This study is the first to describe physical fitness data in relation to body composition and objectively measured physical activity in a diverse sample of adolescent girls. In this cross-sectional analysis, quintiles of fatness and BMI were associated with fitness (kg·m·min−1·kg−1). The assessment of cardiorespiratory fitness in this study was non-weight-bearing, submaximal test (PWC170). Some researchers have suggested that examining fitness relative to body weight penalizes heavier individuals. In primary analysis, we expressed fitness in terms of fitness (kg·m·min−1·kg−1), and we used fat-free mass to account for size differences among girls in their muscle mass. Accounting for the fat-free mass × race interaction adds significantly to the variation among girls in their fitness level after accounting for differences in fat-free mass.
Consistent with previous research that has included diverse samples, our study findings suggest that black adolescent girls have lower fitness levels than their white counterparts (5
). Pivarnik et al. (26
) examined aerobic fitness levels among black and white adolescents with a mean age 13.5 yr participating in a motorized treadmill exercise test. Their findings suggest that the treadmill time to exhaustion among black adolescents was significantly less than their white counterparts at any fat-free mass. Similar racial trends have been found among younger (18
) and older populations (5
Our findings indicate that body composition affects fitness, in addition to race. It is clear from the scatter plots (, ) that the interaction is apparent for higher levels of fat-free mass, fat mass, and BMI, and we speculate that black girls with higher fat-free mass and fat mass may have been less motivated to continue with submaximal testing, resulting in lower PWC-170 than expected based on their body composition. As well, black girls could have had a higher perceived effort which may also contribute to discontinuing cycling at an earlier time. Although not examined in our study, iron deficiency anemia has been associated with reduced physical fitness (14
). Some black adolescent girls may have iron deficiency and the lower fitness levels may have been the result. In addition, Shaibi et al. (31
) measured aerobic fitness in a multiethnic adolescent group using a progressive treadmill test. Results show a reduced
peak in blacks when compared to whites after adjustment for body fat measured by DXA, gender and maturation (31
). Thus, there may be factors aside from body composition that are related to lower fitness in black girls.
Our study results suggest that black girls also were engaged in less moderate to vigorous levels of activity than other ethnic groups, findings consistent with other research (9
). Research by Felton et al. (9
) suggests that black adolescent girls report lower physical activity, both vigorous and moderate-to-vigorous intensity, than their white counterparts. The lower activity levels may explain, to some extent, the lower fitness levels also evident in blacks.
There are limitations to our study. First, measurement of fitness occurred only at one time, 2 yr after an intervention was implemented at half of the schools. No fitness measurements were taken at the time of the baseline; thus, we were not able to examine how changes in physical activity were related to changes in fitness or how changes in fitness were related to changes in measures of body composition. Second, measurements were obtained on only a small subset of girls at each school. With a larger study it may have been possible to detect more subtle cross-sectional relationships. Also, differences among other ethnic groups may have been found with a larger sample size. Third, cardiorespiratory fitness as measured with a submaximal test using HR is an indirect fitness measure and has lower validity than direct measurements. Fourth, the fitness test was of rather short duration. Measurements were obtained for only 6 min and were used to predict a workload at a HR of 170 beats·min−1. It is more difficult in the school environment to use a higher duration or intensity fitness test. Finally, it was necessary to provide extra motivation to get girls to exercise to a HR of 165 beats·min−1. Some girls might not have worked as hard as possible.
The findings from our study add to previous adolescent fitness research concerning ethnic differences. The large sample of ethnically and geographically diverse 6th grade girls is a unique contribution. The few studies which have examined physical fitness and the influence of body composition and ethnicity have not also accounted for the interaction of physical activity levels. Additional research examining longitudinal associations with these variables will further enhance our knowledge derived from our cross-sectional study.