Final answer:
The brachialis is a shoulder flexor muscle, while the latissimus dorsi is a shoulder extensor muscle. The infraspinatus acts as a shoulder external rotator, and the subscapularis acts as a shoulder internal rotator.
Step-by-step explanation:
a) Shoulder Flexor: One muscle that acts as a shoulder flexor is the brachialis. This muscle is located on the anterior side of the arm and plays a major role in flexing the forearm at the elbow joint.
b) Shoulder Extensor: An example of a shoulder extensor muscle is the latissimus dorsi, commonly referred to as the lats. The lats are large muscles that originate from the back and help extend the arm at the shoulder joint.
c) Shoulder External Rotator: One muscle that acts as a shoulder external rotator is the infraspinatus. It is located on the posterior side of the shoulder and plays a key role in rotating the arm externally.
d) Shoulder Internal Rotator: An example of a muscle that acts as a shoulder internal rotator is the subscapularis. This muscle is located on the anterior side of the shoulder and is responsible for rotating the arm internally.