Simplify the following text for a Grade 9 native English-speaking person.
Original: 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 ( flexibility , muscular strength , and cardio-vascular fitness ) .
Simplified: Aerobics or rhythmic fitness is a physical exercise performed under a rhythmic music . It includes running , jumping , stretching and strength training routines .