The upper trapezius lifts the shoulder towards your ear.
The middle trapezius squeezes the shoulder blades together.
The lower trapezius along with the upper trapezius rotate the shoulder blade up. Think of reaching over your head. The lower trapezius muscle works slightly different than most muscles.
As the shoulder blade rotates upward, the muscle gets longer. This is called an eccentric contraction and it takes much more strength than a contraction where the muscle gets shorter which is called a concentric contraction. Keeping your trapezius strong, especially the lower and middle trapezius will help with proper posture. Talk to Mike, Chip, or Sharon to learn how to strengthen your trapezius.