What is one of the primary actions of the biceps brachii?

The biceps brachii is primarily known for its role in flexing the elbow. This muscle, situated at the front of the upper arm, consists of two heads (the long head and short head) that converge into a single muscle belly. When the biceps brachii contracts, it pulls the forearm toward the shoulder, resulting in elbow flexion. This action is crucial for many everyday movements, such as lifting objects or performing pull-ups.

While the biceps also assists in shoulder flexion and supination of the forearm, its most significant action is indeed elbow flexion. The other options refer to movements associated with different muscles; shoulder extension is primarily facilitated by muscles such as the deltoids and latissimus dorsi, knee flexion involves the hamstrings, and wrist extension is accomplished by muscles in the forearm. Thus, identifying elbow flexion as the primary action accurately captures the main function of the biceps brachii.