Working a certain group of muscles for an allotted amount of time to achieve your target heart rate is the objective of aerobic exercise. This works the heart more efficiently and has the body expend a greater amount of calories. Often people will hit the aerobic curve. This is when you begin working out and raise your intensity to the top then decrease gradually. Keeping a continuous heart rate is more effective. The lungs and heart endure for more time and work more efficiently when they are trained. People who perform aerobic exercise on a routine basis will have to exercise longer to reach their target heart rate as their endurance is increased. People who are just beginning will reach their target heart rate faster until their body gets used to the workload.