Exercise may be good for the body, but too much of it may be bad for the bones.
Teenage girls who participate in high-impact physical activity -- such as basketball, running, gymnastics and cheerleading -- have a higher risk of developing stress fractures, according to a report in the online edition of the Archives of Pediatrics & Adolescent Medicine.
Young girls with family histories of osteoporosis have an even greater risk, according to the study.
"Weight-bearing activity stimulates bone remodeling and thus increases bone mass density, but very high levels of activity may be detrimental to bone health and increase the risk of stress fractures," the authors of the study wrote.
Stress fractures are still pretty uncommon, affecting about 20 percent of young girls.
The study looked at 6,831 girls between the ages of 9 and 15 over an eight-year period beginning in 1996. During that time, 267 girls, or 3.9 percent, developed a stress fracture.
Girls reporting a family history of osteoporosis or low bone mass density were almost twice as likely to develop a stress fracture.
Girls who participated in eight or more hours of physical activity a week were also twice as likely to develop a stress fracture as those who engaged in less than four hours of activity per week.
Specifically participating in basketball, running, and gymnastics/cheerleading led to the fractures.
The older a girl was when she started her menstrual period also resulted in an increased risk of developing a stress fracture.
The study concluded that parents should continue to encourage young girls to participate in sports, but to make sure they are not training excessively.
The study was conducted by Alison E. Field, of Brigham and Women's Hospital and Harvard Medical School in Boston, along with her colleagues.