Taylor Custer's gymnastics career has been dominated less by vaults, uneven bars and balance beams than by braces, casts and crutches.

At 14, and as a freshman-to-be at Hudson High School, Custer has managed to compete in only one full season in the last four years.

The problem: stress fractures, sometimes in her ankles, sometimes in her feet and most recently in her shins.

''It's very frustrating,'' she said. ''I always get upset when I'm injured. It's rough. I don't get to compete.''

Some might see this as a sign to hang up the leotards and find a less demanding sport.

Not Custer. And not her doctor.

Dr. Keith Loud, a specialist in adolescent health and sports medicine at Akron Children's Hospital, has found that in many situations like Custer's, it's not the gymnastics (or other demanding sports) causing stress fracture problems, it's heredity.

In a study published in the current issue of the journal Pediatrics, Loud found that a family history of low bone density more than any other factor increased adolescent girls' risk of stress fractures.

A family history of osteoporosis or osteopenia made girls three times more likely to suffer stress fractures. No other factors not training volume, training intensity, or menstrual irregularities put girls at an increased risk.

The take-away message from these findings, Loud said, is not to discourage sports for girls with such a family history. In fact, it's just the opposite.

''We want girls to exercise,'' he said. ''Impact-loading exercise is good and should be encouraged for all, including adolescent girls.''

Impact-loading or weight-bearing exercises include those that require the athlete to support their own body weight, especially any sport that includes running and jumping.

''We know that weight-bearing exercise is good for building bones and adding calcium to bones, but there can be a downside,'' Loud said. ''People are always wondering how much is too much. At what point do we undo the positive effects of exercise?''

In other words, when does the constant pounding of a sport start to do more harm to the bones than good?

After all, a stress fracture occurs because a repetitive motion slowly causes the bone to break down over time.

In the general population, stress fractures are rare, with a lifetime prevalence of about 2.7 percent in adolescent girls. However, among female athletes, somewhere between 6.9 percent and 21.1 percent suffer stress fractures.

More research is needed to find out at what point exercise becomes damaging to bone, Loud said. In his study, girls who exercised more than 20 hours a week showed a dramatically greater risk of stress fracture, while girls who exercised more than 16 hours showed a slightly higher risk.

Custer is right around that 16-hour-a-week range, with four practices a week year-around.

She's now taking calcium supplements, prescription-strength Vitamin D and over-the-counter Vitamin C, while using exercise bands to keep the muscles around her ankles and shins strong. She's been pain-free for the past four months and is looking forward to competing in the upcoming season with West Side Gymnastics in Copley Township.

Her mom, Nichole Becher, said it's unclear whether a family history of low bone density is contributing to her daughter's problems. ''I've never had (a bond density test) done, though I probably should,'' she said. ''I've never really had any fractures like this.''

Another key finding in Loud's study was that the girls who suffered stress fractures had bone density scores lower than the average of the general population, which is less active overall than the girls in the study.

''You would expect an athletic population to be higher than the norm,'' Loud said.

That finding signals a need for girls with stress fractures to see a doctor and have their bone density tested, because they likely have a long-term risk of osteoporosis.