Aerobics is a form of physical exercise that combines rhythmic aerobic exercise with stretching and strength training routines with the goal of improving all elements of fitness. It is usually performed to music and may be practiced in a group setting led by an instructor. With the goal of preventing illness and promoting physical fitness, practitioners perform various routines. Formal aerobics classes are divided into different levels of intensity and complexity and will have five components: warm-up, cardiovascular conditioning, muscular strength and conditioning, cool-down and stretching and flexibility. Aerobics classes may allow participants to select their level of participation according to their fitness level. Many gyms offer different types of aerobic classes. Each class is designed for a certain level of experience and taught by a certified instructor with a specialty area related to their particular class.