Bellew, James W. PT, EdD; Gehrig, Laura MD, FACOS
An increasingly prevalent, latent pathology, osteoporosis is characterized by low bone mass and volume, compromised architectural stability, and increased risk of fracture. While osteoporosis is a widely recognized pathology, it continues to be thought of as a disease of old age. While current medical practice remains focused on minimizing further bone loss and managing osteoporosis-related fractures, considerably less is being done to address prevention of osteoporosis. Ironically, this is where conventional consideration of the disease as one of old age and evidence of maximizing bone density collide.
Prevention of osteoporosis through physical activity has gained more attention in recent years.1 As a means of prevention and intervention, physical activity has been shown to positively influence properties of bone throughout the life span.1,2 During childhood when bone development is greatest, physical activity increases bone accrual, while in middle age, physical activity lessens menopause-related bone loss, and, finally, in the later years, physical activity aids in partial restoration of previously lost bone.1 Recent data suggest, though, that the skeletal system may be most responsive to the stimulus of physical activity during the early childhood growth periods—when bone growth and expansion are greatest.1,3–6
The prevention of osteoporosis in advanced age necessarily requires the recognition that bone integrity is a product, at least in part, of factors determined during childhood. To reduce the risk of developing deficient bone mineral density (BMD) in later adulthood, conventional thinking suggests maximizing BMD in adolescence when individuals are most capable of performing the intense physical activities that are known to increase BMD. The role of physical activity in the formation and preservation of BMD appears to be supported by the literature. Differences, however, in bone development associated with participation in various sporting activities in young females are not fully understood and require greater attention.
Swimming, soccer, and weight lifting are popular activities attracting children even at very young ages.1,7–9 The amount of skeletal loading, especially in the lower extremities, in these sports varies greatly as swimming is non-weight-bearing, weight lifting involves additional skeletal loading beyond body weight but without impact, and soccer involves repetitive impact loading of the skeleton. The extent, however, to which the bony stresses from these sports affect BMD in adolescent females is not clear. Therefore, the intent of this study was to compare BMD of adolescent female athletes involved in swimming, soccer, or weight lifting to determine whether the differences in bony stress inherent to each of these sports are manifested in measures of BMD.
Sixty-four female athletes 10 to 17 years of age involved in swimming, soccer, or Olympic-style weight lifting were examined. Swimmers (n = 29) were recruited from a local city-sponsored swim team that competes at both the intra- and interstate level. Soccer players (n = 16) were recruited from a local soccer training club and all were part of school-sponsored teams and select traveling teams playing year round. The weight lifters (n = 19) were competitors examined during the U.S.A. Weightlifting National Junior Olympic/School Age Championships. Subjects were included only if they had been involved in their respective sport for at least one year, trained at least five hours per week and a minimum of 10 months per year, and were not currently involved in any other organized sport. This investigation was approved by the Institutional Review Board of the Louisiana State University Health Sciences Center. Because all subjects were minors, written informed consent to participate was obtained from both the subjects and the parents or legal guardians of the subjects.
Areal BMD (g/cm2) of the dominant limb calcaneus was measured in a single session using the Lunar PIXI™ densitometer. The validity of the PIXI has been calculated at r = 0.990; SEE = 0.015 g/cm2.10 Prior to data collection, our densitometer was calibrated using known densities (Lunar Phantoms™) to an accuracy of greater than 99% and reliability testing on 20 young adult females in our lab yielded an intraclass correlation coefficient (ICC 3,1) of 0.98.
SPSS (v. 12.0 for Windows) was used for analyses. Conventional statistical methods were used for the calculation of means and standard deviations of BMD for each group. Two methods of analysis were used to examine potential differences in BMD. First, comparisons between sport groups were completed using a univariate analysis of covariance with body mass index (BMI) and age as covariates because of the known relationship between body weight, height, age, and bone density. Second, each sport group's mean BMD was compared to the normative value for adult females 25 years of age from the World Health Organization (WHO) using one-sample t tests.11
Demographic data including training and physical characteristics for each sport group are shown in Tables 1 and 2. Between-sport comparisons of BMD are presented in Figure 1. These comparisons show sport type is a significant factor (F = 6.21, p = 0.004, power = 0.878). BMD in the soccer group was significantly greater than the weight-lifting group (p = 0.025) and swimming group (p = 0.001) with no difference between the weight-lifting and swimming groups (p = 0.209). Compared to normative data from the WHO for adult females (0.500 g/cm2), the soccer group was the only sport with BMD significantly greater than adult norms (p = 0.003), while those of the swimmers were significantly less (p < 0.001) than adult females, and the weight-lifters were not different (p = 0.103) from the WHO norms (Fig. 2).
Participation in physical activity or regular sporting activity during childhood is associated with increased BMD when comparing active to inactive groups. However, the degree to which different sport activities influence bone development is not fully understood.12,13 The findings of this study provide more specific information suggesting that the inherent differences in the magnitude of skeletal loading between sports—more specifically, swimming, soccer, and weight lifting—are reflected in the BMD of adolescent female athletes.
The difference in skeletal loading between the three sports examined served as the primary factor in selecting these sports for study. Swimming represents a non-weight-bearing sport without direct impact loading to skeletal structures while weight lifting exposes the skeleton to intermittent loads in excess of normal body weight but lacks the repetitive, prolonged impact and ground reaction forces seen with soccer. The evidence from this study and the differences in the subjects' BMD in these three sports provide support for previous data suggesting that the magnitude of skeletal loading is critical to bone development.
Previous investigations have examined the effect of sport and exercise on BMD in prepubertal and adolescent female athletes.14,15 To date, activities producing minimal impact (ie, ground reaction forces) or providing minimal skeletal loading, such as swimming, have shown no differences in BMD between young female athletes and age-matched and inactive controls.7,8 In contrast, other data suggest that activities with a higher magnitude of skeletal loading and repetitive impact, such as soccer, result in greater increased regional, as well as whole body, BMD in young females.2,9,13
Repetitive exposure to physical loading exceeding normal activities of daily life has been shown to maximize the potential for bone formation.17 The propensity for and rate of bone formation or breakdown are dependent on mechanical loading (ie, exercise) or unloading (ie, inactivity). Significant increases in bone mineral content following longitudinal studies of exercise using impact-loading activities in children have been reported in recent literature. Fuchs et al18 investigated the effect of high-intensity jumping activities on hip and spine bone mass in prepubescent children before and after a 7-month program. Significant increases in bone mineral content were noted in the lumbar spine and hip. Similarly, MacKelvie et al19 studying children nine to 12 years old, investigated the effects of a two-year randomized, controlled trial of a high-impact training program at three sessions per week for only 10 minutes. After two years, the exercise group showed a significant increase in bone mineral content of the femur and lumbar spine. Most recently, Gero et al14 examined bone accrual in premenarcheal gymnasts over one- and two-year periods. After one year, BMD was significantly greater in the forearm only of gymnasts compared to age-matched controls. After two years, BMD was 1.5- to twofold greater at the forearm, hip, and femoral neck for the gymnasts. These findings offer further evidence of the effect of impact loading on bone development, especially given the changes in forearm density in gymnasts, a sport with repetitive and impact loading of the arms.
The effect of impact and jumping activities on bone health in older women has also been examined. Kemmler et al20 reported a three-year longitudinal study of exercise on bone health but in postmenopausal women with low BMD. The training protocol used was a 14-month, progressive-intensity design comprising jumping activities, aerobic exercise, resistance training, and stretching. Significant increases in lumbar spine BMD were noted following training.
Further evidence of the sport-specific differences in BMD observed in this study is found in the comparisons to normative data from adult females from the WHO. While the mean BMD of the swimmers was significantly less than norms for adult women, the weight lifters were not different, and even more surprising is that the soccer players showed a mean BMD significantly greater than WHO norms. What is most significant is that these WHO norms are for adult women 25 years of age and the subjects in this study are still preadolescent or in early adolescence. That the weight lifters, with a mean age of 14 years, were not different from adult females of 25 years suggests that weight lifting, with skeletal loading in excess of body weight but with limited impact, imparts some benefit to bone development such that at their current age, this group has a BMD similar to that of adult women. Likewise, the greater BMD of the soccer players compared to WHO norms suggests additional benefit of repetitive and substantial impact loading of the skeleton. That the weight lifters and soccer players in this study are in the prime ages for bone development and expansion and already have BMDs comparable to or exceeding those of adult females is evidence of the role of these sports for bone health.
Whether bone density in each of these three sport groups will continue to increase is unknown. At their present age, the swimmers have a mean BMD less than that of adult females. Perhaps the swimmers will reach BMD measures consistent with normative adult levels by adulthood, but this is speculative at best. That the BMD of the swimmers was less than adult norms should not be surprising if considered simply in the context of age. But when comparing female athletes of similar age from soccer and weight lifting whose BMD meets or exceeds adult norms, greater consideration of sport selection during adolescence is warranted.
In any study, the ability to accurately detect differences between groups is critical to the credibility of the results. The statistical power of an experiment represents this credibility. The power of this study was 0.878, which suggests that the probability was 88% that we would correctly identify a difference in BMD between these sport groups if such a difference existed. For the comparison of BMD between sport groups, the null hypothesis would state that there are no differences in BMD between the sport groups. The data of this investigation convincingly fail to support the null hypothesis and suggest that the inherent differences in skeletal loading among swimming, soccer, and weight lifting are manifested in BMD.
A potential criticism of this study may be that the differences in BMD noted between the three sport groups may not be due to differences in sport-specific skeletal loading but rather due to a genetic predisposition to higher BMD and more robust musculoskeletal systems. This may increase the likelihood of choosing participation in an impact sport such as soccer or a high-intensity anaerobic sport such as weight lifting versus a nonimpact sport such as swimming.21 However, findings of greater BMD in the playing arms versus nonplaying arms of athletes from asymmetric sports, such as racquet sports, lends support to the findings of this study—that impact-loading activity promotes bone accrual to a greater degree than nonimpact sports.3,4
While considered a disease primarily of the old, prevention of osteoporosis can begin in the preadolescent and adolescent years. Because there currently is no known cure for osteoporosis, prevention becomes paramount and the period of adolescence, when bone growth is most robust, appears to be a period of significant importance in the development of BMD. Technological advancements in passive, electronic home entertainment in the past two decades have undoubtedly had a profound impact on the participation of modern youths in sports and physical activity. Participation in sport and physical activity should be encouraged. In the context of bone health, the results of this study offer support for participation in impact-loading sports such as soccer or sports such as weight lifting with skeletal loading in excess of body weight.
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