The incidence of type 2 diabetes mellitus (T2DM) increased in children and adolescents in the United States 10-fold from 1982 to 1994 (28), paralleling the abrupt increase in the prevalence of childhood obesity (27). Along with intra-abdominal obesity, a consistent body of evidence suggests that the pathogenesis of insulin resistance in obesity and T2DM is associated with greater fat content within skeletal muscle in adults (14,24) and children (31), even after adjusting for total body adiposity. Recent studies in children with disabilities that limit physical activity (17,22) and otherwise healthy adults (9,23) have suggested a link between physical inactivity and skeletal muscle fat content. Furthermore, prospective studies in adults have reported that regular physical activity prevents excess accumulation of fat within skeletal muscle (12,29,32). Taken together, these findings suggest that lower physical activity may be partly responsible for the link between greater skeletal muscle fat content and insulin resistance. Given that sedentary behavior is a risk factor for the development of childhood obesity, insulin resistance, and T2DM (27), it is imperative to better understand the relationship between physical activity and skeletal muscle fat content in youths.
Few studies in youths have examined the association of physical activity with skeletal muscle fat content. Johnson et al. (17) recently reported that children with quadriplegic cerebral palsy have greater skeletal muscle fat content than healthy children, which may be related to their lower level of physical activity. These findings are consistent with another study in boys, age 9–12 yr, with other disabilities that limit physical activity (22). Taken together, these studies suggest that lower physical activity contributes to greater skeletal muscle fat content, even in children and adolescents. Whether lower physical activity is associated with skeletal muscle fat content in otherwise healthy youths remains unclear.
The purpose of this study was to examine the association between physical activity and skeletal muscle fat content of the calf and thigh in girls. A unique feature of the study was the use of peripheral quantitative computed tomography (pQCT) because of its ability to differentiate tissues on the basis of attenuation characteristics, which are directly related to tissue composition and density (13,19). Controlled studies in sedentary obese and T2DM groups have demonstrated that lower muscle density (mg·cm−3), which can be assessed with pQCT, is a valid measure of greater skeletal muscle fat content (13,19). On the basis of observations that lower physical activity is associated with greater skeletal muscle fat content in children with disabilities (17,22) and otherwise healthy adults (9,23), we hypothesized that lower physical activity would be associated with higher skeletal muscle fat content in girls.
Baseline data were analyzed for 464 girls, age 8–13 yr, who were participants in the “Jump-In: Building Better Bones” study (5–7). Girls who were in school grade 4 or 6 were recruited from 14 elementary and 4 middle schools around Tucson, AZ. Exclusion criteria included learning disabilities (identified by schools) that made it impossible to complete questionnaires or otherwise made the girls unable to comply with assessment protocols; medications known to affect bone, medical conditions, or a disability that limited participation in physical exercise as defined by the Committee on Sports Medicine and Fitness (2); exclusion (or excuse) from participation in physical education; and the inability to read and understand English. The protocol was approved by the University of Arizona Human Subjects Protection Committee, and the study was conducted in accordance with the Helsinki Declaration. All guardians and girls provided written informed consent.
The study population and methods for obtaining measures of body mass, height, trunk height, leg length, and nondominant femur and tibia lengths have been described in detail previously (6). Coefficients of variation (CVs) for tibia and femur lengths were 0.51% and 0.33%, respectively (n = 464). Guardians completed a questionnaire that inquired about participant ethnicity and race. Maturity was assessed from a self-report of breast development (Tanner stages) using a questionnaire that presents illustrations of stages of development that has been validated (26). Tanner staging is common in developmental studies, but its ability to accurately assess maturation is limited (35). Thus, we also used an alternative index of maturation (maturity offset), on the basis of the estimated years from peak height velocity (PHV) using an equation developed by Mirwald et al. (25), which was derived from data from a 6-yr longitudinal study in boys and girls (3). In the sample of Mirwald et al. (25), the maturity offset equation for girls explained 89% of the variance in years from PHV.
Soft tissue composition.
Soft tissue composition was assessed at the 66% tibia (calf) and 20% femur (thigh) sites relative to the respective distal growth plates of the nondominant limb using pQCT (XCT 3000; StraTec Medizintechnik GmbH, Pforzheim, Germany; Division of Orthometrix, White Plains, NY). A detailed description of the scanner and our protocol has been published (6). Briefly, scout scans were performed to locate the distal growth plates, with the scanner programmed to subsequently find the sites of interest. Scanner speed was set at 25 mm·s−1. Slice thicknesses were 2.3 mm, and voxel sizes were set at 0.4 mm. Image processing and calculation of bone parameters were performed using the StraTec software package (version 6.0). Contour mode 3 (−101 mg·cm−3) and Peel mode 2 (40 mg·cm−3) were used to separate adipose (<40 mg·cm−3) and muscle/bone (≥40 mg·cm−3), respectively. Images were subsequently filtered with a 7 × 7 image filter that clearly defined the edge of the muscle and eliminated all bone above 120 mg·cm−3. Further details on image processing, calculations, and analysis, including descriptions of the Contour and Peel modes, are published elsewhere (36). We have previously used this technique to report the relationship between skeletal muscle fat content and bone strength in our sample of girls (6). Soft tissue parameters obtained at the calf and thigh regions included muscle cross-sectional area (MCSA (mm2)) and muscle density (mg·cm−3). CVs estimated from 29 subjects scanned twice within the same day after subject repositioning, calculated as described by Glüer et al. (10), for muscle density (mg·cm−3) and MCSA (mm2) were 0.9% and 1.4%, respectively, at the calf site (6); CVs for the same parameters at the thigh were 0.4% and 1.3%, respectively (6). Total body mass, total body fat mass, percent total body fat, and total body lean mass were obtained from whole-body dual-energy x-ray absorptiometry scans using the GE Lunar Prodigy (software version 5.60.003) fan beam densitometer (GE Lunar Corp., Madison, WI). Subjects were positioned following standard GE/Lunar protocols. Dualenergy x-ray absorptiometry CVs in our laboratory estimated from 261 women scanned twice within the same day after subject repositioning, expressed as a percent of mean bone mineral density (BMD), were ±1.8%, ±2.4%, ±2.4%, and ±0.8% for the lumbar spine, femoral neck, trochanter, and total body bone mineral density, respectively (11). The CV for repeated measures of percent total body fat was 2.8%.
The past-year physical activity questionnaire (PYPAQ) (5,7) was used to collect information about the average duration and frequency of physical activity participation. The PYPAQ has been validated in adolescents (1). Total PYPAQ score was computed using the validated equation from Shedd et al. (34): PYPAQ score = Σ1 − n [duration (minutes per session) × frequency (d·wk−1) × intensity (METs (30)) × load (peak strain score (15))], where n was the number of activities a subject reported during the past year. Average daily time spent in moderate-to-vigorous physical activity (MVPA (min·d−1)) was determined from the duration and frequency of each at least moderate-intensity activity reported. The average daily time spent in MVPA (min·d−1) was used to calculate the proportion of girls who achieved US Centers for Disease Control and Prevention physical activity recommendations for children and adolescents of 60 min of MVPA per day (37).
Data were checked for outliers and normality using histograms, and all variables were tested for skewness and kurtosis. Bivariate correlations were computed using the Pearson r for continuous and the Spearman ρ for categorical variables to examine relationships between maturity, ethnicity, anthropometric characteristics, and percent total body fat and muscle density, MCSA, and total body lean and fat masses. Multiple linear regression was used to examine the independent associations between physical activity and muscle densities of the calf and thigh, after controlling for ethnicity, maturity offset, and MCSA (model 1). Regression analyses were then repeated after substituting MCSA with total body lean mass (model 2). MCSA and total body lean mass were not included in the same model to protect against collinearity. MCSA and total body lean mass were used as covariates to determine whether the association between physical activity and skeletal muscle fat content was independent of muscle size and total body lean mass. Body mass and height were excluded to protect against collinearity with MCSA and total body lean mass. Ethnicity (non-Hispanic = 0, Hispanic = 1) and maturity offset were included as covariates in both models to adjust for the potential confounding effects of ethnicity and physical maturation on skeletal muscle fat content. Before the multiple linear regression analyses, all variables were checked for normality, linearity, and homoscedasticity using residual plots. Postestimation procedures showed that none of the final regression models had any substantial collinearity that would affect the results of the conclusions. Quartiles were subsequently used to divide the sample into four physical activity groups (fourths). Muscle densities of the two middle fourths of physical activity were similar; thus, we collapsed these groups into a single group and used ANCOVA, after adjusting for the same covariates included in the regression models described above, to determine whether there were differences in muscle densities of the calf and thigh between the middle group (average of the middle two fourths) and the lowest and highest groups of physical activity. Bonferroni post hoc tests were used to adjust for multiple comparisons. All analyses were also performed within maturity offset and Tanner stage maturity categories (maturity offset <0 yr from PHV (before) and ≥0 yr from PHV (after) and Tanner stage I (prepubertal), Tanner stages II–III (early pubertal), and Tanner stages IV–V (late pubertal)). A significance level of P = 0.05 was used in all tests. All analyses were performed using the Statistical Package for the Social Sciences for Windows, version 18.0 (SPSS, Chicago, IL).
Descriptive characteristics are shown in Table 1. Sample ethnicity was 23% Hispanic and 77% non-Hispanic. Sample race was 88% white, 7% Asian, 3% black or African American, 0.5% Native American or Alaskan native, 0.5% native Hawaiian or other Pacific Islander, and 1.0% other. On the basis of US National Center for Health Statistics/Centers for Disease Control percentiles for body mass index (BMI (kg·m−2)) (20), 3.0% of the sample was underweight (BMI < fifth percentile), 73.8% of the sample was of healthy weight (BMI = 5th–85th percentile), 15.1% of the sample was overweight (BMI = 85th–95th percentile), and 8.1% of the sample was obese (BMI > 95th percentile). Tanner stage distributions for the total sample were 33% prepubertal (stage I, n = 155), 60% early pubertal (stages II–III, n = 280), and 7% late pubertal (stages IV–V, n = 30). Maturity offset values indicated that girls were, on average, 1.1 yr before PHV, with a range from 3.2 yr before PHV to 1.4 yr after PHV.
As expected, muscle densities of the calf and thigh were inversely correlated with percent total body fat (r = −0.37 and −0.48, P values < 0.001) and total body fat mass (r = −0.33 and −0.40, P values < 0.001). Unadjusted bivariate correlations (Pearson r) showed that physical activity was significantly correlated with muscle densities of the calf (r = 0.13, P = 0.004) and thigh (r = 0.16, P = 0.001). Muscle densities of the thigh and calf were not significantly associated with MCSA (r = −0.06 and 0.07, P values > 0.05) or total body lean mass (r = −0.01 and 0.02, P values > 0.05). There were moderate to strong correlations between maturity (maturity offset), anthropometric characteristics, and percent total body fat and MCSA and total body lean and fat masses (Table 2). Lower correlations were found between maturity, anthropometric characteristics, percent total body fat, and muscle densities of the calf and thigh (Table 2). Ethnicity was not significantly (P values > 0.05) correlated with muscle density, MCSA, or total body lean and fat masses.
Multiple linear regression analyses with physical activity, ethnicity, maturity offset, and MCSA as independent variables showed that physical activity was independently associated with muscle densities of the calf (β = 0.14, P = 0.002) and thigh (β = 0.15, P < 0.001) (Table 3, model 1). Similarly, physical activity was independently associated with muscle density of the calf (β = 0.14, P = 0.003) and thigh (β = 0.16, P < 0.001) when ethnicity, maturity offset, and total body lean mass were covariates (Table 3, model 2). Thus, higher physical activity was significantly associated with less skeletal muscle fat content of the calf and thigh. Multiple linear regression analyses also showed an inverse relationship between Hispanic ethnicity and muscle densities of the calf (model 1: β = −0.126, P = 0.006; model 2: β = −0.127, P = 0.006) and thigh (model 1: β = −0.091, P = 0.046; model 2: β = −0.089, P = 0.053), indicating that Hispanic ethnicity was independently associated with higher skeletal muscle fat content. Of the total variance in calf and thigh skeletal muscle fat content explained by each full regression model, the percent explained by physical activity was 43%–65% (Table 3). Analyses within maturity categories (maturity offset <0 yr from PHV (before) and ≥0 yr from PHV (after) and Tanner stage I (prepubertal), Tanner stages II–III (early pubertal), and Tanner stages IV–V (late pubertal)) gave similar results and did not markedly change the magnitude or direction of the observed relationships between physical activity and muscle density (data not shown).
Comparisons of muscle densities across the lowest, middle (average of the middle two), and highest groups of physical activity were performed using ANCOVA, after adjusting for ethnicity, maturity offset, and MCSA (Fig. 1, model 1). Calf muscle density was 1.7% (P < 0.001) lower in the lowest compared with the highest group of physical activity. Similarly, muscle density of the thigh was 1.8% (P < 0.001) lower in the lowest compared with the highest group of physical activity. After substituting MCSA with total body lean mass (Fig. 1, model 2), muscle densities of the calf and thigh were 1.6% (P < 0.001) and 1.9% (P < 0.001), respectively, lower in the lowest compared with the highest group of physical activity. Taken together, these results indicate that a lower level of physical activity was significantly associated with greater skeletal muscle fat content of the calf and thigh, independent of maturity, ethnicity, MCSA, and total body lean mass. Girls in the lowest, middle, and highest groups of physical activity averaged 9.1 ± 5.3, 33.4 ± 10.8, and 96.2 ± 32.5 min of MVPA per day, respectively. Twenty-five percent of girls achieved the US Centers for Disease Control and Prevention recommendation for children and adolescents of 60 min of MVPA per day (37).
We used pQCT in a large sample of girls, 8–13 yr, to examine the relationship between physical activity and skeletal muscle fat content. To our knowledge, this is the first study to investigate this relationship in girls. Our data demonstrate that lower physical activity is associated with higher skeletal muscle fat content, independent of maturity, ethnicity, muscle size, and total body lean mass. These findings suggest that a lower level of physical activity contributes to greater fat infiltration within skeletal muscle in healthy girls as early as the peripubertal years.
Observations that lower physical activity is associated with greater skeletal muscle fat content in children with disabilities that limit physical activity (17,22) and otherwise healthy adults (9,23) have generated interest in better understanding the relationship between physical activity and skeletal muscle fat content. Johnson et al. (17) used magnetic resonance imaging (MRI) to assess skeletal muscle fat content of the midthigh in 24 children (12 per group, 5–14 yr old) and found that children with quadriplegic cerebral palsy had greater skeletal muscle fat content than healthy children, which was significantly associated with their lower level of physical activity. The findings of Johnson et al. (17) are consistent with findings from another study in boys, 9–12 yr, with low muscle mass and disabilities that limited physical activity participation (22). Until now, an association between lower physical activity and skeletal muscle fat content has not been shown previously in otherwise healthy youth. Importantly, our findings are consistent with the studies in children with disabilities that limited participation in physical activity (17,22) and indicate that lower physical activity is associated with higher skeletal muscle fat content in healthy girls. Our findings are also consistent with a study by Manini et al. (23) in which 6 men and 12 women (19–28 yr) underwent MRI assessments of skeletal muscle fat content of the thigh and calf after a 4-wk control period and then again after 4 wk of unilateral limb suspension to reduce physical activity. In that study, 4 wk of lower limb suspension resulted in increases in skeletal muscle fat content of the thigh and calf of 14.5% and 20%, respectively, suggesting that reduced physical activity leads to marked increases in skeletal muscle fat content (23). Our results are also consistent with findings from a study by Gilsanz et al. (9), who showed in 90 postpubertal females (16–22 yr) that physical inactivity was independently associated with skeletal muscle fat content of the midthigh, assessed by computed tomography (CT). Finally, our findings are consistent with studies in older adults (12,29,32) that have demonstrated that reduced physical activity is associated with increased skeletal muscle fat content, assessed by CT. In toto, our data and data from previous studies across a wide age range (9,12,17,22,23,29,32) suggest that a lower level of physical activity is a risk factor for greater fat infiltration within skeletal muscle.
The US Centers for Disease Control and Prevention recommend that children and adolescents engage in at least 60 min of at least moderate-intensity physical activity per day (37). Twenty-five percent of girls in our sample achieved this recommendation. Whereas we found no differences between the highest fourth (25%) of physical activity and the average of the middle two fourths of physical activity, the lowest fourth of physical activity had significantly higher skeletal muscle fat content, supporting the notion that a lower level of physical activity and sedentary behavior may be important risk factors for greater skeletal muscle fat content. Although we did not measure sedentary behavior, the amount of time spent in sedentary behaviors is undoubtedly related to a lower level of physical activity and increased risk of metabolic syndrome, T2DM, and heart disease (8,16). Future studies should examine the independent association between sedentary time and skeletal muscle fat content.
Fat compartments within skeletal muscle are dynamic; they can be depleted during exercise and used for storage during periods of elevated energy availability (33). Regular physical activity is associated with rapid depletion and repletion of skeletal muscle fat compartments, which contributes to greater insulin sensitivity in skeletal muscle (14). In contrast, sedentary behavior results in downregulation of fat oxidation enzymes and, subsequently, results in a lower capacity for fat oxidation (33). Consequently, sedentary obese and obese T2DM individuals have a diminished ability to use skeletal muscle fat stores during exercise (4,18), which contributes to the development of insulin resistance. Thus, regular turnover of skeletal muscle fat stores during exercise and subsequent recovery periods may improve insulin sensitivity.
The present study was not without limitations. First, the cross-sectional design makes it impossible to establish a causal relationship between lower physical activity and greater skeletal muscle fat content. Second, there is a well-known difficulty in assessing physical activity via self-report questionnaires in children and adolescents. We acknowledge that this approach is susceptible to errors, although we attempted to minimize the limitations of administering self-report questionnaires in youth by encouraging guardian assistance with physical activity recall and by limiting recall to past-year activities. Nonetheless, any misclassifications of physical activity would have likely led to an underestimation of the true association between lower physical activity and higher skeletal muscle fat content. Another potential limitation is that pQCT cannot directly measure the lipid content of skeletal muscle. However, controlled studies in obese sedentary and T2DM groups have demonstrated that lower muscle density is associated with greater skeletal muscle lipid content (13,19). Furthermore, pQCT and CT have both been used in many previous studies (9,12,14,24,29,32) to differentiate tissues of the midthigh and calf regions on the basis of attenuation characteristics, which are directly related to tissue composition and density (13,19). Our study obtained a single slice at the 66% tibia and 20% femur sites relative to the respective distal growth plates of the nondominant leg. These regions have a smaller depot of skeletal muscle adipose tissue than at the midthigh, a potential limitation, although relatively strong correlations between skeletal muscle fat content of the midthigh and calf have been reported using MRI (21). Although MRI has better contrast resolution than pQCT, its high cost and higher radiation prevents its use in large samples. pQCT, because of its relatively low cost, fast speed, and low radiation dose, has promise for application in future large-scale studies. These features make pQCT uniquely suited to safely estimate skeletal muscle fat content of the calf and thigh. Future applications of this technique should help to further clarify the relationship between physical activity and skeletal muscle fat content. Lastly, we acknowledge that the total variance in skeletal muscle fat content explained by the full regression models was low. Nevertheless, of the total variance in skeletal muscle fat content explained by each full regression model, the percent explained by physical activity was relatively large (43%–65%), and more variance in skeletal muscle fat content was explained by physical activity compared with the other variables (maturity, ethnicity, MCSA, lean mass) included in the models. The significant relationship between inactivity and skeletal muscle fat content at the young age of these girls underscores the importance of physical activity and the need for future studies, including prospective observational studies, with a more direct measure of muscle triglyceride, and intervention studies, to test whether skeletal muscle fat content would be reduced with physical activity and the dose required to do so.
In conclusion, our results indicate that lower physical activity is significantly associated with higher skeletal muscle fat content in otherwise healthy girls, independent of maturity, ethnicity, muscle size, and total body lean mass. Importantly, this finding is consistent with available data in children with disabilities that limit physical activity (17,22), healthy adults (9,23), and older adults (12,29,32) and suggests that the relationship between lower physical activity and greater skeletal muscle fat content begins, in females at least, as early as the peripubertal years. Interventions designed to incorporate physical activity into the lives of sedentary girls are a critical need.
The project described was supported by award number HD-050775 (S.G.) from the National Institute of Child Health and Human Development. J.F. is supported by National Institutes of Health National Institute of General Medical Sciences grant T32 GM-08400, Graduate Training in Systems and Integrative Physiology.
The authors thank the principals, teachers, parents, and students from the schools in the Catalina Foothills and Marana school districts for their participation and support. They also thank the radiation technicians, program coordinators, and all other members of the Jump-In Study team for their contributions.
None of the authors had a conflict of interest. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institute of Child Health and Human Development or the National Institutes of Health. The study’s Clinical Trials number is NCT00729378, and its registration date is July 17, 2008.
The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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