Final answer:
The most lateral head of the biceps brachii is the long head. This muscle is implicated in various movements of the upper arm, including forearm flexion and supination.
Step-by-step explanation:
The biceps brachii is a two-headed muscle that plays a crucial role in the movement of the upper arm. The head of the biceps brachii that is most lateral is known as the long head. This muscle crosses both the shoulder and elbow joints, contributing to flexion of the forearm, supination of the forearm at the radioulnar joints, and flexion of the arm at the shoulder joint. Beneath the biceps brachii lies the brachialis muscle, which adds power to the flexing of the forearm, while the brachioradialis muscle located in the forearm can flex the forearm swiftly or aid in lifting a load slowly.