Final answer:
When the fibers of the biceps brachii shorten, flexion of the elbow occurs.
Step-by-step explanation:
The correct answer is flexion.
When the fibers of the biceps brachii shorten, flexion of the elbow occurs. Flexion is the movement that decreases the angle between the bones (bending of the joint) in the upper limb. In this case, the biceps muscle contracts and pulls the forearm towards the upper arm, resulting in the bending of the elbow.