Final answer:
The action shared by both a rotator cuff muscle (infraspinatus) and the deltoid muscle is the lateral rotation of the arm.
Step-by-step explanation:
The student is asking about a specific action that is shared by one of the rotator cuff muscles and the deltoid muscle. The deltoid is responsible for major movements of the arm including abduction, flexion, and medial and lateral rotation. Among the rotator cuff muscles, the infraspinatus is known for its role in lateral rotation of the arm, which is a motion that is also supported by the deltoid muscle. This common action of these two muscles is lateral rotation of the arm.