Aerobic exercise and activities are also called cardio, short for "cardiovascular." During aerobic activity, you repeatedly move large muscles in your arms, legs and hips. Your heart rate increases and you breathe faster and more deeply. This maximizes the amount of oxygen in your blood and ultimately helps you use oxygen more efficiently. How well you use oxygen is called your aerobic capacity. When your aerobic capacity is high, your heart, lungs and blood vessels efficiently deliver large amounts of oxygen throughout your body. As a result, you feel more energized and don't tire as quickly. Cardio is ESSENTIAL to any well balanced health regimen.