The muscle that causes rapid supination of the forearm is called the brachii muscle.
The correct answer and explanation is:
Correct Answer: The muscle that causes rapid supination of the forearm is the biceps brachii.
Explanation (Approximately 300 words):
The biceps brachii is the primary muscle responsible for rapid supination of the forearm, particularly when the elbow is flexed. This muscle is located in the anterior compartment of the upper arm and consists of two heads: the long head and the short head, both of which originate at different points on the scapula and converge into a single tendon that inserts at the radial tuberosity of the radius.
Supination is the rotational movement of the forearm that turns the palm upward or forward (as if holding a bowl of soup — a useful mnemonic). The biceps brachii is uniquely positioned to act on both the elbow joint and the radioulnar joint, which allows it to perform elbow flexion and forearm supination.
While other muscles like the supinator muscle also contribute to supination, the biceps brachii is especially important for powerful or rapid supination, such as turning a doorknob quickly or using a screwdriver. The supinator muscle, in contrast, is more active when the movement is slow or when the elbow is fully extended.
The effectiveness of the biceps brachii in supination is closely related to elbow flexion. When the elbow is bent at about 90 degrees, the mechanical advantage of the biceps brachii increases, making it the most effective position for this muscle to supinate the forearm.
In summary, while multiple muscles assist in supination, the biceps brachii is the dominant and most powerful supinator, especially during rapid or forceful movements when the elbow is partially flexed. Its dual role in elbow flexion and supination makes it one of the most functionally significant muscles of the upper limb.