Warming up before exercising prepares the entire body for your workout. It helps your heart and lungs adjust to the activity, and the increased blood flow warms the muscles and makes them less vulnerable to injury. In fact, a good warm up can make your whole workout a little easier. If your body isn't ready for intense exercise, your muscles may cramp, and your heart rate and breathing may increase too quickly, making it difficult to continue the activity. Furthermore, cramped muscles in the calves, thighs, and back can take days to heal, delaying future workouts. Your warm up doesn't need to be elaborate, so long as you increase your heart rate for a few minutes before transitioning into more intense activity. If your workout includes moderate exercise, such as walking or swimming, you'll only need a minimal warm up. If you're going to run, swim, or perform some other intense exercise, you may want to increase your warm up time up to 10 minutes. After your warm up it's important to stretch your muscles. Remember to talk to a doctor before beginning any fitness program, and to stop if you experience pain, dizziness, or an injury.