Final answer:
The biceps brachii is pivotal in forearm flexion, supination, and arm flexion at the shoulder. It works in tandem with the deeper brachialis muscle and the brachioradialis for these movements. Muscle force depends on muscle length and joint angle.
Step-by-step explanation:
The biceps brachii, a two-headed muscle, plays a crucial role in the movement of the forearm and arm. It crosses both the shoulder and elbow joints, primarily functioning to flex the forearm. Additionally, the biceps brachii is involved in supinating the forearm at the radioulnar joints and flexing the arm at the shoulder joint. Alongside the biceps brachii, the brachialis muscle, located beneath, provides significant additional power for forearm flexion. Another key muscle in this group is the brachioradialis, which can either flex the forearm rapidly or assist in lifting heavy loads more gradually.
When you perform the action of bending your elbow, your biceps contract while the triceps relax; the biceps act as the flexor and the triceps as the extensor of the elbow joint. It is interesting to note that the force which these muscles, especially the biceps brachii, can exert is contingent upon their length and the angle of the joint.