passively or with momentum, but active initiation is lost — this is the **drop arm test** finding. **Mnemonic:** **"Supra starts, Deltoid finishes"** — Supraspinatus initiates (0–15°), Deltoid completes (15–180°). ### Why Supraspinatus Initiates Abduction 1. Supraspinatus is positioned on the superior aspect of the scapula and pulls the humerus upward. 2. It stabilizes the humeral head in the glenoid during the critical initial phase. 3. Deltoid alone cannot initiate abduction because it would cause superior migration of the humeral head without rotator cuff support.