The padding position can be hard on your shoulders, especially when the muscles surrounding your shoulder joint and upper back lack strength and stability. Conditioning your rotator cuff muscles and the muscles that help stabilize your shoulder blades will guide your humeral head (at the top of your upper arm bone) to be positioned properly, which is key to relieving stress from your shoulders.
For starters, try performing external shoulder rotation exercises with cables or bands as well as the movements that follow two or three times a week. This will help you reduce pain and paddle strong.
-- Craig Friedman, Director of Methodology, Athletes' Performance