Final answer:
The origins of the biceps brachii include the supraglenoid tubercle for the long head and the coracoid process for the short head. The biceps is responsible for flexing the forearm at the elbow, supinating the forearm, and flexing the arm at the shoulder. The muscle force varies with arm angles and it works in concert with the brachialis and brachioradialis.
Step-by-step explanation:
The long head of the biceps brachii originates from the supraglenoid tubercle of the scapula, while the short head originates from the coracoid process of the scapula. Both heads converge to form a single muscle belly, which is responsible for multiple actions at the shoulder and elbow.
When the biceps brachii contracts, it primarily functions to flex the forearm at the elbow joint, aids in supinating the forearm at the radioulnar joints (turning the palm upwards), and flexes the arm at the shoulder. The muscle's force output changes depending on the angle between the forearm and the upper arm; when the arm is at a 90° angle, this position allows for maximal force production.
Other muscles such as the brachialis, located deep to the biceps, and the brachioradialis, found in the forearm, also help in flexing the forearm. The biceps brachii is a part of the anterior compartment of the arm, which includes muscles, vessels, and nerves involved in the flexion of the forearm.