The trapezius. This one's a big boy, and can carry a lot of tension, so it really is helpful to be aware of what it is and what it does. For starters, its origin is the skull and spine. It starts at the back base of your skull, and rides all along your spine to just above the lumbar area. All of these fibers stretch across your back to insert into your scapula (information in my October section, haven't figured out how to link my posts yet - will do that soon!). So this one muscle connects your head, spine, and shoulder. You better be nice or it will f*$k you up! Heh. The primary functions of the trapezius are to support the arm and move the scapula around. The top part is used when you shrug your shoulders. The middle part is used when squeezing the shoulder blades together in the middle of the back. The lower part is used mostly for throwing, with some assistance from the deltoids. If you are so talented as to be able to keep you shoulder blades stable in one spot, then you can use your traps to move your spine and extend your neck. I just tried this and it was not so easy! When working together with some neighbors such as the rhomboids and serratus anterior, the trapezius can rotate the scapula (to lift your arms above your head) and bring it in towards the spine.