Adapting the Intensity Gradient for Use with Count-Based Accelerometry Data in Children and Adolescents.

The intensity gradient is a new cutpoint-free metric that was developed to quantify physical activity (PA) measured using accelerometers. This metric was developed for use with the ENMO (Euclidean norm minus one) metric, derived from raw acceleration data, and has not been validated for use with count-based accelerometer data. In this study, we determined whether the intensity gradient could be reproduced using count-based accelerometer data. Twenty participants (aged 7-22 years) wore a GT1M, an ActiGraph (count-based), and a GT9X, ActiGraph (raw accelerations) accelerometer during both in-lab and at-home protocols. We found strong agreement between GT1M and GT9X counts during the combined in-lab activities (mean bias = 2 counts) and between minutes per day with different intensities of activity (e.g., sedentary, light, moderate, and vigorous) classified using cutpoints (mean bias < 5 min/d at all intensities). We generated bin sizes that could be used to generate IGs from the count data (mean bias = -0.15; 95% LOA [-0.65, 0.34]) compared with the original IG. Therefore, the intensity gradient could be used to analyze count data. The count-based intensity gradient metric will be valuable for re-analyzing historical datasets collected using older accelerometer models, such as the GT1M.

Differences in bone mineral density and associated factors in dancers and other female athletes.

Dancers are susceptible to relative energy deficiency in sport (RED-S), specifically low bone mineral density (BMD). Little is known about how dancers' BMD compares to other athletic populations. The objective of this study was to examine the association between participant characteristics and total body areal BMD (aBMD) among female pre-professional dancers compared to other female athletes. Two hundred sixty-nine females (132 pre-professional dancers (17.6 (3.2) years) and 137 sport participants (22.8 (2.6) years) were included in this study. aBMD (g/cm2) was estimated using dual X-ray absorptiometry. Multivariable linear regression was used to examine the association between height-adjusted z-scores of total body aBMD (aBMD-Z) and age (years), body mass index (BMI) (z-score), supplement intake, history of stress fracture, irregular menses, MRI/bone scan, 1-year injury history, oral contraceptives, and activity (dance/sport). Total body aBMD and aBMD-Z were lower in dancers than athletes (dancers: aBMD = 1.03 g/cm2 (95% CI: 1.01, 1.05); aBMD-Z = -0.28 (-0.43, -0.12) (p < 0.001); athletes: aBMD = 1.14 g/cm2 (95% CI: 1.12, 1.16); aBMD-Z = 0.41 (0.25, 0.57) (p < 0.001)). aBMD-Z increased with age (ß = 0.054, 95% CI: 0.017, 0.092; p = 0.004) and BMI (ß = 0.221, 95% CI: 0.006, 0.415; p = 0.043). Activity type modified the relationship between BMI and aBMD-Z (ß = 0.323, 95% CI: 0.025, 0.621; p < 0.033) with a stronger positive association in dancers, compared to other female athletes. Dancers had lower total body aBMD and aBMD-Z than other female athletes. aBMD-Z increases with age in female pre-professional dancers and other female athletes. A stronger association exists between aBMD-Z and BMI in dancers than athletes. Future studies should consider changes in aBMD-Z during adolescence and associations with increased risk of bone injury.

Bone health and physical activity in adolescents with juvenile idiopathic arthritis: a cross-sectional case-control study.

BACKGROUND: Adolescents with juvenile idiopathic arthritis (JIA) tend to engage in less physical activity than their typically developing peers. Physical activity is essential for bone development and reduced physical activity may detrimentally effect bone health. Thus, we examined differences in total body bone mineral content (BMC) and areal bone mineral density (aBMD) between adolescents with JIA and adolescent controls without JIA. We also examined associations between moderate-to-vigorous physical activity (MVPA), lean mass, and bone outcomes. METHODS: Participants included 21 adolescents with JIA (14 females, 7 males) and 21 sex- and age-matched controls aged 10-20 years. Assessments included: height; weight; triple-single-leg-hop distance (TSLH); MVPA by accelerometry; and total body BMC, aBMD, and lean mass measured using dual X-ray absorptiometry. Height-adjusted z-scores were calculated for BMC and aBMD and used for all analyses. Multiple linear mixed effects models examined group differences in BMC and aBMD, adjusting for sex, maturity, MVPA, TSLH, and lean mass. Participants clusters, based on sex and age (within 18 months), were considered random effects. RESULTS: Adolescents with JIA had lower total body aBMD z-scores [ß (95% CI); -0.58 (-1.10 to -0.07), p = 0.03] and BMC z-scores [-0.47 (-0.91 to -0.03), p = 0.04] compared with controls. Mean daily MVPA was 22.0 min/day lower in adolescents with JIA than controls; however, MVPA was not associated with aBMD [-0.01 (-0.01 to 0.01), p = 0.32] or BMC [0.00 (-0.01 to 0.00), p = 0.39]. Lean mass was positively associated with aBMD [0.05 (0.01 to 0.09) g/cm2, p = 0.03] and BMC [0.06 (0.03 to 0.10) g, p < 0.001]. CONCLUSION: Adolescents with JIA had lower total body aBMD and BMC compared with sex- and age-matched controls without JIA. Group differences in bone outcomes were not associated with the lower MVPA participation of adolescents with JIA. Despite this, physical activity should still be encouraged as it promotes physical well-being.

Predicting Bone Adaptation in Astronauts during and after Spaceflight.

A method was previously developed to identify participant-specific parameters in a model of trabecular bone adaptation from longitudinal computed tomography (CT) imaging. In this study, we use these numerical methods to estimate changes in astronaut bone health during the distinct phases of spaceflight and recovery on Earth. Astronauts (N = 16) received high-resolution peripheral quantitative CT (HR-pQCT) scans of their distal tibia prior to launch (L), upon their return from an approximately six-month stay on the international space station (R+0), and after six (R+6) and 12 (R+12) months of recovery. To model trabecular bone adaptation, we determined participant-specific parameters at each time interval and estimated their bone structure at R+0, R+6, and R+12. To assess the fit of our model to this population, we compared static and dynamic bone morphometry as well as the Dice coefficient and symmetric distance at each measurement. In general, modeled and observed static morphometry were highly correlated (R2> 0.94) and statistically different (p < 0.0001) but with errors close to HR-pQCT precision limits. Dynamic morphometry, which captures rates of bone adaptation, was poorly estimated by our model (p < 0.0001). The Dice coefficient and symmetric distance indicated a reasonable local fit between observed and predicted bone volumes. This work applies a general and versatile computational framework to test bone adaptation models. Future work can explore and test increasingly sophisticated models (e.g., those including load or physiological factors) on a participant-specific basis.

Characterizing Bone Phenotypes Related to Skeletal Fragility Using Advanced Medical Imaging.

PURPOSE OF REVIEW: Summarize the recent literature that investigates how advanced medical imaging has contributed to our understanding of skeletal phenotypes and fracture risk across the lifespan. RECENT FINDINGS: Characterization of bone phenotypes on the macro-scale using advanced imaging has shown that while wide bones are generally stronger than narrow bones, they may be more susceptible to age-related declines in bone strength. On the micro-scale, HR-pQCT has been used to identify bone microarchitecture phenotypes that improve stratification of fracture risk based on phenotype-specific risk factors. Adolescence is a key phase for bone development, with distinct sex-specific growth patterns and significant within-sex bone property variability. However, longitudinal studies are needed to evaluate how early skeletal growth impacts adult bone phenotypes and fracture risk. Metabolic and rare bone diseases amplify fracture risk, but the interplay between bone phenotypes and disease remains unclear. Although bone phenotyping is a promising approach to improve fracture risk assessment, the clinical availability of advanced imaging is still limited. Consequently, alternative strategies for assessing and managing fracture risk include vertebral fracture assessment from clinically available medical imaging modalities/techniques or from fracture risk assessment tools based on clinical risk factors. Bone fragility is not solely determined by its density but by a combination of bone geometry, distribution of bone mass, microarchitecture, and the intrinsic material properties of bone tissue. As such, different individuals can exhibit distinct bone phenotypes, which may predispose them to be more vulnerable or resilient to certain perturbations that influence bone strength.

Zoledronic acid after spinal cord injury mitigates losses in proximal femoral strength independent of ambulation ability.

Rapid bone loss can occur after spinal cord injury (SCI) and a standard of care to prevent or treat this phenomenon is an active area of research. Using advanced analysis techniques, this study demonstrates that zoledronic acid, a possible treatment, prevented loss of bone strength at the hip following SCI. INTRODUCTION: Bone loss below the level of neurological lesion is a well-known complication of spinal cord injury (SCI), and effective preventive treatment for this phenomenon is an active area of research. Zoledronic acid has demonstrated efficacy to attenuate bone loss at the hip after SCI, but previous studies relied on measurements from dual-energy X-ray absorptiometry. The purpose of this investigation was to more thoroughly characterize changes to bone mineral and strength at the proximal femur in individuals receiving zoledronic acid in the acute SCI stage; we also examined the influence of ambulatory ability on bone outcomes. METHODS: Participants randomized to either zoledronic acid (n = 29) or placebo (n = 30) received computed tomography (CT) scans and ambulatory assessments at baseline and 6 and 12 months following drug infusion. CT-based finite element (FE) modeling was used to predict changes in proximal femoral strength associated with treatment. RESULTS: After 12 months, FE-predicted bone strength was reduced by a mean (SD) of 9.6 (17.9)% in the zoledronic acid group versus 24.6 (24.5)% in the placebo group (p = 0.007). These differences in strength were explained by reductions in CT measurements of both trabecular (p < 0.001) and cortical (p ≤ 0.021) bone at the femoral neck and trochanteric region. Ambulation ability influenced select trabecular and cortical parameters, but we were unable to detect an impact on FE-predicted bone strength. CONCLUSION: These findings demonstrate that treatment with zoledronic acid in acute SCI attenuates losses in proximal femoral strength, which may reduce the risk of hip fractures across patients with varying degrees of ambulatory abilities.

A 4-Yr Mixed Longitudinal Study of Health Behaviors and Fat Mass Accrual during Adolescence and Early Adulthood.

PURPOSE: Physical activity (PA), sedentary time (SED), and energy intake (EI) are associated with fat mass accrual in children and youth. Previous studies relied primarily on cross-sectional designs and proxy measures of body composition such as body mass index. We aimed to prospectively investigate associations between PA, SED, EI, and total body fat mass accrual using dual-energy x-ray absorptiometry. METHODS: This analysis of the mixed longitudinal Healthy Bones III Study included data from 312 participants (138 boys age 9 to 21 yr at baseline). For each participant, we acquired a maximum of four annual total body dual-energy x-ray absorptiometry scans from which we determined fat mass (in kilograms; n = 748 observations). We assessed total PA, moderate-to-vigorous PA (MVPA) and SED with accelerometers (ActiGraph GT1M) and measured EI via 24-h dietary recall. We fit sex-specific multilevel models adjusting for maturity (years from age at peak height velocity (APHV)), weight status, ethnicity, total PA, MVPA, SED, and EI. RESULTS: Boys and girls demonstrated divergent trajectories of fat mass accrual; rate of fat mass accrual in girls was four times greater than boys at APHV and increased across adolescence, whereas boys' fat mass plateaued after APHV. In boys, within-person change in MVPA negatively predicted fat mass independent of SED; each annual increase in MVPA of 6 min·d -1 was associated with a 0.21-kg lower fat mass. In girls, between-person average MVPA negatively predicted fat mass accrual independent of SED; greater MVPA of 4 min·d -1 across adolescence was associated with a 0.31-kg lower fat mass. CONCLUSIONS: MVPA demonstrates an independent and negative effect on fat mass in boys and girls. Given different trajectories of fat mass accrual and movement behaviors between boys and girls, PA interventions aimed at preventing obesity in youth may benefit from a sex and gendered approach.

Incomplete recovery of bone strength and trabecular microarchitecture at the distal tibia 1 year after return from long duration spaceflight.

Determining the extent of bone recovery after prolonged spaceflight is important for understanding risks to astronaut long-term skeletal health. We examined bone strength, density, and microarchitecture in seventeen astronauts (14 males; mean 47 years) using high-resolution peripheral quantitative computed tomography (HR-pQCT; 61 µm). We imaged the tibia and radius before spaceflight, at return to Earth, and after 6- and 12-months recovery and assessed biomarkers of bone turnover and exercise. Twelve months after flight, group median tibia bone strength (F.Load), total, cortical, and trabecular bone mineral density (BMD), trabecular bone volume fraction and thickness remained - 0.9% to - 2.1% reduced compared with pre-flight (p ≤ 0.001). Astronauts on longer missions (> 6-months) had poorer bone recovery. For example, F.Load recovered by 12-months post-flight in astronauts on shorter (< 6-months; - 0.4% median deficit) but not longer (- 3.9%) missions. Similar disparities were noted for total, trabecular, and cortical BMD. Altogether, nine of 17 astronauts did not fully recover tibia total BMD after 12-months. Astronauts with incomplete recovery had higher biomarkers of bone turnover compared with astronauts whose bone recovered. Study findings suggest incomplete recovery of bone strength, density, and trabecular microarchitecture at the weight-bearing tibia, commensurate with a decade or more of terrestrial age-related bone loss.

Response to High-Dose Vitamin D Supplementation Is Specific to Imaging Modality and Skeletal Site.

High-dose vitamin D supplementation (4000 or 10,000 IU/d) in vitamin D-sufficient individuals results in a dose-dependent decrease in radius and tibia total bone mineral density (Tt.BMD) compared with 400 IU/d. This exploratory analysis examined whether the response to high-dose vitamin D supplementation depends on imaging modality and skeletal site. Participants were aged 55 to 70 years, not osteoporotic, with serum 25(OH)D 30 to 125 nM. Participants' radius and tibia were scanned on high-resolution peripheral quantitative computed tomography (HR-pQCT) to measure Tt.BMD, trabecular bone volume fraction (Tb.BV/TV), trabecular separation (Tb.Sp), cortical thickness (Ct.Th), and finite element analysis (FEA) estimated failure load. Three-dimensional image registration was used. Dual-energy X-ray absorptiometry (DXA) scans of the hip, spine, and radius measured areal BMD (aBMD) and trabecular bone score (TBS). Constrained linear mixed-effects models determined treatment group-by-time and treatment group-by-time-by-sex interactions. The treatment group-by-time interaction previously observed for radial Tt.BMD was observed at both ultradistal (UD, p < 0.001) and 33% (p < 0.001) aBMD sites. However, the treatment group-by-time-by-sex interaction observed for radial Tt.BMD was not observed with aBMD at either the UD or 33% site, and the 4000 and 400 groups did not differ. Registered radial FEA results mirrored Tt.BMD. An increase in Tb.Sp and decrease in Ct.Th underpinned dose-dependent changes in radial BMD and strength. We observed no effects in DXA-based aBMD at the hip or spine or TBS. At the tibia, we observed a time-by-treatment group effect for Tb.BV/TV. Given that DXA measures at the radius did not detect sex differences or differences between the 4000 and 400 groups, HR-pQCT at the radius may be more sensitive for examining bone changes after vitamin D supplementation. Although DXA did not reveal treatment effects at the hip or spine, whether that is a true skeletal site difference or a lack of modality sensitivity remains unclear. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Pre-flight exercise and bone metabolism predict unloading-induced bone loss due to spaceflight.

OBJECTIVES: Bone loss remains a primary health concern for astronauts, despite in-flight exercise. We examined changes in bone microarchitecture, density and strength before and after long-duration spaceflight in relation to biochemical markers of bone turnover and exercise. METHODS: Seventeen astronauts had their distal tibiae and radii imaged before and after space missions to the International Space Station using high-resolution peripheral quantitative CT. We estimated bone strength using finite element analysis and acquired blood and urine biochemical markers of bone turnover before, during and after spaceflight. Pre-flight exercise history and in-flight exercise logs were obtained. Mixed effects models examined changes in bone and biochemical variables and their relationship with mission duration and exercise. RESULTS: At the distal tibia, median cumulative losses after spaceflight were -2.9% to -4.3% for bone strength and total volumetric bone mineral density (vBMD) and -0.8% to -2.6% for trabecular vBMD, bone volume fraction, thickness and cortical vBMD. Mission duration (range 3.5-7 months) significantly predicted bone loss and crewmembers with higher concentrations of biomarkers of bone turnover before spaceflight experienced greater losses in tibia bone strength and density. Lower body resistance training volume (repetitions per week) increased 3-6 times in-flight compared with pre-spaceflight. Increases in training volume predicted preservation of tibia bone strength and trabecular vBMD and thickness. CONCLUSIONS: Findings highlight the fundamental relationship between mission duration and bone loss. Pre-flight markers of bone turnover and exercise history may identify crewmembers at greatest risk of bone loss due to unloading and may focus preventative measures.

The Assessment of Skeletal Muscle and Cortical Bone by Second-generation HR-pQCT at the Tibial Midshaft.

BACKGROUND: Peripheral quantitative computed tomography (pQCT) is the current densitometric gold-standard for assessing skeletal muscle at the 66% proximal tibia site. High resolution peripheral quantitative computed tomography (HR-pQCT) is a leading technology for quantifying bone microarchitecture at the distal extremities, and with the second-generation HR-pQCT it is possible to measure proximal limb sites. Therefore, the objectives of this study were to: (1) assess the feasibility of using HR-pQCT to assess skeletal muscle parameters at the 66% proximal tibia site, and (2) test HR-pQCT skeletal muscle measurement reproducibility at this site. METHODS: Adult participants (9 males; 7 females; ages 31-75) received 1 pQCT scan and 2 HR-pQCT scans at the 66% proximal site of the nondominant tibia. Participants were repositioned between HR-pQCT scans to test reproducibility. HR-pQCT and pQCT scans were analyzed to quantify muscle cross-sectional area (CSA) and muscle density. Coefficients of determination and Bland-Altman plots compared muscle parameters between pQCT and HR-pQCT. For short-term reproducibility, root-mean-square of coefficient of variance and least significant change were calculated. RESULTS: HR-pQCT and pQCT measured muscle density and muscle CSA were positively correlated (R2 = 0.66, R2 = 0.95, p < 0.001, respectively). Muscle density was equivalent between HR-pQCT and pQCT; however, there was systematic and directional bias for muscle CSA, such that muscle CSA was 11% lower with HR-pQCT and bias increased with larger muscle CSA. Root-mean-square of coefficient of variance was 0.67% and 0.92% for HR-pQCT measured muscle density and muscle CSA, respectively, while least significant change was 1.4 mg/cm3 and 174.0 mm2 for muscle density and muscle CSA, respectively. CONCLUSION: HR-pQCT is capable of assessing skeletal muscle at the 66% site of the tibia with good precision. Measures of muscle density are comparable between HR-pQCT and pQCT.

Postural Balance Effects Associated with 400, 4000 or 10,000 IU Vitamin D3 Daily for Three Years: A Secondary Analysis of a Randomized Clinical Trial.

Vitamin D supplementation is proposed as a fall prevention strategy, as it may improve neuromuscular function. We examined whether three years of vitamin D supplementation (400, 4000 or 10,000 IU daily) affects postural sway in older adults. Three hundred and seventy-three non-osteoporotic, vitamin D-sufficient, community-dwelling healthy adults, aged 55-70 years, were randomized to 400 (n = 124), 4000 (n = 125) or 10,000 (n = 124) IU daily vitamin D3 for three years. Sway index was assessed at baseline, 12-, 24- and 36-months using the Biosway machine. We tested participants under four conditions: eyes open or eyes closed with firm (EOFI, ECFI) or foam (EOFO, ECFO) surfaces. Secondary assessments examined sway in the anterior-posterior (AP) and medio-lateral (ML) directions. Linear mixed effects models compared sway between supplementation groups across time. Postural sway under EOFO and ECFO conditions significantly improved in all supplementation groups over time. Postural sway did not differ between supplementation groups at any time under any testing conditions in normal, AP or ML directions (p > 0.05 for all). Our findings suggest that high dose (4000 or 10,000 IU) vitamin D supplementation neither benefit nor impair balance compared with 400 IU daily in non-osteoporotic, vitamin D-sufficient, healthy older adults.

Associations Between Breastfeeding History and Early Postmenopausal Bone Loss.

This study aimed to evaluate associations of parity and breastfeeding history with postmenopausal bone loss. Early postmenopausal women from the Canadian Multicentre Osteoporosis Study were divided into three groups based on their reproductive histories: nulliparous (NP, n = 10), parous with < 6 months breastfeeding (P-NBF, n = 14), and parous with > 6 months breastfeeding (P-BF, n = 21). Women underwent dual X-ray absorptiometry and high-resolution peripheral quantitative computed tomography imaging at baseline and after 6 years to evaluate bone mineral density (BMD), bone microstructure, and finite element-estimated failure load. Average age at baseline was 57 years. Baseline density, microstructure, and failure load were not different among groups. In all women, total and cortical BMD decreased significantly at the tibia and radius. P-BF women only experienced a significant decline in tibial trabecular BMD, with a greater magnitude of change for P-BF than NP women (p = 0.002). Overall, results suggest that early postmenopausal bone health did not differ based on parity or breastfeeding history. Over the 6-year follow-up period, postmenopausal bone loss was evident in all women, with subtle differences in the rate of postmenopausal change among women with varying breastfeeding histories. Parous women who had breastfed for at least 6 months showed an elevated rate of trabecular BMD loss at the tibia. Meanwhile, correlation analyses suggest that longer durations of breastfeeding may be associated with reduced cortical bone loss at the radius. The lack of differences among groups in FE-derived failure load suggests that parity and breastfeeding history is unlikely to significantly affect postmenopausal risk of fracture.

Estimation of Peak Muscle Power From a Countermovement Vertical Jump in Children and Adolescents.

Gomez-Bruton, A, Gabel, L, Nettlefold, L, Macdonald, H, Race, D, and McKay, H. Estimation of peak muscle power from a countermovement vertical jump in children and adolescents. J Strength Cond Res 33(2): 390-398, 2019-Several equations to predict muscle power (MP) from vertical jump height (VJH) have been developed in adults. However, few have been derived in children. We therefore aimed to: (a) evaluate the validity of existing MP estimation equations from a vertical countermovement jump (CMJ) in children and adolescents and (b) develop and validate a new MP estimation equation for use in children and adolescents. We measured peak MP (in watts) and VJH (in centimeters) during a CMJ using a force platform in 249 children and adolescents (9-17 years; 119 boys and 130 girls). We compared actual (force platform) with predicted (12 existing prediction equations) MP using repeated-measures analysis of variance and estimated bias using modified Bland-Altman plots. We developed a new prediction equation using stepwise linear regression, assessed predictive error using leave-one-out and 10-fold cross-validation, and externally validated the equation in an independent sample (n = 100). All existing prediction equations demonstrated some degree of bias, either systematic bias (mean differences ranging 178-1,377 W; 8-64%) or bias at the extremes or interactions with sex. Our new prediction equation estimates MP from VJH and body mass: Power (W) = 54.2 × VJH (cm) + 34.4 × body mass (kg) - 1,520.4. With this new equation, there was no difference between actual and predicted MP (0%) and negligible differences (0.2-0.9%) in R and root mean square error between our observed and cross-validated sets. Actual and predicted MP were not different in our external validation (p = 0.12). The new equation demonstrates excellent validity and can be used to predict MP from a CMJ in children and adolescents.

Physical activity, but not sedentary time, influences bone strength in late adolescence.

Physical activity is essential for optimal bone strength accrual, but we know little about interactions between physical activity, sedentary time, and bone outcomes in older adolescents. Physical activity (by accelerometer and self-report) positively predicted bone strength and the distal and midshaft tibia in 15-year-old boys and girls. Lean body mass mediated the relationship between physical activity and bone strength in adolescents. PURPOSE: To examine the influence of physical activity (PA) and sedentary time on bone strength, structure, and density in older adolescents. METHODS: We used peripheral quantitative computed tomography to estimate bone strength at the distal tibia (8% site; bone strength index, BSI) and tibial midshaft (50% site; polar strength strain index, SSIp) in adolescent boys (n = 86; 15.3 ± 0.4 years) and girls (n = 106; 15.3 ± 0.4 years). Using accelerometers (GT1M, Actigraph), we measured moderate-to-vigorous PA (MVPAAccel), vigorous PA (VPAAccel), and sedentary time in addition to self-reported MVPA (MVPAPAQ-A) and impact PA (ImpactPAPAQ-A). We examined relations between PA and sedentary time and bone outcomes, adjusting for ethnicity, maturity, tibial length, and total body lean mass. RESULTS: At the distal tibia, MVPAAccel and VPAAccel positively predicted BSI (explained 6-7% of the variance, p < 0.05). After adjusting for lean mass, only VPAAccel explained residual variance in BSI. At the tibial midshaft, MVPAAccel, but not VPAAccel, positively predicted SSIp (explained 3% of the variance, p = 0.01). Lean mass attenuated this association. MVPAPAQ-A and ImpactPAPAQ-A also positively predicted BSI and SSIp (explained 2-4% of the variance, p < 0.05), but only ImpactPAPAQ-A explained residual variance in BSI after accounting for lean mass. Sedentary time did not independently predict bone strength at either site. CONCLUSION: Greater tibial bone strength in active adolescents is mediated, in part, by lean mass. Despite spending most of their day in sedentary pursuits, adolescents' bone strength was not negatively influenced by sedentary time.

Sex-, Ethnic-, and Age-Specific Centile Curves for pQCT- and HR-pQCT-Derived Measures of Bone Structure and Strength in Adolescents and Young Adults.

There are presently no adolescent centile curves for bone parameters at the tibial midshaft using peripheral quantitative computed tomography (pQCT) or at the distal radius and tibia using high-resolution pQCT (HR-pQCT). Thus, we aimed to develop sex-, ethnic-, site-, and age-specific centile curves for pQCT and HR-pQCT-derived bone outcomes for youth and young adults aged 10 to 21 years. We acquired pQCT scans (XCT3000 or XCT2000) at the tibial midshaft (50% site) and HR-pQCT scans (XtremeCT) at the distal radius (7% site) and tibia (8% site) in a convenience sample of participants in the mixed-longitudinal University of British Columbia Healthy Bones III Study. We scanned 778 10- to 21-year-olds annually for a maximum of 11 years using pQCT (413 girls, 56% Asian; 365 boys, 54% Asian; n = 3160 observations) and 349 10- to 21-year-olds annually for a maximum of 4 years using HR-pQCT (189 girls, 51% Asian; 165 boys, 50% Asian; n = 1090 observations). For pQCT, we report cortical bone mineral density (BMD), total bone cross-sectional area, and polar strength-strain index. For HR-pQCT, we report standard measures (total BMD, trabecular number, thickness, and bone volume fraction) and automated segmentation measures (total bone cross-sectional area, cortical BMD, porosity, and thickness). We applied finite element analysis to estimate failure load. We applied the lamda, mu, sigma (LMS) method using LMS ChartMaker Light (version 2.5, The Institute of Child Health, London, UK) to construct LMS tables and centile plots. We report sex- and age-specific centiles (3rd, 10th, 25th, 50th, 75th, and 97th) for whites and Asians for pQCT bone parameters at the tibial midshaft and HR-pQCT bone parameters at the distal radius and tibia. These centile curves might be used by clinicians and scientists to interpret values or better understand trajectories of bone parameters in clinical populations, those from different geographic regions or of different ethnic origins. © 2018 American Society for Bone and Mineral Research.

Bone Strength in Girls and Boys After a Distal Radius Fracture: A 2-Year HR-pQCT Double Cohort Study.

We recently reported impaired bone strength in girls with low- to moderate-energy distal radius fractures (Fx) compared with girls with no history of forearm fractures (Non-Fx). We aimed to determine whether bone strength deficits observed at baseline were still present after 2 years. We assessed bone strength, microarchitecture, and bone mineral density (BMD) of the non-fractured (Fx) and non-dominant (Non-Fx) distal radius (7% site) at baseline, 12, and 24 months using high-resolution pQCT (Scanco Medical, Brüttisellen, Switzerland) in 104 girls (aged 11.0 ± 1.7 years; 47 Fx, 57 Non-Fx) and 157 boys (aged 12.7 ± 1.7 years; 86 Fx, 71 Non-Fx). Bone outcomes included total area (Tt.Ar) and bone mineral density (Tt.BMD), trabecular bone volume ratio (BV/TV), thickness (Tb.Th), separation (Tb.Sp), and number (Tb.N), and cortical BMD (Ct.BMD), thickness (Ct.Th), and porosity (Ct.Po). We used finite element analysis to estimate bone strength (failure load [F.Load]; ultimate stress [U.Stress]; load-to-strength ratio). We used sex-specific mixed-effects models to compare bone outcomes between Fx and Non-Fx over 2 years. In girls, those with fractures had 18% to 24% lower U.Stress and 5% to 9% lower Tt.BMD than Non-Fx at all time points (p < 0.017). In secondary analysis by fracture degree, girls with low-energy (LE) fractures had 19% to 21% lower F.Load, 25% to 47% lower U.Stress, 11% to 14% lower Tt.BMD, and 11% to 15% lower BV/TV than Non-Fx at all time points (p < 0.017). In contrast, boys' bone outcomes were similar between Fx and Non-Fx at all time points. In secondary analysis by fracture degree, boys with LE fractures had 10% lower Tt.BMD and 10% lower Ct.Th compared with Non-Fx at 12 months only. Deficits in distal radius bone strength and trabecular bone microarchitecture appear to track across 2 years after a forearm fracture in girls but not in boys. Longer follow-up is needed to determine whether deficits persist into adulthood in women and how they may influence future risk of fragility fracture. © 2017 American Society for Bone and Mineral Research.

Bouts of Vigorous Physical Activity and Bone Strength Accrual During Adolescence.

PURPOSE: We examined the influence of vigorous physical activity (VPA) bout frequency on bone strength accrual across adolescence, independent of total volume of VPA. METHODS: We measured VPA (6 metabolic equivalents; total volume and bout frequency <5 min in duration) annually using waist-worn accelerometers (ActiGraph GT1M) in 309 adolescents (9-20 y at baseline: 99, <13 y; 126, 13-18 y; 84, >18 y) over a maximum of 4 years. We applied finite element analysis to high-resolution peripheral quantitative computed tomography scans of the distal tibia (8% site) to estimate bone strength (failure load; F.Load, Newtons). We fit a mixed effects model with maturity offset (years from age at peak height velocity) as a random effect and sex, ethnicity, tibia length, lean body mass, and VPA (volume and bout frequency) as fixed effects. RESULTS: VPA volume and bout frequency were positively associated with F.Load across adolescence; however, VPA volume did not predict F.Load once VPA bout frequency was included in the model. Participants in the upper quartile of VPA bout frequency (∼33 bouts per day) had 10% (500 N) greater F.Load across adolescence compared with participants in the lowest quartile (∼9 bouts per day; P = .012). Each additional daily bout of VPA was associated with 21 N greater F.Load, independent of total volume of VPA. CONCLUSION: Frequent VPA should be promoted for optimal bone strength accrual.

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. © 2017 American Society for Bone and Mineral Research.