Leigh Gabel

Associations of sedentary time patterns and TV viewing time with inflammatory and endothelial function biomarkers in children.

Investigate associations of TV viewing time and accelerometry‐derived sedentary time with inflammatory and endothelial function biomarkers in children.

Physical Activity, Sedentary Time, and Bone Strength From Childhood to Early Adulthood: A Mixed Longitudinal HR-pQCT study

Bone strength is influenced by bone geometry, density, and bone microarchitecture, which adapt to increased mechanical loads during growth. Physical activity (PA) is essential for optimal bone strength accrual; however, less is known about how sedentary time influences bone strength and its determinants. Thus, our aim was to investigate the prospective associations between PA, sedentary time, and bone strength and its determinants during adolescence. We used HR‐pQCT at distal tibia (8% site) and radius (7% site) in 173 girls and 136 boys (aged 9 to 20 years at baseline). We conducted a maximum of four annual measurements at the tibia (n = 785 observations) and radius (n = 582 observations). We assessed moderate‐to‐vigorous PA (MVPA) and sedentary time with accelerometers (ActiGraph GT1M). We aligned participants on maturity (years from age at peak height velocity) and fit a mixed‐effects model adjusting for maturity, sex, ethnicity, leg muscle power, lean mass, limb length, dietary calcium, and MVPA in sedentary time models. MVPA was a positive independent predictor of bone strength (failure load [F.Load]) and bone volume fraction (BV/TV) at the tibia and radius, total area (Tt.Ar) and cortical porosity (Ct.Po) at the tibia, and negative predictor of load‐to‐strength ratio at the radius. Sedentary time was a negative independent predictor of Tt.Ar at both sites and Ct.Po at the tibia and a positive predictor of cortical thickness (Ct.Th), trabecular thickness (Tb.Th), and cortical bone mineral density (Ct.BMD) at the tibia. Bone parameters demonstrated maturity‐specific associations with MVPA and sedentary time, whereby associations were strongest during early and mid‐puberty. Our findings support the importance of PA for bone strength accrual and its determinants across adolescent growth and provide new evidence of a detrimental association of sedentary time with bone geometry but positive associations with microarchitecture. This study highlights maturity‐specific relationships of bone strength and its determinants with loading and unloading. Future studies should evaluate the dose‐response relationship and whether associations persist into adulthood.

Bone architecture and strength in the growing skeleton: the role of sedentary time.

PURPOSE Todays youths spend close to 60% of their waking hours in sedentary activities; however, we know little about the potentially deleterious effects of sedentary time on bone health during this key period of growth and development. Thus, our objective was to determine whether sedentary time is associated with bone architecture, mineral density, and strength in children, adolescents, and young adults. METHODS We used high-resolution peripheral quantitative computed tomography (Scanco Medical) to measure bone architecture (trabecular and cortical microstructure and bone macrostructure) and cortical and total bone mineral density (BMD) at the distal tibia (8% site) in 154 males and 174 females (9-20 yr) who were participants in the University of British Columbia Healthy Bones III study. We applied finite element analysis to high-resolution peripheral quantitative computed tomography scans to estimate bone strength. We assessed self-reported screen time in all participants using a questionnaire and sedentary time (volume and patterns) in a subsample of participants with valid accelerometry data (89 males and 117 females; ActiGraph GT1M). We fit sex-specific univariate multivariable regression models, controlling for muscle cross-sectional area, limb length, maturity, ethnicity, dietary calcium, and physical activity. RESULTS We did not observe independent effect of screen time on bone architecture, BMD, or strength in either sex (P > 0.05). Likewise, when adjusted for muscle cross-sectional area, limb length, maturity, ethnicity, dietary calcium, and physical activity, accelerometry-derived volume of sedentary time and breaks in bouts of sedentary time were not a determinant of bone architecture, BMD, or strength in either sex (P > 0.05). CONCLUSIONS Further study is warranted to determine whether the lack of association between sedentary time and bone architecture, BMD, and strength at the distal tibia is also present at other skeletal sites.

Sex Differences and Growth-Related Adaptations in Bone Microarchitecture, Geometry, Density and Strength from Childhood to Early Adulthood: A Mixed Longitudinal HR-pQCT Study.

Sex differences in bone strength and fracture risk are well documented. However, we know little about bone strength accrual during growth and adaptations in bone microstructure, density, and geometry that accompany gains in bone strength. Thus, our objectives were to (1) describe growth related adaptations in bone microarchitecture, geometry, density, and strength at the distal tibia and radius in boys and girls; and (2) compare differences in adaptations in bone microarchitecture, geometry, density, and strength between boys and girls. We used HR‐pQCT at the distal tibia (8% site) and radius (7% site) in 184 boys and 209 girls (9 to 20 years old at baseline). We aligned boys and girls on a common maturational landmark (age at peak height velocity [APHV]) and fit a mixed effects model to these longitudinal data. Importantly, boys showed 28% to 63% greater estimated bone strength across 12 years of longitudinal growth. Boys showed 28% to 80% more porous cortices compared with girls at both sites across all biological ages, except at the radius at 9 years post‐APHV. However, cortical density was similar between boys and girls at all ages at both sites, except at 9 years post‐APHV at the tibia when girls’ values were 2% greater than boys’. Boys showed 13% to 48% greater cortical and total bone area across growth. Load‐to‐strength ratio was 26% to 27% lower in boys at all ages, indicating lower risk of distal forearm fracture compared with girls. Contrary to previous HR‐pQCT studies that did not align boys and girls at the same biological age, we did not observe sex differences in Ct.BMD. Boys’ superior bone size and strength compared with girls may confer them a protective advantage. However, boys’ consistently more porous cortices may contribute to their higher fracture incidence during adolescence. Large prospective studies using HR‐pQCT that target boys and girls who have sustained a fracture are needed to verify this.

Reexamining the Surfaces of Bone in Boys and Girls During Adolescent Growth: A 12-Year Mixed Longitudinal pQCT Study.

We revisit Stanley Garns theory related to sex differences in endocortical and periosteal apposition during adolescence using a 12‐year mixed longitudinal study design. We used peripheral quantitative computed tomography to examine bone parameters in 230 participants (110 boys, 120 girls; aged 11.0 years at baseline). We assessed total (Tt.Ar, mm2), cortical (Ct.Ar, mm2), and medullary canal area (Me.Ar, mm2), Ct.Ar/Tt.Ar, cortical bone mineral density (Ct.BMD, mg/cm3), and polar strength‐strain index (SSIp, mm3) at the tibial midshaft (50% site). We used annual measures of height and chronological age to identify age at peak height velocity (APHV) for each participant. We compared annual accrual rates of bone parameters between boys and girls, aligned on APHV using a linear mixed effects model. At APHV, boys demonstrated greater Tt.Ar (ratio = 1.27; 95% confidence interval [CI] 1.21, 1.32), Ct.Ar (1.24 [1.18, 1.30]), Me.Ar (1.31 [1.22, 1.40]), and SSIp (1.36 [1.28, 1.45]) and less Ct.Ar/Tt.Ar (0.98 [0.96, 1.00]) and Ct.BMD (0.97 [0.96, 0.97]) compared with girls. Boys and girls demonstrated periosteal bone formation and net bone loss at the endocortical surface. Compared with girls, boys demonstrated greater annual accrual rates pre‐APHV for Tt.Ar (1.18 [1.02, 1.34]) and Me.Ar (1.34 [1.11, 1.57]), lower annual accrual rates pre‐APHV for Ct.Ar/Tt.Ar (0.56 [0.29, 0.83]) and Ct.BMD (–0.07 [–0.17, 0.04]), and similar annual accrual rates pre‐APHV for Ct.Ar (1.10 [0.94, 1.26]) and SSIp (1.14 [0.98, 1.30]). Post‐APHV, boys demonstrated similar annual accrual rates for Ct.Ar/Tt.Ar (1.01 [0.71, 1.31]) and greater annual accrual rates for all other bone parameters compared with girls (ratio = 1.23 to 2.63; 95% CI 1.11 to 3.45). Our findings support those of Garn and others of accelerated periosteal apposition during adolescence, more evident in boys than girls. However, our findings challenge the notion of greater endocortical apposition in girls, suggesting instead that girls experience diminished endocortical resorption compared with boys.

Reply to: Challenges in the Acquisition and Analysis of Bone Microstructure During Growth

We would like to express our thanks to Drs. Seeman and Ghazem-Zadeh for their interest in our study. We are grateful for the opportunity to discuss challenges associated with assessing bone microstructure during growth and expand upon some of ourmethods, as we believe this is at the heart of their comments. First, Drs. Seeman and Ghazem-Zadeh address the key issue of long-term precision of HR-pQCT in the growing skeleton. They correctly identify a fundamental challenge confronting pediatric bone researchers is that the fixed distance region of interest (ROI), used in adults, represents a “moving target” in children and adolescents. Therefore, we used a percent distance from a fixed anatomical region in our prospective study, fromwhich we are able to locate the same “relative” ROI. Although the same piece of bone will not be measured within a participant (this is not possible as bone migrates proximally during growth), the same relative region will be measured. We suggest that researchers consider the most appropriate ROI as it relates to their research question and study population. If the goal is to conduct prospective studies during growth or comparisons between participants of different body sizes, we believe the relative ROI approach is most appropriate. However, we recognize this approach has limitations. For example, manual assessment of limb length may introduce measurement error. Importantly, such imprecision would be associated with random error (noise) rather than a systematic difference between sexes. Second, we thank Drs. Seeman and Ghazem-Zadeh for summarizing body growth velocities—the intricacies of growth and maturation are fascinating. There are well-known maturational differences between boys and girls of the same chronological age, and controlling for these differences remains a primary challenge in pediatric studies. Several methods are available to assess maturity, the specifics of which are summarized in detail elsewhere. As a continuous measure, age at peak height velocity (APHV) can be used to align boys and girls on a common maturational landmark—the age when an individual is experiencing peak linear growth. We agree that anatomical regions vary with respect to timing of peak growth (ie, peak leg length velocity precedes peak height velocity, which precedes peak trunk velocity). Thus, truncal growth extends over a longer period and contributes more to the adolescent growth spurt compared with leg length growth. However, these processes occur at a similar relative time in both sexes. For example, peak leg velocity occurs approximately 6 months prior to APHV in boys and girls, whereas peak trunk velocity occurs approximately 2 months post-APHV in boys and girls. Importantly, estimated velocities of many performance tasks in both sexes peak at the same time as APHV. Thus, APHV is an important relative marker of function in boys and girls. Further, APHV approximates that of peak skeletal age velocity. Ideally researchers would assess endocrine markers that represent activation of the hypothalamic-gonadalpituitary axis or hand-wrist X-rays of skeletal maturity; however, this is not always possible in pediatric studies. Therefore, although not without limitations, we contend that APHV is an acceptable measure of maturation and recommend its use in pediatric bone studies. We disagree with the comment that we did not account for ethnic differences. On the contrary, we carefully included ethnicity in our longitudinal analyses given known ethnic differences in bone accrual and maturation (see results for Asian and white participants in Tables 4 and 5 in our article). Finally, we agree that long bones are complex, heterogeneous structures. The number of pores, including their shape and size, vary along the length and cross-section of the cortex. A current challenge faced by investigators is accurately defining the periosteal and endocortical surfaces, the latter of which presents a greater challenge due to the gradual transition from cortical to trabecular bone at metaphyseal sites. A debate regarding the most appropriate approach to segment cortical bone is beyond the scope of this response. We agree that porosity and BMD typically go hand in hand; ie, greater porosity is usually indicative of lower BMD. However, if we accept our measures as valid (observing greater cortical porosity but similar BMD in boys compared with girls), one speculation is that boys may have higher tissue mineral density compared with girls.

Bone strength accrual across adolescent growth and the influences of physical activity and sedentary time

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Exercise and the Female Skeleton

Exercise and physical activity are essential for developing and maintaining a strong and healthy skeleton. In this chapter, we aim to review the current literature that addresses the central role that exercise plays in promoting bone health throughout childhood, adolescence, and young adulthood with a focus on the female skeleton. A large body of evidence supports the osteogenic benefits of regular weight-bearing exercise, particularly during the growing years. However, we do not yet know the specific exercise prescription for optimal bone strength accrual during growth. With advances in imaging technologies, we are beginning to understand the complex hierarchy of bone, and how bone adapts not only its mass but also its structure, microarchitecture, and ultimately, its strength to withstand the loads imposed upon it. Based on current data, pre- and early puberty appear to offer a “window of opportunity” during which the female skeleton is most responsive to weight-bearing activity. With the exception of elite athlete populations, maintenance of bone strength benefits achieved during growth appears dependent on continued participation in weight-bearing activities. Importantly, such benefits may not be attained in girls and young adult women with menstrual dysfunction, as amenorrhea and oligomenorrhea may significantly attenuate the skeleton’s ability to adapt to exercise-induced loads. As use of advanced imaging tools becomes more widespread, we will further our understanding of the skeleton’s ability to regain its strength following resumption of normal menses. Such tools will also help to enhance our understanding of bone structural adaptations to physical activity and allow us to explore whether bone strength gains achieved during childhood are sustained into older age.