What happens to the lower arm at the elbow joint when the biceps brachii muscle is activated?

When the biceps brachii muscle is activated, it causes flexion at the elbow joint. Flexion refers to the bending of the joint, which reduces the angle between the bones that form the joint. In this case, the biceps brachii contracts to pull the forearm towards the upper arm, resulting in the lower arm moving in a flexed position. This movement is critical in various activities, such as lifting or pulling actions.

The other options represent movements that the biceps brachii does not primarily facilitate. For instance, extending the lower arm would involve the triceps muscle, which works oppositely to the biceps. Rotating the lower arm involves the muscles responsible for pronation and supination, such as the pronator teres and supinator, rather than the biceps brachii itself. Lastly, locking the arm in place would imply stabilizing it without movement, which is not the function of the biceps during activation.