The trapezius is a muscle every overhead athlete (specifically swimmers) is familiar with. In fact, this is the muscle most attempt to massage out or have their friends massage out when they are sore. The upper trapezius is the main spot of soreness and will receive the bulk of the discussion.
Trigger points at the upper trapezius are believed to occur from excessive elevation of the shoulder girdle. Trigger points can occur on the lateral neck or superior to the shoulder blade. The upper trapezius can refer pain to the bottom of the skull or anywhere along this large muscle. The upper trapezius has been shown to have increased activation in overhead motions in those with shoulder pain (Ludewig 2000, Moraes 2008).