Our study, which is the first large general population-based study of incident stress fractures among adolescent and young adult girls, found that approximately 4% of the girls developed a stress fracture. Because it is the first prospective study in a general population sample, it is difficult to compare our results with estimates from other studies. Not surprisingly, we observed a lower incidence than has been observed in smaller studies of collegiate athletes38
, who represent a high risk group.
Age at menarche was a significant predictor of risk of developing a stress fracture. The older a girl was when menses began, the higher her risk. This result was not unexpected since there is a pronounced increase in BMD towards the end of the pubertal growth spurt, which is typically when menses begin.2
BMD is one of the main determinants of a bone’s ability to withstand loading.2,17
Moreover, a later age at menarche would cause a prolonged period of low serum estrogen, which should increase the risk of developing a stress fracture. However, the results from other studies have been mixed. Bennell and colleagues prospectively studied a small sample of young adult female track-and-field athletes and found that lower bone density and a history of menstrual disturbances were significant predictors of stress fracture.22
Whereas, a prospective study of 50 U.S. collegiate track and field athletes found that menstrual history was not predictive of stress fracture24
and other studies have been too small to detect associations.39,40
Despite our large sample size, we did not observe associations between weight status (i.e., being underweight or overweight) or disordered eating with risk of stress fracture. Thus, our results are not consistent with those from some previous cross-sectional41
studies. Barrack et al.42
failed to observe an association between disordered eating and low BMD among adolescent females. Since low BMD is related to fracture risk in children,43
our findings are consistent with several other studies not findings an association between disordered eating and stress fracture or high risk of stress fracture. Moreover, only lowest adult weight, but not current weight, was associated with risk of stress fracture in a prospective study of 3,758 female military recruits undergoing basic training.44
It is possible that we did not observe an association with disordered eating or body weight because we had a relatively small number of females who were underweight or engaging in disordered eating, and we controlled for family history of low bone density and osteoporosis, which others have found to be a strong predictor of risk.40
Approximately 80% of the variability in BMD is attributable to genetic factors.2
Thus if the risk associated with being underweight is small and correlated with the weight and family history of the mother, controlling for family history would attenuate the association with stress fracture.
Our strongest findings relate to the association between physical activity and fracture risk. These results have important practical implications. We observed that girls engaging in eight or more hours of activity per week were about twice as likely as their less active peers to develop a stress fracture. The association was driven by hours per week engaged in three activities: running, basketball, and cheerleading/gymnastics. These results are consistent with other studies that have observed high rates of stress fracture among collegiate female runners.45
Previous studies of gymnasts and cheerleaders have been too small to have sufficient statistical power, but there is a biological plausibility to such an association. Cheerleading and gymnastics involve repeatedly jumping and landing, which cause particularly high stresses on bone 46
, thus there is physiological support for our observed association. Our results imply that clinicians, athletic trainers, coaches, and others who supervise athletic programs for young female runners, basketball players, gymnasts, and cheerleaders should promote including varied training in non- or intermediate impact activities to decrease the cumulative amount of impact, as well as reducing the hours spent training in their high-impact sport.
There are several limitations to this study. Our cohort is > 90% white and thus it is not clear whether the results are generalizable to non-Caucasian female adolescents and young adults. However, Caucasians are at high risk for stress fracture 47
and are thus an important group to study. In addition, we relied on self-reports from the mothers of the participants to ascertain stress fracture. This may have resulted in some misclassification with small numbers of individuals mistakenly included as cases and a small number of cases that the mother did not know about and, therefore, did not report. However, requiring radiographic confirmation, which would been extremely difficult to obtain in this sample which lives throughout the US, might also result in some misclassification because of the difficulty in distinguishing stress fractures from stress reactions. Nevertheless, the misclassification due to self-reported stress fracture likely slightly biased our results towards the null. Also we did not have information on some of the other predictors of stress fracture, such as factors related to biomechanics and bone microarchitecture. The strengths of the study, however, far outweigh the limitations. This is the first large prospective study of stress fracture. Moreover, this is the largest prospective study of adolescents. Another strength is that exposure information was collected every 12–24 months and was ascertained before the onset of the stress fracture. We also had information on the mother’s history of low bone density and osteoporosis, as well as information from the participant on age at menarche, body weight, disordered eating, and physical activity.
Our study observed that high impact activities, specifically basketball, running and gymnastics/cheerleading, significantly increase risk for stress fracture among adolescent girls. Thus, there is a need to establish training programs that are rigorous and competitive, but include varied training in lower impact activities to decrease the cumulative amount of impact in order to reduce the risk of stress fracture.
More research is needed to understand whether dietary intake, disordered eating, and weight history affects risk. Physical activity during childhood and moderate activity during adolescence may increase bone density and thus help to protect against osteoporosis in adulthood. Therefore, clinicians, parents, and coaches should continue to promote activity to young girls, but should make sure that training hours are not excessive and thereby, not compromising bone health. This is particularly critical since youth sports have moved away from playing a different sport in each seasons to focusing on a single sport throughout the year.