Impact of a School-Based Physical Activity Intervention on Fitness and Bone in Adolescent Females

Background : Many female adolescents participate in insuf ﬁ cient physical activity to maintain cardiovascular ﬁ tness and promote optimal bone growth. This study evaluates the impact of a school-based intervention on ﬁ tness, activity, and bone among adolescent females. Methods: Subjects were assigned to an intervention ( n = 63) or comparison ( n = 59) group, and underwent assessments of cardiovascular ﬁ tness (VO 2peak ), physical activity, body composition, bone mineral density (BMD), bone mineral content (BMC), and serum markers of bone turnover at baseline and at the end of each of two school semesters. Results: The intervention increased physical activity, VO 2peak , and BMC for the thoracic spine ( P values < 0.05). Bone turnover markers were not affected. In longitudinal analyses of the combined groups, improvements in cardiovascular ﬁ tness predicted increased bone formation ( P < 0.01) and bone resorption ( P < 0.05). Conclusion: A school-based intervention for adolescent females effectively increased physical activity, cardiovascular ﬁ tness, and thoracic spine BMC.

Epidemiological trends among adolescent females in the US suggest that the future health of the current generation is seriously at risk owing to the effects of physical inactivity. Data from the Youth Behavioral Risk Factor Survey 1 indicate that 37% of 9th grade females and 54% of 12th grade females are insuffi ciently active to maintain cardiovascular fi tness. Among adults, physical inactivity increases the risk of obesity, 2 diabetes, 3 and osteoporosis. 4 Given that activity patterns established in youth tend to track into adulthood, 5,6 it is critical that steps be taken to reverse the downward trend in physical activity presently evidenced among adolescents in general and adolescent females in particular.
Schools offer a natural environment for intervening to increase physical activity among adolescents, and a number of studies support the effi cacy of schoolbased programs for doing so. [7][8][9] Most of these studies have been implemented and evaluated on a school-wide basis, rather than targeting high-risk groups, although a few have targeted overweight children. 7,10 Within the literature, there has been very little work addressing the needs of female adolescents who are insuffi ciently active to maintain or promote physical fi tness; a subgroup at heightened risk for a future sedentary lifestyle and its associated health problems. Thus, the present research set out to determine whether a school-based intervention to promote physical activity and physical fi tness would be effective among insuffi ciently active adolescent females.
The Task Force on Community Preventive Services 11 has strongly recommended using physical education (PE) classes to promote physical activity and fi tness in school-age youth, and several studies involving curricular change in PE have published encouraging but generally modest fi ndings. 7,9,12 For example, an 8-wk school-based intervention with junior high school students 9 had small but positive effects on cardiovascular fi tness. A 16-wk school-based intervention for overweight and low-active high school females 7 resulted in a positive shift in stage of change for exercise but had no effect on levels of physical activity. Another study of low-active female adolescents 12 found that a one-semester PE-based intervention increased vigorous physical activity and maintained cardiovascular fi tness. An intervention of a longer duration may be required to demonstrate an absolute increase in fi tness within adolescent females who are insuffi ciently active to maintain physical fi tness.
School-based interventions also have been associated with signifi cant increases in bone density 13 and improved bone mineral content 14 in children, but the osteogenic effects of exercise in post-pubertal adolescents are less well documented. Some researchers 15,16 have suggested that the "window of opportunity" for enhancing bone growth with physical activity has largely closed by mid-to late-adolescence; yet a few trials have produced suggestive evidence that vigorous activity is capable of enhancing bone growth in post-pubertal adolescents. 17,18 These initial fi ndings regarding the role of physical activity in bone development among post-pubertal adolescents are not compelling, however, and further investigation is warranted to determine whether increasing activity in post-pubertal adolescents will have a benefi cial impact on bone.
A multifaceted PE-based intervention was designed to increase physical activity and improve physical fi tness among post-pubertal adolescent females. The program combined supervised physical activity with an educational component that encouraged subjects to increase their out-of-school activity. Moreover, in order to examine the population for whom increased activity is the most relevant, we focused on insuffi ciently active female adolescents with relatively low cardiovascular fi tness. Thus, the present study investigates the potential for intervening during post-pubertal adolescence to enhance physical activity participation and physical fi tness among insuffi ciently active adolescent females. Secondary outcomes included body composition (percent body fat, lean body mass, and bone mass) and serum indicators of bone turnover.

Study Design
Two public high schools within a single school district participated in the study. Both schools were similar in terms of facilities and equipment available to the students, and in terms of the structure of the physical education programs. One school was assigned, based on convenience factors, to receive the intervention, and the other functioned as a comparison group. Students were recruited after the assignment of schools to groups, and so were informed in advance whether or not they would be participating in the intervention. To maintain a manageable class size for the intervention, the study was conducted over three consecutive school years, involving sequential cohorts of n = 43 (year 1; 22 intervention and 21 comparison), n = 39 (year 2; 20 intervention and 19 comparison) and n = 40 (year 3; 21 intervention and 19 comparison). The comparison group received no instructions with regard to physical activity, and school records were reviewed to document enrollment in PE.
To minimize group differences in calcium consumption, each subject was asked to consume one fl avored 500 mg calcium chew (Viactive) per day. Average daily dietary calcium intake estimates range between 850 to 1200 mg for adolescents 19 and pilot data for this study indicated a mean daily dietary calcium intake of 800 mg among insuffi ciently active adolescent females (unpublished data). Therefore, a supplement of 500 mg was determined to suffi ciently boost subjectsʼ average daily intake to the recommended level of 1300 mg. 20 Assessments were conducted at a university-based general clinical research center at baseline (summer), semester 1 (the end of fall semester) and semester 2 (the end of spring semester). Intensity and duration of the supervised exercise sessions were estimated using periodic heart rate monitoring. Dietary calcium consumption was measured at baseline and semester 2, and compliance with calcium supplementation was assessed monthly via telephone call. Study subjects received monetary compensation for completing the clinic assessments, and schools awarded community service credit to subjects who completed all three clinical assessments. The study protocol was reviewed and approved by the universityʼs institutional review board, and all subjects and their parent or guardian provided written informed consent.
Recruitment and Setting. The two participating schools were similar demographically (see Table 1). Subject inclusion criteria were: 1) enrollment in the 10th or 11th grade; 2) fewer than three 20-min bouts per week of vigorous physical activity and fewer than fi ve 30-min bouts per week of moderate physical activity; 3) VO 2peak at or below age-specifi c 75th percentile; 4) ability to exercise without restrictions; 5) eumennorheic; and 6) not taking any medications known to infl uence bone health (e.g., steroids).
At baseline, 146 eligible subjects enrolled in the study [n = 79 (intervention); n = 67 (comparison)]. Subsequently, 6 students in each condition changed schools, and so dropped out of the study. Fifteen intervention students were removed from the study because they missed more than 10 d of the PE class in the fi rst 2 months of the study (n = 6) or because they exhibited a negative attitude that threatened to infl uence other studentsʼ participation in class activities (n = 9). One intervention student was dropped because she joined a sports team, and two comparison group subjects discontinued participation. Final analyses were conducted on 122 adolescent females [n = 63 (intervention); n = 59 (comparison)].
Intervention. The intervention goal was to increase studentsʼ levels of physical activity through supervised in-class activity, health education, and Internet-based self-monitoring. The intervention class met 5 d per week for 60 min each day (approximately 40 min of activity time). One day per week was devoted to an educational discussion related to the health benefi ts of exercise and strategies for adopting an active lifestyle. Supervised activities were selected based on student input, and included a variety of aerobic (3 times per week, including aerobic dance, kickboxing, and brisk walking) and strength-building (1 time per week, including weightlifting and yoga) activities. A detailed description of the class has been provided elsewhere. 8

Measures
Cardiovascular Fitness. Cardiovascular fi tness was obtained through a ramptype progressive exercise test on an electronically-braked cycle ergometer. Subjects were encouraged to maintain a pedaling rate of 70 rpm during the test phase of the protocol. The ramp power output increased continuously until subjects reached voluntary fatigue. The test portion of the protocol lasted between 8 to 12 min. Each test was followed by an appropriate cool-down period. Peak oxygen consumption (VO 2peak in L/min and VO 2peak in mL · min -1 · kg -1 ) was obtained using the Sen-sorMedics Vmax 229 metabolic cart (SensorMedics, Yorba Linda, CA), through a method designed for children and adolescents. 21 Gas exchange was measured breath-by-breath throughout the exercise protocol. 22 As noted by Vanhees et al. 23 "in the laboratory, exercise capacity is preferentially assessed through maximal incremental exercise testing. . . The peak oxygen uptake is the gold standard in the assessment of exercise tolerance." (p. 102) The majority of the activities practiced by the intervention subjects in this study were intended to increase cardiovascular fi tness; thus, we believe that the cycle ergometer test was appropriate for assessing whether improvements in fi tness had occurred as a result of the intervention.
Physical Activity. Self-reported physical activity was measured using a 3-Day Physical Activity Recall (3DPAR) validated by Motl, Dishman, Dowda, and Pate. 24 Activities were converted into metabolic equivalents (METs) using the compendium published by Ainsworth et al., 25 and grouped to calculate the average daily minutes spent engaged in moderate (between 3 and 6 METs) and vigorous (greater than 6 METs) activity. Steps were taken to ensure that the 3DPAR excluded any supervised activity that was part of the intervention.
Bone Mass and Body Composition. Percent body fat, lean body mass and bone mass [i.e., bone mineral content (BMC in g) and bone mineral density (BMD in g/cm 2 )] were assessed by dual X-ray absorptiometry (DEXA) using a hologic QDR 4500 densitometer (Hologic, Inc. Bedford, MA). A series of scans were performed by a licensed X-ray technician and analyzed using software designed for a pediatric population. Standard procedures were employed to yield BMC and BMD values for the following regions: lumbar spine (L1-L4), hip, total body, thoracic spine, femoral neck, and trochanter. Subjects were scanned in a hospital patient gown while lying fl at on their backs. On each day of testing, the DEXA machine was calibrated using the procedures provided by the manufacturer. Height was measured to the nearest 0.01 cm using a stadiometer and weight was measured to the nearest 0.1 kg using a calibrated scale. Bone Turnover. Subjects had blood drawn from an antecubital vein to assess levels of serum indicators of bone turnover. Serum was immediately isolated and small aliquots prepared and stored at -80 °C until needed. Bone formation was measured using ELISAs for osteocalcin (sensitivity 0.4 ng/mL), bone-specifi c alkaline phosphatase (BSAP, sensitivity 0.7 U/L), and C-terminal procollagen peptide (CICP, sensitivity 0.2 ng/mL). Bone resorption was measured using ELISAs for deoxypyridinoline cross-links (PYD, sensitivity 0.4 nM/L). All assay kits were obtained from QUIDEL, Inc., Santa Clara, CA.
Intensity and Duration of PE Class Activity. Heart rates were monitored for one class period every other week within the intervention class using a Polar heart rate monitor. This device consists of an elastic belt, worn around the lower portion of the chest, and a watch-like receiver worn on the wrist. Monitors recorded average heart rate and number of minutes at or above 120 beats per min. Monitoring sessions varied by day of the week in order to obtain an equal representation of the different class activities. Students participated in the monitoring on a rotating basis.
Dietary Intake. Daily caloric consumption in kilocalories and daily calcium consumption were measured with a 3-d diet diary (one weekend day and two weekdays) at baseline and semester 2. Registered dieticians trained subjects to complete the diary, contacted each subject midway through the assessment period to answer questions, and reviewed diaries with subjects upon completion. Data were analyzed using First Database Nutrition Pro.

Calcium Supplementation Compliance.
Compliance with calcium supplementation was assessed monthly via a telephone call. Researchers asked subjects to report the number of days in the past week that the calcium supplement was taken (0 to 7 d). Average compliance was calculated for the 9-month intervention.

Data Analyses
Non-normal distributions required transformation of several variables prior to analyses: moderate activity (square root transform); vigorous activity (dichotomized into "some" and "none"); CICP and trochanter BMC (log transforms). To account for the dependence of observations within each study group, baseline comparisons and group × time interactions were assessed using multi-level random coeffi cient modeling (HLM, version 6.0, Scientifi c Software International, Lincolnwood, IL). 26,27 The extent to which the intervention infl uenced vigorous activity ("some" versus "none") was tested using the hierarchical generalized linear model (HGLM) function, a non-linear analysis for binary outcomes using the Bernoulli distribution. All multi-level models controlled for cohort (i.e., year 1, year 2, or year 3) and ethnicity. Additional covariates included baseline percent body fat (for models testing baseline group differences and the group × time interactions for moderate, vigorous, and VO 2peak ), height and weight (for models testing baseline group differences in BMC, BMD, and serum indicators of bone turnover), and changes in height and weight (for models testing group × time interactions for BMC, BMD, and serum indicators of bone turnover). A more conservative approach of using robust standard errors was taken for all analyses owing to the fact that many of the outcome variables were moderately skewed. Multiple regressions examined whether individual improvements in cardiovascular fi tness and physical activity predicted benefi cial changes in secondary outcomes when the intervention and comparison groups were combined (N = 122). These analyses also controlled for changes in height and weight.

Participation in Physical Education Within the Comparison Group.
Twentyfour percent (n = 14) of the comparison group subjects were enrolled in PE for one semester, and 76% (n = 44) were enrolled in PE for two semesters during the intervention year (data were missing for one comparison subject). ANOVAs showed that whether comparison group subjects were enrolled in PE for one or two semesters had no effect on change over time in any of the study outcome variables (P values > 0.05). Calcium Intake and Compliance. Daily dietary calcium intake decreased by about 20 mg in both the intervention and the comparison groups; there was no signifi cant difference between the two groups (P > 0.10). Similarly, overall compliance with daily calcium supplementation was comparable between groups, with subjects reporting consuming the supplement an average of 5 d per week.

Effect of the Intervention
Cardiovascular Fitness, Physical Activity, and Body Composition. Improvements in VO 2peak and vigorous activity were signifi cantly larger for the intervention group (Table 2; P values < 0.001). Results of the analysis did not change when VO 2peak mL · min -1 · kg -1 was used as the dependent variable. Within the intervention group, participation in vigorous activity increased from 59% at baseline to 84% at semester 2. In addition, average daily minutes in moderate activity remained stable in the intervention group, whereas average minutes of moderate activity declined in the comparison group (P = 0.04). The intervention group experienced a signifi cantly larger weight increase between baseline and semester 2 (P = 0.04); yet, changes in height, percent body fat, and lean body mass were similar across groups.

Longitudinal Relationships Among Fitness, Physical Activity, and Secondary Outcomes
Changes in cardiovascular fi tness across the combined groups (N = 122) were positively related to changes in BSAP (β = 0.172, P = 0.004) and PYD (β = 0.156, P = 0.025). No other relationships between changes in physical activity/fi tness and changes in secondary outcomes emerged.

Discussion
A school-based physical activity promotion intervention successfully enhanced physical activity and physical fi tness within a group of adolescent females who exhibited low levels of physical activity and fi tness at baseline. The two-semester intervention, consisting of a special PE class and Internet-based self-monitoring was found to signifi cantly impact participation in both moderate and vigorous physical activity and to improve cardiovascular fi tness as compared to a nonintervention comparison group. The intervention had very little effect, however, on bone development, with only one of the six measures of BMC and none of the measures of BMD showing any improvement. Importantly, the majority of the comparison group subjects were enrolled in regular PE classes for the duration of the intervention, suggesting that the intervention was effective in promoting activity and cardiovascular fi tness changes as compared to traditional PE. Overall, the intervention was effective in infl uencing key behavioral and physiological outcomes in a population at high risk for becoming sedentary, overweight adults, but was less effective for enhancing bone health. Prior research indicated that a one-semester intervention was not suffi cient to improve cardiovascular fi tness among insuffi ciently active adolescent females. 12 The current study shows that fi tness levels can be signifi cantly increased over the course of a sustained two-semester intervention. The fact that fi tness primarily increased in the later months of the intervention highlights the importance of sustained interventions among insuffi ciently active populations. Neumark-Sztainer et al. (2003) found that a 4-month school-based intervention did not increase physical activity among low-active high school girls, and the duration of school-based programs shown to successfully impact fi tness and activity is typically 6 months or more. 28 It is interesting to note that whereas there was a small increase over time in weight and BMI in the intervention group, relative to the comparison group, there was no differential change in percent body fat. Visual inspection of the pattern of change in lean body mass (LBM) suggests that the intervention group may have experienced a larger increase in muscle mass, although this increase was not large enough to reach statistical signifi cance (perhaps owing to the large standard deviation in LBM). We hypothesize, therefore, that the intervention stimulated greater increases in muscle mass through increased activity, and that the weight gain evidenced in the intervention group was due to enhanced LBM.
In terms of the secondary outcomes investigated, the results are ambiguous. On the one hand, the magnitude of the change in thoracic spine BMC (6% in the intervention vs. 1% in the comparison group) is comparable to or even greater than the changes in bone mass found by investigators working with pre-pubertal girls.
For example, McKelvie et al. 14 found a difference of 2 to 4% in BMC between girls who engaged in high-impact jumping three times per week for 2 y and a control group, and Heinonen et al. 15 found a difference of 3 to 4% in BMC between girls in a 9-month step aerobic program and a control group. On the other hand, the present intervention resulted in a signifi cant change in bone development in only one of the six skeletal regions analyzed. Moreover, there was no signifi cant impact of the intervention on the serum indicators of bone turnover. These fi ndings suggest that the effect of the intervention on bone development overall was limited in scope.
The minimal impact on bone does not appear to be attributable to problems with intervention fi delity. Based on heart rate monitoring, in-class activities were suffi ciently intense to elevate subjectsʼ heart rates to at least 60% of their maximum level for close to 20 min four times per week. Subjects also increased their vigorous activity outside the supervised setting. The most compelling evidence for the effectiveness of the intervention, however, is provided by group differences in cardiovascular fi tness. Over the course of the 9 months, intervention group subjects increased cardiovascular fi tness by 8%, whereas the control group decreased fi tness by 4%.
Our fi ndings are in line with controlled studies of young adult females that have failed to yield substantial improvements in bone parameters as a result of increased physical activity. 29,30 As suggested by previous authors, these results are likely due to a limited developmental window of opportunity when exercise is able to effectively infl uence bone growth. 31 Heinonen et al. 15 propose that exercise-related effects on bone are restricted to periods of rapid body growth (i.e., early and middle adolescence) when levels of insulin-like growth factor 1 (IGF-1), growth factor, estrogen, and androgen are high. In terms of reducing the risk of osteoporosis, then, it would appear that the most effective time to increase physical activity, at least among girls, would be in the pre-menarchal years.
The fi nding that individual gains in cardiovascular fi tness were associated with greater bone turnover-as indicated by signifi cant increases in serum markers-raises some interesting questions. Among older women, increased bone turnover has been related to a greater risk for osteoporotic fracture. 32 Among female adolescents, however, higher levels of formation and resorption markers have been associated with increased BMD. 33 The evidence for a relationship between increased concentrations of serum markers of bone turnover and enhanced bone density is not compelling, however, and additional research is necessary to determine whether higher circulating levels of PYD and BSAP are predictive of enhanced bone growth among adolescents.
The strengths of this study include the longitudinal design, use of standardized and clinical measures, and the focus on insuffi ciently active female adolescents. Nevertheless, the lack of random assignment and selective attrition are limitations that should be acknowledged. With respect to most of the major study criteria, the intervention and comparison groups were comparable at baseline, and we controlled statistically for those differences that did emerge. Moreover, the two schools that participated in the study were similar based on publicly available demographic and academic performance statistics. Nevertheless, there may have been unmeasured differences between the two groups or in the two school environments that affected the fi ndings. In terms of attrition, we took a proactive approach to the intervention subjects, and terminated participation for individuals who either were absent from school an unusually high number of days (thus limiting their exposure to the intervention) or manifested a behavior pattern in the intervention class that threatened to disrupt the intervention for other subjects. This proactive approach may have increased our probability of having an impact on cardiovascular fi tness among those who completed the study, and may therefore limit the generalizability of the fi ndings.
Overall, a school-based physical activity intervention signifi cantly increased levels of physical activity and fi tness, had only minimal effects on bone development, and had no effect on body composition among insuffi ciently active, postpubertal adolescent females. These fi ndings support the use of specialized PE classes (because we found that regular PE classes were ineffective) as an appropriate venue for promoting physical activity among insuffi ciently active adolescent females, and highlight the possibility that physical activity may have minimal impact on bone development in post-pubertal females.