The most common cause of injury is exercising too agressively—the "too much, too soon" syndrome. Start any new exercise at a relatively low intensity and gradually increase your level of exertion over a number of weeks. Use the 10 percent rule: In general, don’t increase your training load—the length or frequency of workouts, the intensity or the distance—by more than 10 percent a week.
“No pain, no gain” is a myth. If you have continuing pain during an exercise, stop and don’t continue unless you can do so painlessly. (If the pain occurs in the chest or neck area, contact your doctor immediately.) General muscle soreness after exercise is another matter: It usually indicates that you are not warming up sufficiently or are exercising too long or strenuously.
"Ballistic” stretching—where you bounce in and out of a stretching position—can increase the chance of muscle tears and muscle soreness. Instead, perform static stretches, in which you gradually stretch through a muscle’s full range of movement until you feel resistance. This helps to loosen muscles without straining them.
Avoid high-impact aerobics. Most aerobics instructors and many aerobics students suffer injuries to their shins, calves, lower back, ankles and knees as a result of the repetitive, jarring movements of some of the aerobics routines they perform. You may want to stick to the marching or gliding movements of low-impact aerobics instead of the jolting, up-and-down motion of typical aerobics.
Keep yourself well hydrated: replace fluids lost through sweating. This is particularly important in hot weather, when you can easily lose more than a quart of water in an hour through sweating. Neglecting to compensate for fluid loss can cause lethargy and nausea, interfering with your performance. Even if you don’t feel thirsty, it’s important to drink at regular intervals when exercising. Your thirst may be satisfied long before you have replenished lost fluids.