Kung fu is a fighting style that have developed over the centuries in China. This fighting style is often classified according to common traits, identified as "families", "sects" or "schools" of martial arts. Examples of such traits include physical exercises involving animal mimicry, or training methods inspired by Chinese philosophies, religions and legends. Styles that focus on qi manipulation are called internal while others that concentrate on improving muscle and cardiovascular fitness are called "external" Geographical association, as in northern and "southern" is another popular classification method.