Which muscle has no action on shoulder joint?
The correct answer and explanation is:
Correct Answer: The brachialis muscle has no action on the shoulder joint.
Explanation:
The brachialis is a muscle located in the arm, specifically in the anterior compartment of the upper arm. Its primary function is flexion of the elbow joint, and it acts on the forearm. Unlike other muscles in the arm, such as the biceps brachii, it does not cross the shoulder joint and therefore has no influence or action on it.
Here’s a detailed explanation of why the brachialis does not affect the shoulder joint:
- Anatomical Location: The brachialis originates from the anterior surface of the distal half of the humerus and inserts into the coronoid process and tuberosity of the ulna. Since its origin and insertion are confined to the humerus and ulna, it does not interact with any structures in the shoulder region.
- No Crossing of the Shoulder Joint: For a muscle to act on a joint, it must cross that joint. The brachialis is entirely located below the shoulder and does not cross the glenohumeral joint, the primary joint of the shoulder. Thus, it cannot influence shoulder movements.
- Functionality: The brachialis is a pure elbow flexor and works independently of the position of the shoulder. Its sole focus is on bending the forearm at the elbow, making it highly efficient for this action. Unlike the biceps brachii, which also assists in shoulder flexion due to its crossing of the shoulder joint, the brachialis is not involved in any shoulder movement.
In contrast, muscles like the deltoid, rotator cuff muscles, biceps brachii, and triceps brachii contribute directly or indirectly to shoulder joint actions. The brachialis stands out as a muscle solely dedicated to elbow flexion, with no influence on the shoulder.