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Kathleen F. Janz

Of all the lifestyle strategies for increasing bone strength during the growing years, physical activity is one of the most efficacious. This commentary highlights two exceptional 2016 publications addressing bone strength in children and adolescents with an eye toward reduced fracture risk later in life. The first by Weaver et al. was selected due to its comprehensive approach to understanding bone development. The second by Mitchell et al explores a new field of inquiry, that is, genetic-environment interaction as represented by bone mineral density-lowering alleles and high-impact physical activity. It is a first look at future precision medicine as it may pertain to pediatric bone strength.

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Kathleen F. Janz and Fatima Baptista

The positive effects of physical activity on bone strength are certain. However, researchers have yet to precisely quantify the contribution of specific characteristics of physical activity that affect bone strength in children and adolescents. This commentary highlights 2 noteworthy 2017 publications that addressed osteogenic physical activity dose–response issues. Both papers moved the field forward by providing new insights on physical activity exposures beyond high-impact loading. Koedijk et al’s paper was selected because, to the best of our knowledge, it is the first systematic review to solely examine associations between sedentary behavior and indicators of bone strength. The second selected paper, Gabel et al, used novel approaches in accelerometer processing and statistical modeling to separate the osteogenic effects of frequency of short bouts of physical activity from total volume of physical activity. As such, the authors of this paper begin to explore in youth what animal models have shown for some time, that is, optimal bone adaptation requires the correct combination of intensity, frequency, duration, nonrepetitive movement, and rest. Together, these papers signal new and important approaches for the conceptualization, measurement, and interpretation of osteogenic physical activity.

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Kathleen F. Janz and Shelby L. Francis

Although there is strong and consistent evidence that childhood and adolescent physical activity is osteogenic, the evidence concerning its sustained effects to adult bone health is not conclusive. Therefore the value of interventions, in addition to beneficial bone adaptation, could be exposure to activities children enjoy and therefore continue. As such, interventions should provide skills, pleasure, and supportive environments to ensure continued bone-strengthening physical activity with age. Until the dose-response as well as timing of physical activity to bone health is more fully understood, it is sensible to assume that physical activity is needed throughout the lifespan to improve and maintain skeletal health. Current federal guidelines for health-related physical activity, which explicitly recommend bone-strengthening physical activities for youth, should also apply to adults.

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Kathleen F. Janz and Larry T. Mahoney

To assess the relationship of changes in physical fitness and body composition to heart growth and rising blood pressure (BP) during early puberty, fat-free mass (FFM), body fatness (% fat), physical fitness (peak VO2, peak mechanical power, peak O2 pulse, peak systolic blood pressure [SBP], and grip strength), Tanner stage, resting BP, and echocardiographic left ventricular mass (LVM) were measured in 123 children (age M = 10 years) and remeasured 2 years later. Increased FFM, increased grip strength, and increased peak power explained 28% of the variability in heart growth. Increased FFM, increased % fat, and decreased peak O2 pulse explained 23% of the variability in rising SBP. During puberty, physical fitness is an independent predictor of changing heart size and systolic blood pressure. Results suggest that improvements in physical fitness and decreases in body fatness may have beneficial effects on children’s blood pressure.

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Kathleen F. Janz and Larry T. Mahoney

This study examined average daily physical activity and discrete activity intensities in 102 adolescents (age M = 15 yr). Dependent physical activity variables were constructed from minute-by-minute movement counts measured during 4 consecutive days of accelerometry. Independent variables included gender, sexual maturation, TV viewing, and video playing. The stability of 4 days of activity measures ranged from R = .66 to 30. Video game playing was inversely associated with average daily movement for boys (r = −.38) and girls (r = −.55). Boys at all levels of sexual maturation had higher levels of activity than girls. Late and postpubertal boys and girls were more sedentary, had lower levels of vigorous activity, and lower levels of average daily movement than boys and girls in midpuberty.

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Phyllis J. Wenthe, Kathleen F. Janz, and Stephen M. Levy

This study investigated the relationship between predisposing, reinforcing, and enabling factors conceptualized within the Youth Physical Activity Promotion Model (YPAP) and moderate to vigorous physical activity (MVPA) of adolescent males and females. Specifically, self-efficacy to overcome barriers, enjoyment of physical activity; family support, peer support, perceived school climate, neighborhood safety and access to physical activity were examined. The Physical Activity Questionnaire for Adolescents (PAQ-A) and the Actigraph 7164 were used to obtain three different measures of MVPA in 205 adolescents (102 males, 103 females). Family support emerged as the most significant and consistent factor associated with the MVPA of both adolescent males and females. This relationship was noted even when different methods of measuring MVPA were employed. These findings should increase the confidence of public health officials that family support has the potential to positively alter the physical activity behavior of adolescents.

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Kathleen F. Janz, Jeffrey D. Dawson, and Larry T. Mahoney

To evaluate the effect of changes in aerobic fitness and physical activity on changes in lipoproteins, we measured body composition, peak V̇O2, vigorous and sedentary activity, maturation, and lipoproteins in 125 children (mean baseline age, 10.5 years) for 5 years. Change in variables was analyzed using the slopes of the regression line obtained by plotting the data for each child. No predictor variables were significant for girls. In boys, predictors of favorable changes in lipoproteins included decreases in fatness, increases in fitness, early maturation, and increases in fat-free body mass (FFM). Multivariable analysis, adjusted for baseline age, indicated that change in FFM explained 21% of the variability in change in LDL-C. Results suggest that during puberty, changes in activity and fitness do not predict changes in lipoproteins.

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Joanna L. Morrissey, Phyllis J. Wenthe, Elena M. Letuchy, Steven M. Levy, and Kathleen F. Janz

In a sample of 291 adolescents (mean age 13 yr), seven psychosocial factors, including family support, were examined in relation to accelerometry-derived physical activity (PA) measured after school and during the weekend. Gender-specific stepwise linear regression analyses determined which combinations of factors explained the variance in nonschool moderate to vigorous PA and nonschool total PA after adjusting for % BF, age, and maturity (p ≤ 0.05). Being praised by a family member and % BF explained 13% of the variance in female nonschool MVPA, while being praised and maturity explained 13% of the variance in nonschool total PA. Having a family member watch him participate, % BF, and age explained 11.5% of the variance in male nonschool MVPA, while having a family member participate with him explained 6.4% of the variance in nonschool total PA. Despite adolescents’ growing independence, family support continues to influence PA levels.

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Shelby L. Francis, Ajay Singhvi, Eva Tsalikian, Michael J. Tansey, and Kathleen F. Janz


Determining fitness is important when assessing adolescents with type 1 diabetes mellitus (T1DM). Submaximal tests estimate fitness, but none have been validated in this population. This study cross-validates the Ebbeling and Nemeth equations to predict fitness (VO2max (ml/kg/min)) in adolescents with T1DM.


Adolescents with T1DM (n = 20) completed a maximal treadmill test using indirect calorimetry. Participants completed one 4-min stage between 2.0 and 4.5 mph and 5% grade (Ebbeling/Nemeth protocol). Speed and grade were then increased until exhaustion. Predicted VO2max was calculated using the Ebbeling and Nemeth equations and compared with observed VO2max using paired t tests. Pearson correlation coefficients, 95% confidence intervals, coefficients of determination (R2), and total error (TE) were calculated.


The mean observed VO2max was 47.0 ml/kg/min (SD = 6.9); the Ebbeling and Nemeth mean predictions were 42.4 (SD = 9.4) and 43.5 ml/kg/min (SD = 6.9), respectively. Paired t tests resulted in statistically significant (p < .01) mean differences between observed and predicted VO2max for both predictions. The association between the Ebbeling prediction and observed VO2max was r = .90 (95% CI = 0.76, 0.96), R 2 = .81, and TE = 6.5 ml/kg/min. The association between the Nemeth prediction and observed VO2max was r = .81 (95% CI = 0.57, 0.92), R 2 = .66, and TE = 5.6 ml/kg/min.


The Nemeth submaximal treadmill protocol provides a better estimate of fitness than the Ebbeling in adolescents with T1DM.

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Brad R. Julius, Amy M.J. O’Shea, Shelby L. Francis, Kathleen F. Janz, and Helena Laroche

Purpose: The authors examined the relationship between mother and child activity. Methods: The authors compared moderate–vigorous physical activity (MVPA) and sedentary time of low-income mothers with obesity and their 6- to 12-year-old children on week (WD) and weekend (WE) days. A total of 196 mother–child pairs wore accelerometers simultaneously for a week. Mothers completed questionnaires. Spearman correlation and multivariate regression were used. Results: WE MVPA (accelerometry) was significantly correlated between mothers with children aged 6–7 (r s = .35) and daughters (r s = .27). Self-reported maternal PA time spent with one of their children was significantly correlated with the WE MVPA of all children (r s = .21) and children aged 8–10 (r s = .22) and with the WD MVPA of all children (r s = .15), children aged 8–10 (r s = .23), aged 11–12 (r s = .52), and daughters (r s = .37), and inversely correlated to the WD sedentary time of all children (r s = −.21), children aged 8–10 (r s = −.30), aged 11–12 (r s = −.34), daughters (r s = −.26), and sons (r s = −.22). In multivariate regression, significant associations were identified between reported child–mother PA time together and child MVPA and sedentary time (accelerometry). Conclusions: Mothers may influence the PA levels of their children with the strongest associations found in children aged 6–7 and daughters. Mother–child coparticipation in PA may lead to increased child MVPA and decreased sedentary behavior.