Overhead abduction at shoulder joint is caused by which muscle –
**Core Concept**
The overhead abduction of the shoulder joint involves the movement of the arm away from the body and above the horizontal plane, which requires the coordinated action of multiple muscles. This movement is essential for activities such as throwing, lifting, and reaching.
**Why the Correct Answer is Right**
The primary muscle responsible for overhead abduction is the **Deltoid muscle**, specifically its middle and upper fibers. The deltoid muscle has three parts: the anterior (front), middle, and posterior (back) fibers, each with a unique function. During overhead abduction, the middle and upper fibers of the deltoid muscle contract, generating the necessary force to lift the arm above the horizontal plane. The deltoid muscle works in conjunction with the supraspinatus muscle to stabilize the shoulder joint and prevent excessive movement.
**Why Each Wrong Option is Incorrect**
* **Option A:** The **Pectoralis major** muscle is primarily responsible for adduction and medial rotation of the shoulder joint, not abduction.
* **Option B:** The **Trapezius** muscle assists in upward rotation and elevation of the scapula, but it is not the primary muscle responsible for overhead abduction.
* **Option C:** The **Rhomboids** muscles, including the rhomboid major and minor, help stabilize the scapula and assist in downward rotation, but they are not directly involved in overhead abduction.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the deltoid muscle is responsible for flexion, extension, and rotation of the shoulder joint, in addition to abduction. The middle and upper fibers of the deltoid muscle are often the first to be affected in conditions such as rotator cuff injuries or tendonitis.
**Correct Answer:** D. Deltoid muscle