Knee injuries are common in sports because people’s knees are actually rather fragile. Only a few tight bands, called ligaments, hold your leg bone to your thigh bone. When you play sports, the ligaments are the only things that keep your knees from bending sideways or backward! Torn ligaments are among the most common kinds of athletic knee injuries.

Athletes sometimes tear an important band of knee tissue called the ACL (arrow).

Artville

If an athlete tears a knee ligament, doctors have to do surgery to repair it. Most often, the ligament that tears is the ACL, which stands for “anterior cruciate ligament.” A torn ACL doesn’t heal like your skin does. So, surgeons must replace the injured ACL with a piece of healthy ligament that they remove from another spot in the patient’s leg. It takes a long time to recover from this surgery. It’s painful too.

Cato T. Laurencin and other scientists at the University of Virginia in Charlottesville are working on a new way to repair knee ligaments. They want to make a fake ligament that surgeons can use instead of a real one. They’ve already created such a ligament by putting artificial materials together with cells from rabbits. When they tested the artificial ligament in rabbits, the cells slowly grew and created a new, natural ligament, while the artificial materials dissolved little by little.

A microscope shows that cells (purple) and other important tissue (green) are growing in the spaces between bits of an artificial knee ligament (white).

Proceedings of the National Academy of Sciences

So far, the researchers have tried this new idea only on rabbits. But someday, their research might help injured athletes such as Jackson get back in the game sooner.—P. Barry

Going Deeper:

Barry, Patrick. 2007. Hurt-knees Rx: Surgical method promotes ligament regeneration. Science News 171(Feb. 24):116. Available at http://www.sciencenews.org/articles/20070224/fob3.asp .

Sohn, Emily. 2005. Football scrapes and nasty infections. Science News for Kids (Feb. 9). Available at http://www.sciencenewsforkids.org/articles/20050209/Note2.asp .

______. 2004. Workouts: Does stretching help? Science News for Kids (May 5). Available at http://www.sciencenewsforkids.org/articles/20040505/Feature1.asp .

______. 2003. A framework for growing bone. Science News for Kids (April 30). Available at http://www.sciencenewsforkids.org/articles/20030430/Note3.asp .

______. 2003. Prime time for broken bones. Science News for Kids (Oct. 8). Available at http://www.sciencenewsforkids.org/articles/20031008/Note3.asp .

If you’ve ever played on a sports team or gone to gym class, you probably know the drill. First, you do some warmups. Then you stretch. Exercises and activities follow. At the end of class or practice, you do more stretches.

For decades, coaches and gym teachers have insisted that stretching helps athletes perform better, suffer fewer injuries, and feel less sore the next day. From the health club to the football field to the gymnastics mat, everywhere you look, people stretch.

Is stretching before soccer as helpful as just jogging or doing some other sorts of warmups?

Now, research suggests that stretching may not do your body as much good as people thought. After reviewing more than 350 scientific studies, researchers from the Centers for Disease Control and Prevention (CDC) found that stretching may not reduce the chance of injury.

“There’s insufficient evidence to demonstrate that stretching is effective,” says Stephen Thacker. He’s director of the CDC’s epidemiology program in Atlanta, Ga.

Athletic performance

If it’s athletic performance you’re after, don’t expect stretching to help you run faster, jump higher, or throw a ball farther, either. Some studies show that stretching may actually slow you down, especially if you do it before you play your sport.

To top it off, it now looks as if stretching may actually make you even more likely to get hurt, says Stacy Ingraham.

Ingraham is an exercise physiologist at the University of Minnesota, Twin Cities. Her research focuses on injuries in women and girls, who tend to hurt their muscles and joints more often than men do.

“Certain athletes stretch all the time,” Ingraham says. “They’re the ones who usually get hurt.”

Ingraham points to professional baseball players as an example. They all do stretches before a game, she says. Yet baseball players have one of the highest rates of injury in any sport. Minnesota Twins center fielder Torii Hunter tore his hamstring in his first game of the season.

Ingraham has coached and worked with runners, basketball players, baseball players, and football players. When she persuades them to stop stretching, she says, injury rates tend to go down.

Thacker has noticed a similar pattern. He coaches a high school girl’s basketball team. During his first 5 years of coaching, six girls tore their ACL, a ligament that connects the thigh to the shin and stabilizes the knee. Three years ago, Thacker replaced stretching with specific strengthening exercises and warm-up activities such as jogging and sidestepping. Since then, the team has had no ACL injuries.

Working muscles

To understand why stretching may be a bad idea, it helps to know how muscles work.

It all starts as orders from the brain. Special cells called neurons carry electrical messages from the brain through the nervous system to the muscles you want to activate. If you’re running, your brain tells your legs to move and your arms to pump. As soon as the messages get to their targets, the muscles react. You’re cruising.

How muscles are attached to bones at the knee.

Skeletal muscles are the kind that attach to bones. They do most of the work when you exercise. Skeletal muscles are made up of long, twisted cells called fibers. Proteins inside the fibers help your muscles contract and relax. These muscle movements allow you to run, jump, skip, throw a Frisbee, swim, and more.

With exercise, muscle fibers grow and multiply. The more you work out, the stronger and bigger your muscles get.

When you stretch, you lengthen muscle fibers. It then takes longer for messages from the brain to travel through them. Stretched muscles also seem to be more sluggish than unstretched ones. They don’t spring back as readily. And every time you stretch, you may be tearing your muscle fibers a teeny bit.

Stretching before you exercise is particularly risky, experts say, because stretched muscles are less stable. That makes it harder for them to bounce back from the jarring impact of running, jumping, or weaving around other players on a soccer field.

Instead of stretching before an activity, experts recommend warming up by starting slowly to get blood and oxygen flowing to your muscles. Warming up is also a natural way of stretching your muscles just enough to prepare them for more intense activity.

“If you’re going to play soccer, jog a bit beforehand,” Thacker says. “If you’re going to play baseball, swing the bat before a game.”

Kids don’t get injured as often as adults do because they don’t put the same kind of stresses on their joints, Thacker says. Still, it’s never too early to get into good habits.

Girls, especially, have reason to worry. Among high school athletes, girls are three times more likely than boys to tear the ACL in their knees. By the time they hit the professional level, women tear their ACLs up to 10 times as often as men do, Ingraham says.

Girls hurt their ankles and backs more often than boys, too. Scientists aren’t sure what causes the difference, but girls tend to be more flexible than boys, Ingraham says. Her research suggests that this extra flexibility could be part of the problem. And stretching may only make things worse.

Gentle motions

It may be worth talking about stretching with your coaches and gym teachers.

But if they insist you keep stretching, don’t be too worried, says fitness expert Jay Blahnik. He has written a book called Full-Body Flexibility.

In some cases, gentle stretching can be helpful, Blahnik says, as long as you do the right kind of stretches at the right time and you do them correctly.

Instead of grabbing your ankles and yanking or forcing your body into pretzels, he suggests gentle motions that actively use your muscles.

Try clasping your hands behind your head, Blahnik recommends. Then slowly pull your elbows backward and squeeze your shoulder blades together. “The act of stretching doesn’t decrease injuries,” he says, “but we think being mobile and flexible does.”

As part of a cooldown after a workout, light stretching is also OK, Blahnik says. And, for sports such as gymnastics and dancing, stronger stretching may be appropriate.

Overall, preparation for sports or exercise should involve a variety of activities, not just stretching. Athletes, coaches, trainers, and others need to use the combination of strength training, conditioning, and warming up that’s best for a given sport.

Whatever you decide to do about stretching, don’t stop exercising. Research continues to show that exercise is good for your heart, good for your bones, and good for your muscles. It helps you sleep better and keeps your weight under control. Running around is fun. It can even make you smarter.

Now, stop reading and get a move on!

Word Find: Stretching and Exercise

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Going Deeper:

As it turns out, the choices you make now can make a big difference in how you feel later in life. I recently learned this lesson the hard way.

It started with an injury: a cracked shinbone caused by too much running on hard pavement. My doctor suggested a bone scan, which showed that my bones are weaker than average. I don’t have osteoporosis, a disease that causes older people to shrink in height and break bones easily. But I’m close.

Example of a human bone.

For me, the diagnosis was a scary wakeup call. I’m just 27 years old, but already I’m worried about things that normally happen only to women more than twice my age. Will I break my hip if I slip on a patch of ice? Is it safe for me to go skiing, lift heavy boxes, play Ultimate Frisbee?

Perhaps what upsets me most is the realization that I might have avoided all of this if only I had thought ahead earlier in life. Childhood and adolescence are the most important times to build strong bones. For you, there’s still time. Doctors suggest a variety of foods you can eat and exercises you can do as a teenager to build strong bones for life.

Living tissue

Bones are amazing. They’re hard but flexible, and they’re lightweight but tough. Without bones, we’d be just puddles of skin and guts.

An adult person has 206 bones in his or her body. The outer layer of a typical bone is made of a hard material honeycombed with tunnels. This web of hollow pipes allows a bone to be strong and light. It also allows the passage of nutrients and waste. A protein called collagen gives a bone its elasticity. Chemicals known as calcium salts make a bone hard.

But, even though our bones support us, they’re easy to ignore. Unlike a cut or bruise, a weak bone isn’t visible or painful.

Osteoporosis is sometimes called a silent disease. People often don’t realize they have it until it has progressed so far that they break bones while doing ordinary things, such as walking down stairs or lifting heavy objects.

Osteoporosis happens mostly to older people. But I’m not the only woman in her 20s with weak bones. Increasingly, scientists are finding that weak bones are a problem in teenagers and even younger kids. That’s especially troubling because youth is the critical time for bone growth.

If you’ve ever seen a skeleton in a museum, you might think that bones are dead. In fact, bones are living tissue. They reshape and rebuild themselves many times as you grow and age.

Front and back views of a human skeleton. An adult has 206 bones in his or her body.

The cycle of building and breaking down bone changes over a person’s lifetime. Bone-building is fastest during the first 3 years of life and again during adolescence. By the time you’re in your 20s, the tissue in your bones is about as tightly packed as it’s going to get.

Measuring something called bone density tells you how tightly packed the bone tissue is. A high bone density normally shows that you have strong bones.

Once you get to be about 35 years old, bone tissue gets broken down more quickly than it’s replaced. This means that bones tend to lose tissue, and the bone density goes down. That’s when osteoporosis usually becomes a concern. And it’s a bigger risk for women than for men.

Food concerns

Getting the right kind of bone-building nutrition and exercise as a teenager is like putting money in the bank. Your bones can stay strong as you get older.

Unfortunately, many teenagers don’t think about their bones when they order lunch or decide what to do with their free time. They’d rather snack on chips or slurp soda than think about vitamins.

And parents don’t always set the best example. “I was standing by the elevator at Children’s Hospital,” says Susan Coupey, an adolescent medical specialist at Children’s Hospital at Montefiore in Bronx, New York. “There was a 2-year-old child being fed soda by his parents.”

Unlike milk or fortified juice, a soft drink doesn’t provide calcium for helping to build strong bones.

Junk food has few nutrients. It also fills you up, so you don’t eat enough of the good stuff. That’s one reason why many adults want schools to get rid of soda machines.

Doctors urge kids to get plenty of calcium, the mineral that makes bones strong. Calcium is also essential for keeping nerves, blood, and muscles healthy. When you don’t take in enough calcium, your body takes calcium out of your bones, which weakens your bones even more.

Although calcium is abundant in milk, yogurt, cheese, fortified juices, soy milk, and some nuts and vegetables, few people get enough of it. The Institute of Medicine recommends that kids between the ages of 9 and 18 get 1,300 milligrams of calcium every day. That’s roughly the amount of calcium in a quart of milk.

Yet fewer than 10 percent of girls and 25 percent of boys get that much, according to the National Osteoporosis Foundation. “The average calcium intake of adolescent girls in the United States is somewhere around 900 milligrams,” Coupey says. “Many take in just 600 to 700 milligrams.”

Paying attention

Now that I’ve started paying attention, I realize that getting enough calcium takes some effort. Getting 1,300 milligrams of calcium is equivalent to drinking about four glasses of milk, eating 10 cups of cooked broccoli, or having two glasses of milk, a cup of yogurt, and a glass of orange juice—every day!

And that’s not all you need. To absorb the calcium you eat, you have to take in a variety of other vitamins and minerals, including lots of vitamin D.

In the summertime, you get vitamin D from sunlight on your skin. Where I live in Minnesota, though, it’s too dark and cold much of the year to spend a great deal of time outside. To get the recommended 400 to 800 international units of vitamin D recommended for people my age, I drink 2 cups of fortified milk every day, and I take a vitamin supplement. The American Academy of Pediatrics now recommends that teenagers take a daily multivitamin that has 200 international units of vitamin D.

Getting enough exercise is also crucial. “There have been some really excellent studies showing the effectiveness of weight-bearing exercise and strengthening exercises on bone density,” Coupey says.

Any exercise at all is better than sitting in front of the TV. Walking and lifting weights, in particular, are great for building muscles that support and strengthen bone. Playing soccer, tennis, or basketball are also good options.

A recent study found that elementary school girls who did jumping exercises for 10 to 12 minutes, three times a week, built 5 percent more bone mass than did girls who didn’t do the exercises. That’s enough bone mass to buy women some extra bone strength later in life, said the scientists from the University of British Columbia who did the study.

Even if you’re glued to the TV set, why not do some jumping jacks during the commercials? Have a glass of milk or fortified juice and some almonds instead of a can of soda and chips.

The changes are small, but the payoff could be big. You might even be amazed at how good it feels to take care of your bones. Support them, and they will support you for many years to come.

Going Deeper:

Word Find: Strong Bones

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