When most people discuss the upper body, however -- especially the muscles of the upper body -- they are referring to the muscles on the front of the chest, shoulders, upper back, neck and arms.pper body muscle strains -- partial tearing of upper body muscle fibers -- are another common cause of upper body muscle soreness, especially among athletes. Muscle contusions or bruises, muscle cramps or charley horses, and fibromyalgia can all cause upper body muscle pain, states the MedlinePlus website.