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2016 The Year That Was: Bone Strength

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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Bone Strength and Exercise During Youth—The Year That Was 2017

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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Childhood Physical Activity May or May Not Provide Sustained Effects to Protect Adults From Osteoporosis

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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Two-Year Follow-Up on the Impact of Physical Fitness and Body Fatness on Children’s Heart Growth and Rising Blood Pressure: The Muscatine Study

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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Maturation, Gender, and Video Game Playing Are Related to Physical Activity Intensity in Adolescents: The Muscatine Study

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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Gender Similarities and Differences in Factors Associated with Adolescent Moderate-Vigorous Physical Activity

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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Changes in Physical Fitness and Physical Activity during Puberty Do Not Predict Lipoprotein Profile Changes: The Muscatine Study

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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Association Between Light-Intensity Physical Activity and Adiposity in Childhood

Soyang Kwon, Kathleen F. Janz, Trudy L. Burns, and Steven M. Levy

The purpose of this study was to examine whether the association between daily light-intensity physical activity (LPA) and total body fat mass changes during childhood. The study sample was 577 children participating in the longitudinal Iowa Bone Development Study. Body fat mass and physical activity (PA) were measured using dual energy X-ray absorptiometry (DXA) and accelerometers, respectively, at approximately 5, 8, and 11 years of age. Age- and gender-specific multivariable linear regression models were fit to predict fat mass by LPA, adjusted for actual age, birth weight, fat-free mass, height, moderate- to vigorous-intensity PA, and physical maturity (only for girls). Among boys, LPA was negatively associated with fat mass at age 11, but not age 5 or 8. Among girls, LPA was negatively associated with fat mass at ages 8 and 11, but not at age 5. LPA may have a beneficial effect against excess adiposity among older children.

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Specific Types of Family Support and Adolescent Non-School Physical Activity Levels

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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Measuring Children’s Vertical Ground Reaction Forces with Accelerometry during Walking, Running, and Jumping: The Iowa Bone Development Study

Kathleen F. Janz, Smita Rao, Hope J. Baumann, and Jaime L. Schultz

Ground reaction forces (GRF) are associated with bone hypertrophy; therefore, they are important to understanding physical activity’s role in children’s bone health. In this study, we examined the ability of accelerometry to predict vertical GRF in 40 children (mean age 8.6 yr) during slow walking, brisk walking, running, and jumping. Correlation coefficients between accelerometry-derived movement counts and GRF were moderate to high and significant during walking and running, but not during jumping. Given a large proportion of children’s daily physical activity involves ambulation, accelerometry should be useful as a research method in bone-related research. However, this method underestimates GRF during jumping, an important physical activity for bone modeling and remodeling.