Final answer:
The deltoid is the muscle that abducts the upper arm and is capable of flexing and extending the humerus. It also helps with medial and lateral rotation.
Step-by-step explanation:
The muscle that abducts the upper arm and can both flex and extend the humerus is the deltoid. This muscle creates the rounded lines of the shoulder and is not only the major abductor of the arm but also assists in both flexing and extending the arm. Additionally, the deltoid muscle also facilitates medial rotation and lateral rotation of the humerus.
When it comes to the other muscles, the subscapularis is involved in medial rotation, while the supraspinatus and infraspinatus muscles are responsible for abduction and lateral rotation, respectively. The teres major and teres minor also play their parts in moving the arm. The coracobrachialis is another muscle, and it flexes and adducts the arm.