Girls who spend lots of time playing high-impact sports including gymnastics and basketball are twice as likely to suffer a small crack in their bones than girls who spend less time exercising, or those who play low-impact sports, says a new study.
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Stress fractures are especially worrisome in adolescent girls because puberty is an important time for bones to strengthen, which wards off osteoporosis later in life.
"I certainly don't want to be discouraging any girls from becoming active," Alison Field, the article's lead author who studies adolescent medicine at Children's Hospital Boston, told Reuters Health.
"The problem is there are some girls that are too active," she said. When that's the case, "your body's just not getting the time it needs to heal. And that's particularly true for stress fractures."
The small cracks generally occur in weight-bearing bones in the leg and foot and develop over time when bones don't have a chance to repair themselves during ongoing physical activity.
In the current study, published in Archives of Pediatrics & Adolescent Medicine, Field and her colleagues surveyed almost 7,000 girls ages 9 to 15 over a period of 7 years. Every 1 or 2 years, girls recorded how many hours a week they spent participating in a variety of sports and other physical activities.
At the end of the 7 years, girls' mothers were asked if their daughters had been diagnosed with a stress fracture during the study period. All mothers were registered nurses who were participating in another long-term health study.
A total of 267 girls, or about 4%, were diagnosed with a stress fracture. Spending 1 to 2 hours a day playing any sport meant that girls were twice as likely to get a stress fracture compared to those that spent about 30 minutes or less being active every day.
When the authors examined stress fracture risk by sport, they found that only participating in high-impact activities - basketball, cheerleading, gymnastics and running - made girls more likely to get a stress fracture. The researchers calculated that each extra hour spent playing a high-impact sport every week increased a girl's risk of getting a stress fracture by about 8%.
Girls who played sports considered "medium-impact," such as baseball or hockey did not have an extra risk of stress fracture, nor did those who participated in non-impact activities such as biking and swimming.
Field said that the findings were especially troubling given that more young athletes are specializing in one sport and putting strain on the same muscles and bones every season.
The authors did not find that having low body weight or symptoms of an eating disorder were linked to stress fracture risk, which was a surprise, Field said. But they did find that girls who got their period at a later age were more likely to suffer a stress fracture - and first periods are often delayed in girls who are very thin.
For girls especially, variety is an important part of athletic training, Field said. Runners, for example, can spend some of their time cross-training on a bike or elliptical trainer to use different muscles and avoid damage to their bones, she said.
While some weight-bearing activity is good for bones, the message for young athletes with high-intensity training plans and their coaches is: "can we bring that down a level?" Field said.
"What we're seeing is that some kids are spending a phenomenal amount of time engaged in sports," she added. "As with most things, you can do too much of a good thing."