Most golf fitness focuses on improving muscle function around the mid-section of the body. I like to call it the "strike zone," the range of muscles on the front and back side of the body from the chest down to the kneecaps. No one will ever criticize you for concentrating your workouts here.
That being said, if you're neglecting your shoulders, you're forgetting how important those joints are to the golf swing. Nearly anything you do to move your arms—up, down, fast, slow, inward or outward—starts with muscle activity in the shoulders. Of particular importance to golfers is having healthy rotator cuffs. This group of four muscles and the assisting tendons are primarily responsible for the internal and external motion of the shoulder joint. They allow you to reach for a plate on a high cabinet shelf and win your local arm-wrestling competition. More importantly for golfers, they allow the club to be swung on the correct plane, stabilize it through the impact zone, and generate speed, which translates to better shots. They also assist in safely decelerating the club, which is key to avoiding injuries from the repetitive and somewhat unnatural motion needed to consistently swing a club correctly.
If you want to improve the functionality of your rotator cuffs, nationally-known strength coach Mike Boyle has a two-part program for you to follow. Click on the video below to see him demonstrate it.