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 including flexibility, muscular strength, and cardio-vascular fitness. It is performed to music and may be practiced in a group setting led by an instructor. It can be done solo and without musical accompaniment. With the goal of preventing illness and promoting physical fitness, practitioners perform various routines comprising a number of different dance-like exercises. Classes are divided into different levels of intensity and complexity.