You hear a lot about how kids need to exercise. "Don't be a couch potato," your parents say. "Get outside and run around this weekend," your gym teacher says. "Get in shape," your older brother says. All these people are right. Kids do need to exercise to grow strong and be healthy.

But doctors caution that exercise -- especially contact sports -- needs to be done carefully. Otherwise, you might get hurt. About 8 million kids end up in family doctors' offices with sports injuries each year. Four million are hurt badly enough to require emergency room care.

According to the U.S. Consumer Product Safety Commission, six activities cause nearly three quarters of all children's sports injuries. They are: biking, football, baseball, basketball, roller skating and playground activities. Soccer is beginning to have a high injury rate, too.

What kinds of injuries do kids get? Kids who play football and baseball may hurt their knees, elbows or ankles. They may pull muscles or tendons. Among boys, football causes the most injuries. Among girls, field hockey and gymnastics are the riskiest.

Lyle Micheli, the director of sports medicine at Children's Hospital in Boston, Micheli sees a lot of injured kids in his practice. So he decided to write a book to help parents and coaches teach kids to play sports safely. It's called "Sportswise: An Essential Guide for Young Athletes, Parents and Coaches." Micheli isn't against sports. In fact, he plays rugby, one of the toughest sports there is.

He is a strong believer in sports for kids, too. "A youngster's body is saying, 'Go, go, go' in its growing stages because it needs physical activity to promote proper growth," he writes.

Kids who spend more than 20 hours a week watching TV just don't give their muscles the workout they need, Micheli writes. Spending your days scrunched over a Nintendo game or slumped in front of the tube can make your muscles and joints tighten up.

Bodies were designed to be used, and when they're not used they don't work very well. What happens? These statistics about American kids will give you an idea:

70 percent of boys and girls can't do a single chin-up.

40 percent of boys and 70 percent of girls can do only one push-up.

40 percent of boys can't touch their toes.

25 percent of all kids can't do one proper sit-up.

"Pretty pathetic," he writes. So get out there and play. But play in an environment where you'll learn the fundamentals of your sport and where you'll be supervised by an adult who knows what he or she is doing.

The most effective way to prevent sports injuries is to make sure that coaches are trained in how to teach their sport. Coaches should also know first aid and cardiopulmonary resuscitation so they can act fast if injuries do happen. They should make sure their team has good equipment that fits the kids it is designed to protect.

The field where your games are played needs to be properly cared for, too. It should be checked for potholes and for broken glass and other debris before play begins. Goal posts should be padded.

Coaches should include conditioning exercises, warm-ups and cool-downs in every session. These exercises help increase the body's flexibility and make it less likely that a player will get hurt. Flexible bodies play better, are less prone to injury and have fewer aches and pains after playing.

During a game, there should be rest breaks and lots of water breaks to prevent overheating. Coaches also need to be careful not to get so carried away with winning that they push the kids on the team too hard. A tired player is much more likely to stumble, fall and sprain an ankle.

"I believe that the root cause of most of the problems in children's sports is the preoccupation with competition," Micheli writes. "This 'winning is everything' attitude detracts from the sports experience for kids." It leads, he says, to stress and worry as well as to physical injuries.

Perhaps the most important advice for kids is this. After you finish your game, the first question you or your parents should ask is not "Did you win?" but "Did you have fun?" How would you answer that question about the sport you play? Tips for Parents

Here are five questions that Lyle Micheli, author of "Sportswise: An Essential Guide for Young Athletes, Parents and Coaches." (Houghton, Mifflin: $9.95) and director of sports medicine at Children's Hospital in Boston, suggests parents ask to help determine the safety of children's sports programs in your community: 1. Are the coaches certified in first aid and CPR?. 2. Does the coach have a written emergency plan in case of an accident, and has it been rehearsed? 3. Is there a first aid box and ice on site at all practices and games? 4. Does the coach have the youngsters do warm-ups, stretching and cool-down exercises? 5. Are pre-sports physicals required?

Catherine O'Neill is a freelance children's writer.