Have you been a couch potato all summer? Has it been too hot to play tennis, too much trouble to go swimming, too boring to jog? Do you know every video on MTV by heart? Have you eaten your way through a supermarket's worth of corn chips and drunk a small lake of soda?

Uh-oh! You may be out of shape. With school -- and physical education classes -- about to begin, it's time to think about exercise.

Exercise can be defined as body movement with a purpose. The purpose: to get you in shape. In the United States, many young people are out of shape. They weigh too much for their height, and they aren't strong enough to pass a few simple physical tests.

Experts say that this sorry state of affairs is the result of too much sedentary activity. "Sedentary" means "seated" or "sitting down." Think about it. We sit down to eat; we sit down to watch TV; we sit in cars on the way to the movies, where we sit in a soft chair and eat popcorn while we watch other people race around on the screen. Students sit in school; many adults sit down all day at work.

How sedentary are you? Keep a diary of your activity for just one day. Unless you are quite athletic, you'll probably be surprised at how much sitting you do in a day. Of course, the human body is designed for sitting. We even come equipped with padding to make it comfortable. But the body is also designed for activity -- for running, jumping, climbing, pulling, pushing, lifting and more. To stay in top condition, human beings must engage in the activities we were built to do.

Being a couch potato for one evening may be relaxing; sitting there every afternoon and night all week is just plain bad for your health. "But I'm no couch potato. I play softball," you say. "I must be in good shape." It's great to play softball -- and it is exercise. Softball calls for bursts of energy when you're running the bases or chasing down a line drive. But during other parts of the game you're just standing there -- or sitting on the bench waiting for your turn in the batting order to roll around. If softball is your only exercise, you should combine it with other activities, too.

Analyze your favorite sport: Do you keep moving for long periods, or do you work hard for a few seconds, then rest? Tennis, for example, is good exercise -- if you're a good player. If you're a beginner and spend most of your time picking up balls, you won't get too much benefit from the exercise.

Exercise that promotes fitness makes your heart and lungs stronger. It reduces the amount of extra body fat you carry around. It strengthens you, making your body capable of staying active for longer periods of time without getting tired. Some sports come with built-in benefits. Running, jogging, swimming, bicycling, cross-country skiing and energetic walking all promote fitness. The movements involved in those sports are sustained. That means you do them over and over again for a period of time. Your muscles warm up; your heart and lungs work at a higher rate. The result: fitness.

The President's Council on Physical Fitness asked medical experts to rate some sports and exercises to see how well the activities promoted stamina (the ability to keep going), muscle strength and flexibility. Handball and squash came in first, followed closely by jogging and skiing; bowling came in last. Can you figure out why?

When you go back to school next month, think about taking part in an exercise or sports program that will get you in shape and keep you there. It's probably too late to start training for the 1988 Olympics. But at least you won't be named Couch Potato of the Year.

Tips for Parents

Parents worry about the potential for injury in the sports kids play. Injury prevention can be promoted, according to "The Young Athlete's Health Handbook" by Douglas W. Jackson, MD, and Susan C. Pescar (Everest House; $15.95). Young athletes should: :: Warm up and cool down. Warmed-up muscles are more flexible and thus less prone to injury. A cool-down period at the end of the activity allows heat to dissipate gradually, minimizing stiffness and aching. :: Play fair. Coaches, teachers, parents and officials should keep youngsters from becoming overzealous and destructively aggressive about winning. :: Check playing areas for hazards. :: Use safety equipment, and make sure it fits and is in good shape. :: Wear appropriate clothing. :: Don't continue to play if you're in pain.

Catherine O'Neill is a free-lance children's writer in Baltimore.