Cardio mainly exercises your lungs and heart, which is built differently from most of your muscles (it's designed for constant use). Also if you eat too much, i

Cardio mainly exercises your lungs and heart, which is built differently from most of your muscles (it's designed for constant use). Also if you eat too much, it's good for burning up excess calories. It has mixed benefits for the rest of your body. You'll improve your muscle tone a bit through running or cycling or swimming, but repetitive strain injuries are also a risk. For most people, the benefits of cardio far outweigh the downsides, but on balance it's not necessarily good for your joints and ligaments.